# Deposition of Coating to Protect Waste Water Reservoir in Acidic Solution by Arc Thermal Spray Process

Authors: Han-Seung Lee; Jin-ho Park; Jitendra Kumar Singh; Mohamed A. Ismail
Journal: Advances in Materials Science and Engineering
 (2018)

---

## Abstract

The corrosion characteristics of 304 stainless steel (SS) and titanium (Ti) coatings deposited by the arc thermal spray process in pH 4 solution were assessed. The3b2:?thyc=10Ti-sprayed3b2:?thyc coating exhibits uniform, less porous, and adherent coating morphology compared to the 3b2:?thyc=10SS-sprayed3b2:?thyc coating. The electrochemical study, that is, electrochemical impedance spectroscopy (EIS), revealed that as exposure periods to solution were increased, the polarization resistance (Rp) decreased and the charge transfer resistance (Rct) increased owing to corrosion of the metallic surface and simultaneously at the same time the deposition of oxide 3b2:?thyc=10films/corrosion 3b2:?thycon the 3b2:?thyc=10SS-3b2:?show $132#sprayed3b2:?thyc surface, while Ti coating transformed unstable oxides into the stable phase. Potentiodynamic studies confirmed that both sprayed coatings exhibited passive tendency attributed due to the deposition of corrosion products on SS samples, whereas the 3b2:?thyc=10Ti-sprayed3b2:?thyc sample formed passive oxide films. The Ti coating reduced the corrosion rate by more than six times compared to the SS coating after 312 h of exposure to sulfuric acid- (H2SO4-) contaminated water solution, that is, pH 4. Scanning electron microscope (SEM) results confirmed the uniform and globular morphology of the passive film on the Ti coating resulting in reduced corrosion. On the other hand, the corrosion products formed on 3b2:?thyc=10SS-sprayed3b2:?thyc coating exhibit micropores with a 3b2:?thyc=10net-like3b2:?thyc microstructure. X-3b2:?show $132#ray diffraction (XRD) revealed the presence of the composite oxide film on 3b2:?thyc=10Ti-sprayed3b2:?thyc samples and lepidocrocite (γ-FeOOH) on the 3b2:?thyc=10SS-coated3b2:?thyc surface. The transformation of TiO and Ti3O into TiO2 (rutile and anatase) and Ti3O5 after 312 h of exposure to H2SO4 acid reveals the improved corrosion resistance properties of 3b2:?thyc=10Ti-sprayed3b2:?thyc coating.


---

## Body

## 1. Introduction

3b2:?A3B2 tpt 1ptConcrete is a material that can withstand and sustain exposure to an aggressive environment over long periods and resist deterioration. Therefore, concrete is a reliable and durable construction material with versatile applications in waste water reservoirs, buildings, bridges, towers, roads, and so on. However, it is also a porous material, which means that aggressive ions such as Cl−, CO3−, and SO4− can slowly or steadily penetrate and move toward the embedded steel 3b2:?A3B2 tlsb -.1ptrebar, thus causing corrosion that leads to premature concrete failure [1–3b2:?show $132#3].The major factor in concrete deterioration is acidic impurities in its surrounding [4]. Different external coatings have been applied to protect concrete and the embedded steel rebar. Swamy and Tanikawa used acrylic rubber as an external coating material for concrete in accelerated 3b2:?thyc=10wet-dry3b2:?thyc and saline environments [5]. They found that this coating withstood saline and ozone exposures but suffered extensive cracking when exposed to ultraviolet radiation. Therefore, polymeric coating is not suitable for concrete coating because of vast differences in the thermal contraction and expansion coefficients between the concrete and polymer.The presence of3b2:?thyc=10sulfur-reducing3b2:?thyc bacteria in waste water makes it acidic [6,[tnq_nbsp] 7]. The minimum pH, that is, 4.5, of waste water can be obtained [8] depending on the source and environment where it is produced.The acidity of waste water influences the deterioration of the concrete and steel rebar. Therefore, to protect these from corrosion, SS grouting and anchoring have been widely used [6,[tnq_nbsp] 7]. SS plates are used in waste water reservoirs to protect the concrete and embedded steel rebar. But cost factors are affecting the use of such steel plates on the outer surface of 3b2:?A3B2 tlsb -.18ptconcrete. Nonetheless, SS plates and other protective methods or surface treatments are frequently used to protect the concrete and waste water reservoirs owing to their high resistance to corrosion [9]. Also, the nanostructured and PVD coatings were used in 3b2:?thyc=10high-speed3b2:?thyc drilling and cutting instruments to enhance the mechanical properties of tools [10,[tnq_nbsp] 11].A method to deposit coatings onto the concrete surface is an important factor in the deposition of high melting point3b2:?thyc=10corrosion-resistant3b2:?thyc metals. One view is that the coating process should be considered for deposition on account of its feasibility and applicability. High 3b2:?thyc=10corrosion-resistant3b2:?thyc materials such as SS, nickel (Ni), tungsten (W), molybdenum (Mo), chromium (Cr), and Ti need a specific deposition process. The arc thermal spray process is suitable and feasible for depositing these metals onto any substrate [12–3b2:?show $132#17].3b2:?A3B2 tlsb .1ptThe arc thermal spray process is an easy process for the deposition of coatings onto concrete and steel substrates. This process involves arc spraying with twin metal wires on oppositely charged tips that use atomized hot air to deposit the coating onto the substrate [12]. During the coating process, melted metal droplets are deposited and form[tnq_nbsp] a thick layer on the substrate. During coating deposition, 3b2:?thyc=10pores/defects 3b2:?thycare formed, which is an inherent property of the arc thermal spray process [13–3b2:?show $132#15]. Pore formation depends on the metal to be used and spraying parameters of the process.Our recently published paper showed that the 316L SS coating applied on the concrete substrate, and then sealed with alkyl epoxide, effectively protected the surface from corrosion in pH 4, 5, and 6 solutions. The most destructive was the pH 4 solution because of its higher acidity compared with the other solutions [16]. In the pH 5 solution, the tested coatings exhibited the highest corrosion resistance because of the presence of undissociated water molecules that formed a protective passive film on the coating surface. This experiment was carried out for instantaneous exposure; there is currently no study on prolonged exposure.The pH 4 solution is an aggressive environment, and the surface is expected to deteriorate dramatically. This pH solution increases the risk of corrosion due to its more acidic nature. If a coating can withstand this pH, then it can extend the protection of a waste water reservoir against corrosion.The present investigation aims to protect the concrete of a waste water reservoir from deterioration, spalling, and thawing during prolonged exposure to an acidic environment using surface treatment with 304 SS and Ti metallic coatings by an arc thermal spray process. This communication is an advancement of our earlier published work [16]. The acidic condition was simulated by mixing 0.1 M·H2SO4 (pH = 1) in distilled water to reduce the pH of distilled water from 6.5 to 4.0 at 25°C. Assessments of the corrosion resistance of these coatings were carried out using different electrochemical techniques.


## 2. Experimental Section

### 2.1. Process of Coating

3b2:?A3B2 tlsb -0.1ptThe 304 SS and Ti coatings were applied to 3b2:?thyc=10grit-blasted3b2:?thyc mild steel containing carbon (C) = 0.24, silicon (Si) = 0.26, manganese (Mn) = 0.95, phosphorus (P) = 0.016, sulfur (S) = 0.008, copper (Cu) = 0.02, chromium (Cr) = 0.04, nickel (Ni) = 0.03, and iron (Fe) = balance (wt.%) and to concrete substrates by an arc thermal spray process. The coating deposited onto the steel substrate was used to study its electrochemical and physical characteristics as shown in Figure[tnq_nbsp] 3b2:?show [?"gr1",5,"cond1"]1, while the coating deposited onto the concrete surface was used for bond adhesion measurement.Figure 1
Sketch of coating applied on the steel substrate.Concrete is a3b2:?thyc=10low-conducting3b2:?thyc material that cannot be used for electrochemical studies. Hence, it was not considered for electrochemical studies. A 1.6 mm diameter wire of 304 SS and commercially pure Ti was used for metal spraying in the arc thermal spraying process to deposit the coatings onto the substrates.Prior to the deposition of coatings, the steel substrate was pickled with 10%3b2:?thyc=10v/v 3b2:?thycHCl, thoroughly washed with distilled water, dried, and finally 3b2:?thyc=10grit-blasted3b2:?thyc with 0.7 and 0.8 mm steel balls using a pressure machine at 0.7 MPa. Coating thickness was measured using a nondestructive Elcometer 3b2:?A3B2 tlsb -.1pt456 dry film thickness gauge at different locations on the steel 3b2:?A3B2 tlsb .1ptand concrete substrate; the thickness was approximately 200 µm (±5 µm).3b2:?A3B2 tlsb -.1ptThe coatings were applied to the substrate by the arc thermal spray gun 3b2:?thyc=10(LD/U3 3b2:?thycelectric arc wire spray gun, Oerlikon MetcoTM, Switzerland) process using 304 SS and Ti wires with a circular slit of hot and compressed air [17–3b2:?show $132#20]. The melted metal particles diffused onto the substrate and cooled at room temperature, resulting in the formation of 3b2:?thyc=10pores/defects 3b2:?thycon the coating.The spraying of metallic coating on the target substrate was carried out by keeping the sample 20 cm away from the spray gun at an air pressure of 6 bar. The spraying voltage and current were maintained at approximately 30 V and 200 mA, respectively [15,[tnq_nbsp] 21–3b2:?show $132#23].After applying the coating on the concrete substrate, coating adhesion was measured according to the KS F4716 test method [24]. In this process, a 300 mm × 300 mm section of the coated substrate was taken for the adhesion test (Figure[tnq_nbsp] 1).


### 2.2. Electrochemical Experiments

Electrochemical experiments were carried out on the deposited coatings and 304 SS plate. For the sake of comparison with deposited coating, we have chosen to characterize the 304 SS plate.To perform the electrochemical experiments, a solution was prepared in double distilled water by adding a few drops of 0.1 M·H2SO4 to reduce the pH to 4 at 25°C. These experiments were performed using three electrode systems [17], where the coating acted as the working electrode (WE), the platinum wire acted as the counterelectrode, and the 3b2:?thyc=103b2:?A3B2 tlsb -.1ptsilver/silver 3b2:?thycchloride 3b2:?thyc=10(Ag/AgCl) 3b2:?thycacted as the reference electrode. The area of the WE was 0.78 cm2 and was fixed for all the samples.EIS experiments were carried out by changing the frequency of a 10 mV sinusoidal voltage from 100 kHz to 0.01 Hz. The potentiodynamic experiments were performed from −0.4 V to +0.8 V versus3b2:?thyc=10Ag/AgCl 3b2:?thycat 1 3b2:?thyc=10mV/s 3b2:?thycscan rate.[tnq_nbsp] The potentiostat used in this study was a VersaSTAT (Princeton Applied Research, Oak Ridge, TN, USA), and data analysis was carried out using the Metrohm Autolab Nova 1.10 software.The 304 SS plate was abraded with an emery paper from 60µm up to 300 µm to remove the native oxides from the surface prior to starting electrochemical experiments. All electrochemical experiments were conducted in triplicate at room temperature (27 ± 1°C) to generate reproducible data.


### 2.3. Characterization of Coating

The morphology of the deposited coatings and 304 SS plate was determined by an SEM (Philips XL 30) operated at 15 kV. Prior to taking the images of the samples, these were coated with platinum to increase their conductivity and avoid a charging effect.XRD (Philips X’3b2:?thyc=10Pert-MPD)3b2:?thyc studies of the coatings and 304 SS plate were performed using Cu-Kα radiation (λ = 1.54059 Å) generated at 40 kV and 100 mA. The scanning rate to scan XRD data from 10 to 90° was at 0.5°/min.


## 2.1. Process of Coating

3b2:?A3B2 tlsb -0.1ptThe 304 SS and Ti coatings were applied to 3b2:?thyc=10grit-blasted3b2:?thyc mild steel containing carbon (C) = 0.24, silicon (Si) = 0.26, manganese (Mn) = 0.95, phosphorus (P) = 0.016, sulfur (S) = 0.008, copper (Cu) = 0.02, chromium (Cr) = 0.04, nickel (Ni) = 0.03, and iron (Fe) = balance (wt.%) and to concrete substrates by an arc thermal spray process. The coating deposited onto the steel substrate was used to study its electrochemical and physical characteristics as shown in Figure[tnq_nbsp] 3b2:?show [?"gr1",5,"cond1"]1, while the coating deposited onto the concrete surface was used for bond adhesion measurement.Figure 1
Sketch of coating applied on the steel substrate.Concrete is a3b2:?thyc=10low-conducting3b2:?thyc material that cannot be used for electrochemical studies. Hence, it was not considered for electrochemical studies. A 1.6 mm diameter wire of 304 SS and commercially pure Ti was used for metal spraying in the arc thermal spraying process to deposit the coatings onto the substrates.Prior to the deposition of coatings, the steel substrate was pickled with 10%3b2:?thyc=10v/v 3b2:?thycHCl, thoroughly washed with distilled water, dried, and finally 3b2:?thyc=10grit-blasted3b2:?thyc with 0.7 and 0.8 mm steel balls using a pressure machine at 0.7 MPa. Coating thickness was measured using a nondestructive Elcometer 3b2:?A3B2 tlsb -.1pt456 dry film thickness gauge at different locations on the steel 3b2:?A3B2 tlsb .1ptand concrete substrate; the thickness was approximately 200 µm (±5 µm).3b2:?A3B2 tlsb -.1ptThe coatings were applied to the substrate by the arc thermal spray gun 3b2:?thyc=10(LD/U3 3b2:?thycelectric arc wire spray gun, Oerlikon MetcoTM, Switzerland) process using 304 SS and Ti wires with a circular slit of hot and compressed air [17–3b2:?show $132#20]. The melted metal particles diffused onto the substrate and cooled at room temperature, resulting in the formation of 3b2:?thyc=10pores/defects 3b2:?thycon the coating.The spraying of metallic coating on the target substrate was carried out by keeping the sample 20 cm away from the spray gun at an air pressure of 6 bar. The spraying voltage and current were maintained at approximately 30 V and 200 mA, respectively [15,[tnq_nbsp] 21–3b2:?show $132#23].After applying the coating on the concrete substrate, coating adhesion was measured according to the KS F4716 test method [24]. In this process, a 300 mm × 300 mm section of the coated substrate was taken for the adhesion test (Figure[tnq_nbsp] 1).


## 2.2. Electrochemical Experiments

Electrochemical experiments were carried out on the deposited coatings and 304 SS plate. For the sake of comparison with deposited coating, we have chosen to characterize the 304 SS plate.To perform the electrochemical experiments, a solution was prepared in double distilled water by adding a few drops of 0.1 M·H2SO4 to reduce the pH to 4 at 25°C. These experiments were performed using three electrode systems [17], where the coating acted as the working electrode (WE), the platinum wire acted as the counterelectrode, and the 3b2:?thyc=103b2:?A3B2 tlsb -.1ptsilver/silver 3b2:?thycchloride 3b2:?thyc=10(Ag/AgCl) 3b2:?thycacted as the reference electrode. The area of the WE was 0.78 cm2 and was fixed for all the samples.EIS experiments were carried out by changing the frequency of a 10 mV sinusoidal voltage from 100 kHz to 0.01 Hz. The potentiodynamic experiments were performed from −0.4 V to +0.8 V versus3b2:?thyc=10Ag/AgCl 3b2:?thycat 1 3b2:?thyc=10mV/s 3b2:?thycscan rate.[tnq_nbsp] The potentiostat used in this study was a VersaSTAT (Princeton Applied Research, Oak Ridge, TN, USA), and data analysis was carried out using the Metrohm Autolab Nova 1.10 software.The 304 SS plate was abraded with an emery paper from 60µm up to 300 µm to remove the native oxides from the surface prior to starting electrochemical experiments. All electrochemical experiments were conducted in triplicate at room temperature (27 ± 1°C) to generate reproducible data.


## 2.3. Characterization of Coating

The morphology of the deposited coatings and 304 SS plate was determined by an SEM (Philips XL 30) operated at 15 kV. Prior to taking the images of the samples, these were coated with platinum to increase their conductivity and avoid a charging effect.XRD (Philips X’3b2:?thyc=10Pert-MPD)3b2:?thyc studies of the coatings and 304 SS plate were performed using Cu-Kα radiation (λ = 1.54059 Å) generated at 40 kV and 100 mA. The scanning rate to scan XRD data from 10 to 90° was at 0.5°/min.


## 3. Results and Discussion

### 3.1. Adhesion Test of Sprayed Coatings

Adhesion measurements were carried out after deposition of coatings onto the concrete surface by arc thermal spraying. This was measured for four samples, and the average was calculated. The average adhesion values of 304 SS- and3b2:?thyc=10Ti-sprayed3b2:?thyc surfaces were 3.39 and 2.72 MPa, respectively. The 3b2:?thyc=10SS-sprayed3b2:?thyc coating exhibits higher adhesion values than 3b2:?thyc=10Ti-sprayed3b2:?thyc coating. The standard deviation of SS- and 3b2:?thyc=10Ti-sprayed3b2:?thyc coating was calculated, 3b2:?A3B2 tlsb -.1ptand these were 0.40 and 0.24 MPa, respectively. This indicates that SS coating adhered strongly to the concrete surface, whereas 3b2:?thyc=10Ti-sprayed3b2:?thyc coating adhered 1.25 times lesser than the 304 SS coating. The higher adhesion values of the SS coating may be attributed to small interfacial separation between the concrete substrate and metallic particles, while that for Ti was large [25].3b2:?A3B2 tpb 1.6pt


### 3.2. Morphology of SS Plate and Sprayed Coating

3b2:?A3B2 tlsb .1ptThe morphologies of the 304 SS plate and deposited coatings on[tnq_nbsp] the mild steel substrate were characterized by SEM. Figure[tnq_nbsp] 3b2:?show [?"gr2",5,"cond1"]2 shows the SEM images of the deposited coatings and 304 SS plate. The SS plate surface exhibited a smooth and very finely scratched line (Figure[tnq_nbsp] 2(a)), while the[tnq_nbsp] de3b2:?show $132#3b2:?show $132#posited coatings had many cracks and defects over the surfaces (Figures[tnq_nbsp] 2(b) and (c)).3b2:?tex type="figure" layout="twocols" n="2,3"Figure 2
FE-SEM images of the (a) SS plate, (b) SS-sprayed surface, and (c) Ti-sprayed surface.
(a)
(b)
(c)The scratched line on the plate surface was caused by abrasion with an emery paper up to 300µm because this grade of the emery paper is hard and makes some fine 3b2:?thyc=10defects/lines 3b2:?thycon the surface. The 3b2:?thyc=10SS-sprayed3b2:?thyc surface showed coagulated valleys and uneven deposition, while the 3b2:?thyc=10Ti-sprayed3b2:?thyc surface showed uniform, nanosized globular, and fine elongated cracks. The fine cracks on the 3b2:?thyc=10Ti-sprayed3b2:?thyc surface might be due to formation of thin and nanoscaled brittle oxides. The morphology of 3b2:?thyc=10SS-sprayed3b2:?thyc coating can be attributed due to the sudden cooling of melted metal particles at room temperature (27 ± 1°C).3b2:?A3B2 tlsb -.13ptThe microstructure of the 3b2:?thyc=10SS-sprayed3b2:?thyc coating (Figure[tnq_nbsp] 2(b)) 3b2:?A3B2 tlsb .1ptcould allow the deposition of aggressive ions, water, and moisture particles on valleys, which cause localized or crevice corrosion. The 3b2:?thyc=10Ti-sprayed3b2:?thyc coating also exhibited fine cracking on the surface but had little influence on the deposition of water molecules. Owing to the smooth microstructure of the Ti coating, water molecules may slide off the surface.


### 3.3. Phase Identification of SS Plate and Sprayed Coatings by XRD

3b2:?A3B2 tlsb .1ptXRD was performed to determine the phases present in the coatings and plate surfaces. The results are plotted between 2θ = 10° and 90° versus intensity in counts per second (CPS) and shown in Figure[tnq_nbsp] 3b2:?show [?"gr3",5,"cond1"]3. The SS plate and sprayed surfaces exhibited the austenite phase of the 3b2:?thyc=10Fe-Cr-Ni3b2:?thyc alloy, that is, 304 SS [26,[tnq_nbsp] 27]. Besides this phase in the 3b2:?thyc=10SS-sprayed3b2:?thyc surface, magnetite (Fe3O4) is also observed, and it 3b2:?A3B2 tlsb -.1ptis due to the partial oxidation of coating during the spraying process.Figure 3
XRD of the SS plate and sprayed coatings by the arc thermal spray process (0.5°/min).XRD of3b2:?thyc=10Ti-sprayed3b2:?thyc coating showed Ti [28] and two oxides such as TiO and Ti3O, which were formed due to higher melting point of it than SS where there are possibilities to oxidize the deposited coating. The another reason to oxidize the Ti is high affinity of it with atmospheric oxygen and thus to form surface oxide films such as TiO or Ti3O. The formation of these two oxides of Ti in open atmosphere has also been reported by other researchers [29,[tnq_nbsp] 30]. However, these oxides are amorphous, brittle, and unstable in in vivo conditions which easily can be removed simply by brushing with soft tissues [31–3b2:?show $132#33].


### 3.4. Electrochemical Studies of SS Plate and Sprayed Coating in pH 4 Solution

#### 3.4.1. EIS Studies

3b2:?A3B2 tlsb -.11ptThe samples were immersed in pH 4 solution for different periods of exposure. EIS was carried out to study their corrosion characteristics. These results are shown in Figures[tnq_nbsp] 3b2:?show [?"gr4",5,"cond1"]4 and 3b2:?show [?"gr5",5,"cond1"]5. The electrical equivalent circuit (EEC) is shown in the corresponding Nyquist plots. The EEC of the SS plate exposed to the pH 4 solution for 1 h is shown in Figure[tnq_nbsp] 4(a). In the Nyquist plot of the SS plate, the sample after 1 h of exposure is differentiated by two depressed semicircle loops such as one at high while another at the lower studied frequency. For more clarity of plots at high frequency, the Nyquist result of samples is shown in Figure[tnq_nbsp] S1 (supplementary figure). These results can be explained either by the heterogeneity of the solid surface or by the dispersion of some physical properties. The interface of the surface cannot be considered as an ideal capacitor due to heterogeneity of the surface, and it may involve a 3b2:?thyc=10constant-phase3b2:?thyc element (CPE) as a substitute of the ideal capacitor. The first EEC consists of the[tnq_nbsp] solution resistance (Rs), polarization resistance (Rp), and CPE1 due to the metal surface and nonideal 3b2:?thyc=10double-layer3b2:?thyc capacitance behaviour [34–3b2:?show $132#36]. The Rp and CPE1 are parallel to each other. However, the second EEC contains the charge transfer resistance (Rct) and CPE2. The formation of Rct may be due to formation of the protective passive layer on the SS plate surface in acidic pH solution after 1 h of exposure. These two EECs are connected in series to each other.3b2:?tex type="figure" layout="twocols" n="2"Figure 4
Impedance spectra: (a) Nyquist and (b) Bode |Z| and (θ) of the SS plate and sprayed coatings in pH 4 solution after 1 h of exposure (100 kHz to 0.01 Hz).
bottom
3b2:?A3B2 tpt 4pt(a)
(b)3b2:?tex type="figure" layout="twocols" n="2"Figure 5
Impedance spectra: (a) Nyquist and (b) Bode |Z| and (θ) of the SS plate and sprayed coatings in pH 4 solution after 312 h of exposure (100 kHz to 0.01 Hz).
bottom
3b2:?A3B2 tpt -2pt(a)
(b)The EECs for the SS- and3b2:?thyc=10Ti-sprayed3b2:?thyc coating systems are somewhat different from the SS plate, and they are inserted in Figure[tnq_nbsp] 4(a). The different EECs for these coatings may be attributed to the inherent property of the arc thermal spray process, where coatings suffer from surface defects. Due to reaction on the metal surface, Rct participated owing to initiation of the corrosion process in acidic pH solution. The reaction on the metal surface caused by Rct led to the formation of a 3b2:?thyc=10passive/oxide 3b2:?thyclayer on the metal surface, which increased the resistance and reduced the corrosion reaction. The CPE1 due to a nonideal behaviour of the coating surface and Rp are parallel to each other, while another EEC is connected in series to Rp which contains CPE2 and Rct [15]. The CPE2 and Rct are parallel to each other.The Nyquist plots reveal the real characteristics of the samples after 1 h of exposure (Figure[tnq_nbsp] 4(a)). The samples were exposed to the solution for 1 h to stabilize the potential; thereafter, EIS measurements were performed.Figure[tnq_nbsp] 4(a) shows the two semicircle loops in the Nyquist plots exhibited by coated samples. The SS- and 3b2:?thyc=10Ti-sprayed3b2:?thyc coatings show zigzag and scattered plots which might be due to low conductivity of electrolytes, deposition of defective coating, formation of a defective passive film, and the presence of more resistive elements such as Ti in 3b2:?thyc=10Ti-sprayed3b2:?thyc coating while Cr and Ni in 3b2:?thyc=10SS-sprayed3b2:?thyc samples [37–3b2:?show $132#40]. These results are attributed to the fact that both sprayed samples exhibit capacitive properties due to the presence of defects. However, the Ti coating imposed a resistance greater than the SS coating due to the formation of a 3b2:?thyc=10passive/oxide 3b2:?thycfilm with capacitive behaviour which enabled surface resistance to penetrate the ions of the acidic solution.In the sulfuric acid solution, the Ti surface tends to form defective passive films with high resistance. Similar results have been observed by Baron et[tnq_nbsp] al. on TiAlV and TiAlNb alloys in Tyrode’s solution [41].The dimensions of the semicircle loop of SS plate samples clearly show high capacitive property that enables the surface to resist the penetration of the solution. The capacitive property of the passive film on the SS plate was attributed to the formation of3b2:?thyc=10Cr-enriched3b2:?thyc oxide and NiO in the H2SO4-contaminated water solution [42].The dimensions of the semicircle loops of SS- and3b2:?thyc=10Ti-sprayed3b2:?thyc coatings were less than those of the SS plate because the sprayed samples were more susceptible to corrosion, owing to the formation of 3b2:?thyc=10defects/pores 3b2:?thycon the coating surface in the solution after 1 h of exposure.The impedance at low frequency (0.01 Hz) and the phaseθ (°) of 3b2:?A3B2 tlsb -0.15ptBode plots were plotted against log |Z| (Ω·cm2) ver3b2:?show $132#sus log f (Hz) and θ (°) versus log f (Hz) in Figure[tnq_nbsp] 4(b), re3b2:?show $132#spec3b2:?show $132#tively.[tnq_nbsp] The impedance values of the SS plate sample were greater than[tnq_nbsp] those of the SS and Ti coatings. The SS coating exhibited lower impedance values because of the presence of more 3b2:?thyc=10defects/pores 3b2:?thycat locations where the chances of penetration by the acidic solution are high, and this initiated the deterioration of the coating.3b2:?A3B2 tlsb -.28ptFrom the log |Z| (Ω·cm2)-log f (Hz) Bode plots (Figure[tnq_nbsp] 4(b)), 3b2:?A3B2 tlsb -.1ptit can be seen that, at high frequency (100 kHz), resistance was moreover identical to that observed at low frequency, that is, 0.01 Hz, which might be attributed to the low conductivity of acidic pH solution.In this study, the solution was prepared by adding a few drops of 0.1 M·H2SO4 to distilled water. The solution conductivity was very low, which caused the resistance in total impedance. The conductivity of the solution is an important parameter that must be considered in electrochemical studies. However, the Ti coating exhibited higher impedance than the SS coating due to formation of a thick and protective passive oxide film. The low impedance values of the sprayed samples are due to the presence of more defects or less interfacial resistance between the splats of coating than on the SS plate sample.3b2:?A3B2 tlsb -.1ptThe surface finish and coating microstructure play an important role to determine the corrosion resistance properties of materials in the solution. The pH 4 solution is very aggressive and causes localized or pitting corrosion of the oxide films formed during exposure [43–3b2:?show $132#45].3b2:?A3B2 tlsb -.1ptThe defective parts of coatings can function as an anode, while the remaining acts as a cathode, resulting in the formation of microgalvanic cells on the surface. The presence of microgalvanic cells enhances the corrosion rate of materials; thus, there is a chance of getting low impedance. Such observations are found in SS- and 3b2:?thyc=10Ti-sprayed3b2:?thyc coatings. In view of the above, it can be observed that the SS coating exhibited 3b2:?thyc=10valley-type3b2:?thyc deposits (Figure[tnq_nbsp] 2(b)) where the acidic solution could 3b2:?thyc=10stagnant/deposit 3b2:?thycand cause localized and crevice corrosion. During the initial period of exposure, both sprayed coatings had defects that resulted in lower impedance values than the SS plate surface.3b2:?A3B2 tlsb .1ptThe SS plate shows a −40° shift of the phase angle maxima at the lower studied frequency and reveals high resistance to corrosion in the pH 4 solution (Figure[tnq_nbsp] 4(b)). On the other hand, Ti and SS coatings exhibited −1° and −2° shifts, respectively, which indicate their susceptibility to corrosion during the initial period of exposure [37]. In the middle frequency range, the samples exhibited scattering which might be due to the capacitive response of the defective passive film that was formed during exposure of the samples to H2SO4 solution [38–3b2:?show $132#40].The shifting of maxima at the higher studied frequency (100 kHz) is due to the deposition of corrosion products on the3b2:?thyc=10SS-sprayed3b2:?thyc sample, whereas on the 3b2:?thyc=10Ti-coated3b2:?thyc sample, it is due to formation of the resistive passive film. It can be seen that the Ti coating exhibited approximately −57° shift followed by the SS coating at −38°, while the SS plate had the lowest shift at −23°. These results indicate that the 3b2:?thyc=10Ti-sprayed3b2:?thyc surface formed a protective passive film owing to reaction at the 3b2:?thyc=10coating/solution 3b2:?thycinterface. Thus, the Ti coating exhibited higher resistance to the acidic solution.As the exposure periods were extended, the increased dimensions of semicircle loops in the Nyquist plots showed increased corrosion resistance [46]. The bigger loops in the Nyquist plots reveal high resistance to corrosion in any environment. Such results can be seen from Figure[tnq_nbsp] 5(a) after 312 h of exposure to pH 4 solution. The EEC for the SS plate after 312 h of exposure is inserted in Figure[tnq_nbsp] 5(a). The Warburg impedance (W) is caused by diffusion of the protective passive layer on the SS plate surface in the H2SO4-contaminated solution. Rct and W are parallel to the CPE2[tnq_nbsp] [16].At longer periods of exposure (312 h), many parameters are involved owing to the complex reaction process on the3b2:?thyc=10metal/solution 3b2:?thycinterface.All samples exhibited two loops in the Nyquist plots, one at higher and another at lower frequencies. The loops formed at higher and lower frequencies because of the solution resistance and the reaction at the3b2:?thyc=10metal/solution 3b2:?thycinterface, respectively [47–3b2:?show $132#51]. As the exposure period is increased, strengthening of the passive film on the SS plate may occur [52]. However, in case of Ti, there is a possibility of formation of the protective passive film due to transformation 3b2:?A3B2 tlsb -.1ptof unstable titanium oxides into stable oxides, while on 3b2:?thyc=10SS-sprayed3b2:?thyc samples, it is due to deposition of corrosion products on the coating defects in H2SO4-contaminated water solution [53]. The two semicircle loops of Ti and SS coatings were successfully distinguished from each other for this exposure 3b2:?A3B2 tlsb .1ptperiod. Therefore, the Ti coating had provided greater 3b2:?A3B2 tlsb -.1ptprotection than the SS coating. These two diffused semicircle loops on sprayed coatings are not clearly seen because of the reduced conductivity of the solution and scattered data.The dimensions of semicircle loops in the Nyquist plot of the Ti coating were bigger, indicating that the anodic surface area of the coating was decreased by the formation of the protective oxide film rather than SS coating. The SS plate had higher resistance to the H2SO4-contaminated solution owing to the formation of the protective passive film [42].The SS plate surface exhibited the protective passive film that is resistant to corrosion because the values of bothZ′real and −Z″imaginary axes are increased (Figure[tnq_nbsp] 5(a)). From the initial to the prolonged exposure, the SS plate showed higher resistance to corrosion which can be attributed to the formation of compact and uniform passive layers [42].3b2:?A3B2 tlsb .1ptOn the other hand, the SS and Ti coatings showed less resistance to corrosion than the SS plate because of the formation of surface 3b2:?thyc=10defects/cracks, 3b2:?thycwhich enhanced the corrosion rate due to penetration of aggressive solution. It 3b2:?A3B2 tlsb -.1ptcan be seen from Figure[tnq_nbsp] 5(a) that the SS and Ti coatings exhibit diffused semicircle loops separated by two small loops.The bigger loop shifted toward Z″imaginary because of the formation of the capacitive passive 3b2:?thyc=10film/corrosion 3b2:?thycproducts. The lower frequency loop shifted toward Z′real of the Nyquist plots (Figure[tnq_nbsp] 5(a)) because of the increased resistance to corrosion.3b2:?A3B2 tlsb -.1ptThe nature of corrosion 3b2:?thyc=10products/passive 3b2:?thycfilms plays[tnq_nbsp] a major role in controlling the corrosion of the sprayed samples at prolonged exposure [53]. In case of the SS plate and Ti coating, the passive film controls the corrosion of the samples. There is no role played by chemistry; rather, morphology controls the corrosion of samples.The impedance values measured at lowest frequency (0.01 Hz) in Figure[tnq_nbsp] 5(b) were found to be the highest than those of 1 h of exposure to acidic pH solution for all samples. The impedance values of both sprayed coatings exhibited almost identical characteristics, but those of the Ti coatings were higher. This result is attributed to the fact that the Ti coating is more resistant to corrosion in the H2SO4 solution at pH 4 after 312 h of exposure [45,[tnq_nbsp] 54–3b2:?show $132#56]. The Ti and SS coatings exhibited higher resistance at the highest studied frequency due to formation of passive films and deposition of corrosion products in 3b2:?thyc=10defects/pores, 3b2:?thycrespectively, and showed higher impedance.After 312 h of exposure, the corrosion of SS and Ti coatings in acidic solution was controlled by their respective corrosion products and passive film [53]. The impedance value of Ti coating was greater than that of the 3b2:?thyc=10SS-sprayed3b2:?thyc coating owing to the more stable and adherent passive oxide film formed on its surface after exposure to the solution. The SS plate had the highest impedance values compared to the sprayed coatings.The phase shiftθ (°)-log f (Hz) Bode plots of samples after 312 h of exposure to solution are shown in Figure[tnq_nbsp] 5(b). The scattered data shown in the middle frequency range are attributed due to the 3b2:?thyc=10defective/porous 3b2:?thycoxide film caused by the corrosion products of SS and Ti coatings.The shifting of the phase angle maxima toward −75° for the SS plate was attributed to the formation of the homogeneous passive film on the surface, which revealed the strengthening of the film in the solution. This result indicates that the passive3b2:?thyc=10film/corrosion 3b2:?thycproducts formed on the plate are surface resisted to the attack of corrosive ions [57].The impedance data were validated by Kramers–3b2:?show $132#Kronig (3b2:?thyc=10K-K)3b2:?thyc transformation by transforming the real axis into the imaginary axis and vice versa. The K-3b2:?show $132#K transformations are shown in Figure[tnq_nbsp] S2 and have been described elsewhere [58–3b2:?show $132#60]. These results confirm the agreement between the experimental data and K-3b2:?show $132#K transformations, which is accordance with the linear system theory.Brug’s formula has been widely used to extract effective capacitance values from CPE parameters for studies on double layers [61]. Brug et[tnq_nbsp] al. [62] have established the relationship between CPE parameters and effective capacitance (Ceff) associated with the CPE which can therefore be expressed as follows:3b2:?A3B2 tpb 1pt(1)Ceff=Q1nR1−n3b2:?A3B2 }go/3b2:?A3B2 :go3b2:?A3B2 show \/n,where Q is the CPE parameter such as nonideal 3b2:?thyc=10double-layer3b2:?thyc capacitance, R is a resistance caused by dissolution of the metal or alloy at the 3b2:?thyc=10metal/solution 3b2:?thycinterface in low frequency, and n is the CPE exponent (−1 <n < 1). When n is ∼1, 0.5, 0, and −1, the CPE is equivalent to a capacitor, the Warburg diffusion, a resistor, and an inductor, respectively.After fitting of EIS data to a suitable EEC, the electrochemical parameters are shown in Table[tnq_nbsp] 3b2:?show [?"tbl1",5,"cond1"]1. The Rs is very high for all systems due to low conductively of the solution. The Rs is gradually decreased with increasing exposure periods due to involvement of more ions after reaction of metals in acidic pH solution [53].Table 1
Electrochemical parameters of the SS plate, SS-sprayed coating, and Ti-sprayed coating extracted after fitting of EIS data to suitable EECs with different exposure periods in pH 4 solution.
Time (h)
1
312
3b2:?show "^tbl_width"$01649v
Sample ID
SS plate
SS sprayed
Ti sprayed
SS plate
SS sprayed
Ti sprayed
R
s (kΩ·cm2)
6.77
5.20
8.64
5.02
5.37
8.45
Rp (kΩ·cm2)
166.00
16.02
48.19
22.82
20.15
21.07
CPE1
Q1 (1 × 10−6) (Ω−1·cm−2·sn)
2.7
62.0
7.3
29.6
41.6
37.2
n1
0.99
0.99
0.99
0.95
0.92
0.93
C
eff1 (µF·cm−2)
2.68
62.0
7.22
29.0
41.0
36.5
R
ct (kΩ·cm2)
11.87
9.15
10.97
350.02
11.56
46.33
CPE2
Q2 (1 × 10−6) (Ω−1·cm−2·sn)
91.6
252.3
105.8
20.9
156.1
38.5
n2
0.75
0.44
0.54
0.91
0.46
0.60
C
eff2 (µF·cm−2)
94.19
731.72
120.11
25.44
312.13
56.62
W (1 × 10−6) (Ω·cm2·s−0.5)
—
—
—
4.2
—
—TheRp and Rct values of samples are gradually decreased and increased, respectively, as exposure periods increased. The Rp is emphasizing due to resistance caused by inhomogeneity of the metal surface, and it is decreased due to corrosion. The Rct is increased for SS plate and Ti-sprayed coating due to protective nature of passive film while SS sprayed coating owing to deposition of corrosion products on surface. The corrosion products and passive oxide film[tnq_nbsp] increase their thickness as exposure periods were increased, resulting in high Rct than 1 h of exposure [63]. The capacitance of the 3b2:?thyc=10metal/coating 3b2:?thycsurface and passive 3b2:?thyc=10film/3b2:?show [?tjl=20mm]&tjlpc;[?tjl]corrosion 3b2:?thycproducts is derived as Ceff1 and Ceff2, respectively. The Ceff1 is dramatically increased as Rp is decreased with exposure periods, which indicates that the 3b2:?thyc=10metal/coating 3b2:?thycsurface started to corrode, but as the Rct is increased, Ceff2 is[tnq_nbsp] decreased. The Ceff2 result was attributed to that the surface[tnq_nbsp] became homogenized due to formation of the passive layer or corrosion products on the 3b2:?thyc=10metal/coating 3b2:?thycinterface after 312 h of exposure. However, it is found that the Ceff is[tnq_nbsp] greater for SS coating than the Ti coating and SS plate in all[tnq_nbsp] exposure periods. It indicates that 3b2:?A3B2 tlsb .1ptthe SS coating is more[tnq_nbsp] inhomogeneous and defective than other samples. The thickening of the oxide film was attributed to anodic oxidation and formation of the protective passive 3b2:?thyc=10film/corrosion 3b2:?thycproducts that reduced the penetration of aggressive ions [64]. The corrosion product itself caused resistance to corrosion due to uniform and adherent deposition.After 312 h of exposure, W was observed for the SS plate, possibly resulting from diffusion of the protective passive layer on the surface [65,[tnq_nbsp] 66]. As exposure periods are increased, the passive film strength also increased.After 1 h of exposure,Rp is found to be highest for all samples due to a barrier type of protection exhibited by the coatings. The NiO, Fe2O3, FeO, and Cr2O3 thin films are formed on the SS plate [42] which give the protection against corrosion. Initially, the metal or coating surface does not start to react with solution resulting in high Rp, but once proper reaction has occurred, the surfaces start to corrode. At the time of corrosion initiation, Rct will involve which causes resistance to penetration of the solution toward the metal surface. Therefore, Rp is decreased and Rct is increased as exposure periods are increased. The film formed on the surfaces was imperfect and rough [67,[tnq_nbsp] 68] after 1 h of exposure; thus, the dispersion coefficient (n) is less for CPE2. As the exposure periods were increased, the Rct values increased and CPE decreased for passive 3b2:?thyc=10layer/corrosion 3b2:?thycproducts of the samples. Rct is high for all samples due to deposition of corrosion products on 3b2:?thyc=10SS-sprayed3b2:?thyc coating and the protective passive layer on 3b2:?thyc=10Ti-sprayed3b2:?thyc coatings and the SS plate after 312 h of exposure.


#### 3.4.2. Potentiodynamic Studies

3b2:?A3B2 tlsb -.1ptPotentiodynamic studies were carried out after 312 h of exposure, and results are shown in Figure[tnq_nbsp] 3b2:?show [?"gr6",5,"cond1"]6. The SS plate showed pitting and many breakdown potentials during anodic scanning. The breakdown potentials may be caused by oxidation of the metal surface due to impressed current which form a new phase or a metastable passive film, that altered the passive film properties [69]. Therefore, there is a chance that another oxide phase could form on the surface, which might be protective in nature.Figure 6
Potentiodynamic plots of the SS plate and sprayed coatings in pH 4 solution after 312 h of exposure (1 mV/s).3b2:?A3B2 tlsb -.1ptThe current density of the SS plate is lower than that of Ti and SS coatings during anodic scanning. The interesting observation is found in case of Ti and SS coatings that there is a gradual increase in anodic current density during anodic scanning. It may be due to corrosion or transformation of unstable oxide films of these samples, and whatever corrosion 3b2:?thyc=10products/passive 3b2:?thycfilm formed was deposited on the surface.The anodic and passive corrosion current of the Ti coating was lower than that of the SS coating, which means that, in this case, the former is more likely to form compact, protective, and adherent passive oxide films [70,[tnq_nbsp] 71].3b2:?A3B2 tlsb -.1ptThe passive film of 3b2:?thyc=10Ti-sprayed3b2:?thyc coating resisted the penetration of corrosive species of the solution; thus, the reducing corrosion rate is observed. During cathodic scanning, all samples exhibited hydrogen evolution reaction which dominated over the oxygen reduction reaction [72].The electrochemical parameters were extracted after fitting of potentiodynamic plots to the Tafel region using the Stern–3b2:?show $132#Geary equation3b2:?A3B2 tpb -1.2pt(2)Icorr=BRtotal.The Stern–3b2:?show $132#Geary constant (B) can be calculated by putting the values for corrosion current density (Icorr) and total polarization resistance (Rtotal) in (2). The extracted data on the corrosion potential (Ecorr), Icorr, Rtotal, B, and the corrosion rate of samples after 312 h of exposure to pH 4 solution are shown in Table[tnq_nbsp] 3b2:?show [?"tbl2",5,"cond1"]2.Table 2
Electrochemical parameters extracted after fitting of potentiodynamic plots to the Tafel region.
Sample ID
Electrochemical parameters
E
corr (V) versus Ag/AgCl
I
corr (µA·cm−2)
R
total (kΩ·cm2)
B (V)
Corrosion rate (µm·y−1)
3b2:?show "^tbl_width"$01649v
SS plate
0.138
0.382
379.860
0.15
5.132
SS sprayed
−0.594
19.803
33.792
0.67
266.043
Ti sprayed
−0.403
1.913
68.464
0.13
42.703TheEcorr of the SS plate and SS and Ti coatings are 0.138, −0.594, and −0.403 V versus 3b2:?thyc=10Ag/AgCl, 3b2:?thycrespectively. The SS plate exhibited nobler Ecorr than the Ti coating followed by the SS coating.The nobler potential of the SS plate is due to formation of the3b2:?thyc=10Cr-enriched3b2:?thyc oxide film, whereas others exhibited the active potential. The active Ecorr of SS- and 3b2:?thyc=10Ti-sprayed3b2:?thyc coatings compared to the SS plate is attributed to the presence of defects on the coating surface.Lai et[tnq_nbsp] al. observed that when SS was exposed to H2SO4-contaminated water solution, it formed NiO, Fe2O3, FeO, and Cr2O3 thin films which were protective in nature and noble [42]. The active potential of SS coating was due to the presence of defective or porous 3b2:?thyc=10oxide/corrosion 3b2:?thycfilms that made the sample more susceptible to corrosion and exhibit the mixed potential [73].The studied pH solution was acidic and led to the[tnq_nbsp] deterioration of the samples. During exposure, the formed corrosion products deposited on the sample surface. The corrosion products blocked the 3b2:?thyc=10defects/pores 3b2:?thycof the samples and resisted the penetration of the solution [74,[tnq_nbsp] 75].The iron oxides were more active and therefore exhibited the active potential. The Ti coating exhibited a nobler potential than SS coating because it had only fine and elongated cracks (Figure[tnq_nbsp] 2(c)), which stifled the aggressive species of the solution from reaching the base metal. In contrast, the SS coating contains many connected pores and valley morphology where the acidic solution can accumulate and induce crevice corrosion.These results indicate that the passive film formed on the3b2:?thyc=10Ti-sprayed3b2:?thyc coating after exposure to pH 4 solution is protective, nonporous, compact, and resistant to the penetration of aggressive ions in the solution. The SS coating has porous and nonprotective corrosion 3b2:?thyc=10products/iron 3b2:?thycoxides.TheRtotal values of the SS plate, 3b2:?thyc=10SS-sprayed3b2:?thyc coating, and 3b2:?thyc=10Ti-sprayed3b2:?thyc coating are 379.860, 33.792, and 68.464 kΩ·cm2, respectively. The higher Rtotal value of Ti coating compared to the SS coating suggests that it can be used as a coating to protect the materials in H2SO4-contaminated water solution, even at low pH. The B values were calculated by using (2), and it was found that the SS plate and 3b2:?thyc=10Ti-sprayed3b2:?thyc coating were identical and less active, while 3b2:?thyc=10SS-sprayed3b2:?thyc coating showed 0.67 V which is more pronounced to corrosion [76]. The B value of the SS plate and 3b2:?thyc=10Ti-sprayed3b2:?thyc coating is under 3b2:?A3B2 tlsb -.1ptthe active control, while the 3b2:?thyc=10SS-sprayed3b2:?thyc surface exhibits active dissolution values which influence the corrosion phenomena. The calculated Icorr value of 3b2:?thyc=10SS-sprayed3b2:?thyc samples reveals the 3b2:?A3B2 tlsb .1ptactiveness of coating, while the SS plate and 3b2:?thyc=10Ti-sprayed3b2:?thyc coating control the corrosion process in acidic solution at longer duration of exposure.The corrosion rate (µm·y−1) was calculated by the following equation [77]:3b2:?A3B2 tpb .7pt(3)Corrosion  rateµm⋅y−1=3.27×Icorr×EWd.The corrosion rate in (3) is expressed in micrometres per year (µ3b2:?thyc=10m/year) 3b2:?thycand Icorr in µA·cm−2. The Icorr was obtained by dividing the total surface area of the working electrode under the corrosion current (µA). EW represents the equivalent weight (g·mol−1), and d is the density (g·cm−3).The corrosion rate of the SS coating is 266.043µm·y−1 and is greater than that of the SS plate and Ti coating by 51. 84 and 6.23 times, respectively. This result indicates that the SS is not an effective coating material for deposition by the arc thermal spray process in pH 4 solution and long duration of exposure.The corrosion rate data of the SS coating revealed that it totally3b2:?thyc=10dissolved/corroded 3b2:?thycdown to the base substrate. The initial coating thickness was 200 µm, while the corrosion rate was 266.043 µm·y−1. Thus, it may be reported that the Ti coating was effective in protecting the surface than the SS coating. The Ti can be used as a coating material to protect the waste water reservoir and extend its service life.


#### 3.4.3. Characterization of Corrosion Products after Potentiodynamic Studies in pH 4 by Different Techniques

3b2:?A3B2 tlsb .1ptThe morphology of corrosion products was examined by SEM, and results are shown in Figure[tnq_nbsp] 3b2:?show [?"gr7",5,"cond1"]7. On the SS plate surface, the passive film was adherent, uniform, and regularly deposited, thus preventing the penetration of solution 3b2:?A3B2 tlsb -.1pt(Figure[tnq_nbsp] 7(a)). The edges of the surface show few cracks caused 3b2:?A3B2 tlsb .1ptby the destructive potentiodynamic experiment, and the passive film prevented the cracking. After potentiodynamic studies, the SS plate surface did not show any other type of corrosion products/rust.3b2:?tex type="figure" layout="twocols" n="2,3"Figure 7
SEM images of corrosion products formed on the (a) SS plate, (b) SS-sprayed coating, and (c) Ti-sprayed coating after potentiodynamic studies in pH 4 solution.
(a)
(b)
(b)3b2:?A3B2 tlsb -.1ptThe SS coating exhibited different sizes of corrosion product morphology with micropore formation (Figure[tnq_nbsp] 7(b)). 3b2:?A3B2 tlsb .2ptThe 3b2:?thyc=10net-like3b2:?thyc microstructure of corrosion products is attributed to the presence of porous iron oxides. Through the net and thread morphologies, the acidic solution easily penetrated the substrate and formed corrosion products.The morphology of corrosion products formed on the Ti coating was totally different from that on the SS plate and sprayed coating. The passive films formed on the3b2:?thyc=10Ti-sprayed3b2:?thyc surface exhibit microcracks, plate, and globular morphology (Figure[tnq_nbsp] 7(c)). The globular particles block the micro- and macrocracks on the top surface. Therefore, enhanced corrosion resistance was observed after 312 h of exposure than on 3b2:?thyc=10SS-sprayed3b2:?thyc coating.Passive oxide films of Ti coating contain3b2:?thyc=10plate-like3b2:?thyc microstructures that were uniformly deposited on the surface. Similar morphologies were not observed in the corrosion products of the SS plate and sprayed coating.The phases present in the corrosion products of all samples after potentiodynamic studies were studied by XRD. The identification of phases in corrosion products is shown in Figure[tnq_nbsp] 3b2:?show [?"gr8",5,"cond1"]8. The SS plate exhibits the presence of tetrataenite (FeNi) and Fe. It is reported that FeNi is unstable and can deteriorate into other forms, if it exposes for long term to 3b2:?thyc=10low-temperature3b2:?thyc environments [78].Figure 8
XRD of the SS plate and sprayed coatings after potentiodynamic studies in pH 4 solution (0.5°/min).3b2:?A3B2 tlsb -.1ptThe presence of lepidocrocite (γ-FeOOH) in the corrosion products of 3b2:?thyc=10SS-sprayed3b2:?thyc coatings confirmed that this coating was susceptible to corrosion in acidic solution. However, Ti coating exhibits composite oxides along with Ti and TiO. Therefore, the improved corrosion resistance of 3b2:?thyc=10Ti-sprayed3b2:?thyc coating is observed by formation of TiO2 (rutile and anatase), and this observation corroborates with EIS and potentiodynamic results. The passive oxides of Ti such as TiO2 and Ti3O5 have formed. The TiO2 is thermodynamically more stable than others [79]. Therefore, the 3b2:?thyc=10Ti-sprayed3b2:?thyc coating is attributed to improved corrosion resistance properties of coating in H2SO4 solution. The transformation of Ti into TiO2 in the H2SO4 environment is well documented elsewhere [80,[tnq_nbsp] 81]. TiO3 and some amount of TiO (Figure[tnq_nbsp] 3) may be transformed into TiO2 and Ti3O5 due to a strong oxidizing ability of H2SO4 solution. Thus, corrosion 3b2:?thyc=10products/passive 3b2:?thycfilm of 3b2:?thyc=10Ti-sprayed3b2:?thyc coating exhibits some peaks of TiO (Figure[tnq_nbsp] 8). Therefore, corrosion is observed after 312 h of exposure to H2SO4 solution. Once proper transformation 3b2:?A3B2 tlsb -.1ptof Ti and TiO into the stable form occurred, then the corrosion rate would be completely suppressed.


## 3.1. Adhesion Test of Sprayed Coatings

Adhesion measurements were carried out after deposition of coatings onto the concrete surface by arc thermal spraying. This was measured for four samples, and the average was calculated. The average adhesion values of 304 SS- and3b2:?thyc=10Ti-sprayed3b2:?thyc surfaces were 3.39 and 2.72 MPa, respectively. The 3b2:?thyc=10SS-sprayed3b2:?thyc coating exhibits higher adhesion values than 3b2:?thyc=10Ti-sprayed3b2:?thyc coating. The standard deviation of SS- and 3b2:?thyc=10Ti-sprayed3b2:?thyc coating was calculated, 3b2:?A3B2 tlsb -.1ptand these were 0.40 and 0.24 MPa, respectively. This indicates that SS coating adhered strongly to the concrete surface, whereas 3b2:?thyc=10Ti-sprayed3b2:?thyc coating adhered 1.25 times lesser than the 304 SS coating. The higher adhesion values of the SS coating may be attributed to small interfacial separation between the concrete substrate and metallic particles, while that for Ti was large [25].3b2:?A3B2 tpb 1.6pt


## 3.2. Morphology of SS Plate and Sprayed Coating

3b2:?A3B2 tlsb .1ptThe morphologies of the 304 SS plate and deposited coatings on[tnq_nbsp] the mild steel substrate were characterized by SEM. Figure[tnq_nbsp] 3b2:?show [?"gr2",5,"cond1"]2 shows the SEM images of the deposited coatings and 304 SS plate. The SS plate surface exhibited a smooth and very finely scratched line (Figure[tnq_nbsp] 2(a)), while the[tnq_nbsp] de3b2:?show $132#3b2:?show $132#posited coatings had many cracks and defects over the surfaces (Figures[tnq_nbsp] 2(b) and (c)).3b2:?tex type="figure" layout="twocols" n="2,3"Figure 2
FE-SEM images of the (a) SS plate, (b) SS-sprayed surface, and (c) Ti-sprayed surface.
(a)
(b)
(c)The scratched line on the plate surface was caused by abrasion with an emery paper up to 300µm because this grade of the emery paper is hard and makes some fine 3b2:?thyc=10defects/lines 3b2:?thycon the surface. The 3b2:?thyc=10SS-sprayed3b2:?thyc surface showed coagulated valleys and uneven deposition, while the 3b2:?thyc=10Ti-sprayed3b2:?thyc surface showed uniform, nanosized globular, and fine elongated cracks. The fine cracks on the 3b2:?thyc=10Ti-sprayed3b2:?thyc surface might be due to formation of thin and nanoscaled brittle oxides. The morphology of 3b2:?thyc=10SS-sprayed3b2:?thyc coating can be attributed due to the sudden cooling of melted metal particles at room temperature (27 ± 1°C).3b2:?A3B2 tlsb -.13ptThe microstructure of the 3b2:?thyc=10SS-sprayed3b2:?thyc coating (Figure[tnq_nbsp] 2(b)) 3b2:?A3B2 tlsb .1ptcould allow the deposition of aggressive ions, water, and moisture particles on valleys, which cause localized or crevice corrosion. The 3b2:?thyc=10Ti-sprayed3b2:?thyc coating also exhibited fine cracking on the surface but had little influence on the deposition of water molecules. Owing to the smooth microstructure of the Ti coating, water molecules may slide off the surface.


## 3.3. Phase Identification of SS Plate and Sprayed Coatings by XRD

3b2:?A3B2 tlsb .1ptXRD was performed to determine the phases present in the coatings and plate surfaces. The results are plotted between 2θ = 10° and 90° versus intensity in counts per second (CPS) and shown in Figure[tnq_nbsp] 3b2:?show [?"gr3",5,"cond1"]3. The SS plate and sprayed surfaces exhibited the austenite phase of the 3b2:?thyc=10Fe-Cr-Ni3b2:?thyc alloy, that is, 304 SS [26,[tnq_nbsp] 27]. Besides this phase in the 3b2:?thyc=10SS-sprayed3b2:?thyc surface, magnetite (Fe3O4) is also observed, and it 3b2:?A3B2 tlsb -.1ptis due to the partial oxidation of coating during the spraying process.Figure 3
XRD of the SS plate and sprayed coatings by the arc thermal spray process (0.5°/min).XRD of3b2:?thyc=10Ti-sprayed3b2:?thyc coating showed Ti [28] and two oxides such as TiO and Ti3O, which were formed due to higher melting point of it than SS where there are possibilities to oxidize the deposited coating. The another reason to oxidize the Ti is high affinity of it with atmospheric oxygen and thus to form surface oxide films such as TiO or Ti3O. The formation of these two oxides of Ti in open atmosphere has also been reported by other researchers [29,[tnq_nbsp] 30]. However, these oxides are amorphous, brittle, and unstable in in vivo conditions which easily can be removed simply by brushing with soft tissues [31–3b2:?show $132#33].


## 3.4. Electrochemical Studies of SS Plate and Sprayed Coating in pH 4 Solution

### 3.4.1. EIS Studies

3b2:?A3B2 tlsb -.11ptThe samples were immersed in pH 4 solution for different periods of exposure. EIS was carried out to study their corrosion characteristics. These results are shown in Figures[tnq_nbsp] 3b2:?show [?"gr4",5,"cond1"]4 and 3b2:?show [?"gr5",5,"cond1"]5. The electrical equivalent circuit (EEC) is shown in the corresponding Nyquist plots. The EEC of the SS plate exposed to the pH 4 solution for 1 h is shown in Figure[tnq_nbsp] 4(a). In the Nyquist plot of the SS plate, the sample after 1 h of exposure is differentiated by two depressed semicircle loops such as one at high while another at the lower studied frequency. For more clarity of plots at high frequency, the Nyquist result of samples is shown in Figure[tnq_nbsp] S1 (supplementary figure). These results can be explained either by the heterogeneity of the solid surface or by the dispersion of some physical properties. The interface of the surface cannot be considered as an ideal capacitor due to heterogeneity of the surface, and it may involve a 3b2:?thyc=10constant-phase3b2:?thyc element (CPE) as a substitute of the ideal capacitor. The first EEC consists of the[tnq_nbsp] solution resistance (Rs), polarization resistance (Rp), and CPE1 due to the metal surface and nonideal 3b2:?thyc=10double-layer3b2:?thyc capacitance behaviour [34–3b2:?show $132#36]. The Rp and CPE1 are parallel to each other. However, the second EEC contains the charge transfer resistance (Rct) and CPE2. The formation of Rct may be due to formation of the protective passive layer on the SS plate surface in acidic pH solution after 1 h of exposure. These two EECs are connected in series to each other.3b2:?tex type="figure" layout="twocols" n="2"Figure 4
Impedance spectra: (a) Nyquist and (b) Bode |Z| and (θ) of the SS plate and sprayed coatings in pH 4 solution after 1 h of exposure (100 kHz to 0.01 Hz).
bottom
3b2:?A3B2 tpt 4pt(a)
(b)3b2:?tex type="figure" layout="twocols" n="2"Figure 5
Impedance spectra: (a) Nyquist and (b) Bode |Z| and (θ) of the SS plate and sprayed coatings in pH 4 solution after 312 h of exposure (100 kHz to 0.01 Hz).
bottom
3b2:?A3B2 tpt -2pt(a)
(b)The EECs for the SS- and3b2:?thyc=10Ti-sprayed3b2:?thyc coating systems are somewhat different from the SS plate, and they are inserted in Figure[tnq_nbsp] 4(a). The different EECs for these coatings may be attributed to the inherent property of the arc thermal spray process, where coatings suffer from surface defects. Due to reaction on the metal surface, Rct participated owing to initiation of the corrosion process in acidic pH solution. The reaction on the metal surface caused by Rct led to the formation of a 3b2:?thyc=10passive/oxide 3b2:?thyclayer on the metal surface, which increased the resistance and reduced the corrosion reaction. The CPE1 due to a nonideal behaviour of the coating surface and Rp are parallel to each other, while another EEC is connected in series to Rp which contains CPE2 and Rct [15]. The CPE2 and Rct are parallel to each other.The Nyquist plots reveal the real characteristics of the samples after 1 h of exposure (Figure[tnq_nbsp] 4(a)). The samples were exposed to the solution for 1 h to stabilize the potential; thereafter, EIS measurements were performed.Figure[tnq_nbsp] 4(a) shows the two semicircle loops in the Nyquist plots exhibited by coated samples. The SS- and 3b2:?thyc=10Ti-sprayed3b2:?thyc coatings show zigzag and scattered plots which might be due to low conductivity of electrolytes, deposition of defective coating, formation of a defective passive film, and the presence of more resistive elements such as Ti in 3b2:?thyc=10Ti-sprayed3b2:?thyc coating while Cr and Ni in 3b2:?thyc=10SS-sprayed3b2:?thyc samples [37–3b2:?show $132#40]. These results are attributed to the fact that both sprayed samples exhibit capacitive properties due to the presence of defects. However, the Ti coating imposed a resistance greater than the SS coating due to the formation of a 3b2:?thyc=10passive/oxide 3b2:?thycfilm with capacitive behaviour which enabled surface resistance to penetrate the ions of the acidic solution.In the sulfuric acid solution, the Ti surface tends to form defective passive films with high resistance. Similar results have been observed by Baron et[tnq_nbsp] al. on TiAlV and TiAlNb alloys in Tyrode’s solution [41].The dimensions of the semicircle loop of SS plate samples clearly show high capacitive property that enables the surface to resist the penetration of the solution. The capacitive property of the passive film on the SS plate was attributed to the formation of3b2:?thyc=10Cr-enriched3b2:?thyc oxide and NiO in the H2SO4-contaminated water solution [42].The dimensions of the semicircle loops of SS- and3b2:?thyc=10Ti-sprayed3b2:?thyc coatings were less than those of the SS plate because the sprayed samples were more susceptible to corrosion, owing to the formation of 3b2:?thyc=10defects/pores 3b2:?thycon the coating surface in the solution after 1 h of exposure.The impedance at low frequency (0.01 Hz) and the phaseθ (°) of 3b2:?A3B2 tlsb -0.15ptBode plots were plotted against log |Z| (Ω·cm2) ver3b2:?show $132#sus log f (Hz) and θ (°) versus log f (Hz) in Figure[tnq_nbsp] 4(b), re3b2:?show $132#spec3b2:?show $132#tively.[tnq_nbsp] The impedance values of the SS plate sample were greater than[tnq_nbsp] those of the SS and Ti coatings. The SS coating exhibited lower impedance values because of the presence of more 3b2:?thyc=10defects/pores 3b2:?thycat locations where the chances of penetration by the acidic solution are high, and this initiated the deterioration of the coating.3b2:?A3B2 tlsb -.28ptFrom the log |Z| (Ω·cm2)-log f (Hz) Bode plots (Figure[tnq_nbsp] 4(b)), 3b2:?A3B2 tlsb -.1ptit can be seen that, at high frequency (100 kHz), resistance was moreover identical to that observed at low frequency, that is, 0.01 Hz, which might be attributed to the low conductivity of acidic pH solution.In this study, the solution was prepared by adding a few drops of 0.1 M·H2SO4 to distilled water. The solution conductivity was very low, which caused the resistance in total impedance. The conductivity of the solution is an important parameter that must be considered in electrochemical studies. However, the Ti coating exhibited higher impedance than the SS coating due to formation of a thick and protective passive oxide film. The low impedance values of the sprayed samples are due to the presence of more defects or less interfacial resistance between the splats of coating than on the SS plate sample.3b2:?A3B2 tlsb -.1ptThe surface finish and coating microstructure play an important role to determine the corrosion resistance properties of materials in the solution. The pH 4 solution is very aggressive and causes localized or pitting corrosion of the oxide films formed during exposure [43–3b2:?show $132#45].3b2:?A3B2 tlsb -.1ptThe defective parts of coatings can function as an anode, while the remaining acts as a cathode, resulting in the formation of microgalvanic cells on the surface. The presence of microgalvanic cells enhances the corrosion rate of materials; thus, there is a chance of getting low impedance. Such observations are found in SS- and 3b2:?thyc=10Ti-sprayed3b2:?thyc coatings. In view of the above, it can be observed that the SS coating exhibited 3b2:?thyc=10valley-type3b2:?thyc deposits (Figure[tnq_nbsp] 2(b)) where the acidic solution could 3b2:?thyc=10stagnant/deposit 3b2:?thycand cause localized and crevice corrosion. During the initial period of exposure, both sprayed coatings had defects that resulted in lower impedance values than the SS plate surface.3b2:?A3B2 tlsb .1ptThe SS plate shows a −40° shift of the phase angle maxima at the lower studied frequency and reveals high resistance to corrosion in the pH 4 solution (Figure[tnq_nbsp] 4(b)). On the other hand, Ti and SS coatings exhibited −1° and −2° shifts, respectively, which indicate their susceptibility to corrosion during the initial period of exposure [37]. In the middle frequency range, the samples exhibited scattering which might be due to the capacitive response of the defective passive film that was formed during exposure of the samples to H2SO4 solution [38–3b2:?show $132#40].The shifting of maxima at the higher studied frequency (100 kHz) is due to the deposition of corrosion products on the3b2:?thyc=10SS-sprayed3b2:?thyc sample, whereas on the 3b2:?thyc=10Ti-coated3b2:?thyc sample, it is due to formation of the resistive passive film. It can be seen that the Ti coating exhibited approximately −57° shift followed by the SS coating at −38°, while the SS plate had the lowest shift at −23°. These results indicate that the 3b2:?thyc=10Ti-sprayed3b2:?thyc surface formed a protective passive film owing to reaction at the 3b2:?thyc=10coating/solution 3b2:?thycinterface. Thus, the Ti coating exhibited higher resistance to the acidic solution.As the exposure periods were extended, the increased dimensions of semicircle loops in the Nyquist plots showed increased corrosion resistance [46]. The bigger loops in the Nyquist plots reveal high resistance to corrosion in any environment. Such results can be seen from Figure[tnq_nbsp] 5(a) after 312 h of exposure to pH 4 solution. The EEC for the SS plate after 312 h of exposure is inserted in Figure[tnq_nbsp] 5(a). The Warburg impedance (W) is caused by diffusion of the protective passive layer on the SS plate surface in the H2SO4-contaminated solution. Rct and W are parallel to the CPE2[tnq_nbsp] [16].At longer periods of exposure (312 h), many parameters are involved owing to the complex reaction process on the3b2:?thyc=10metal/solution 3b2:?thycinterface.All samples exhibited two loops in the Nyquist plots, one at higher and another at lower frequencies. The loops formed at higher and lower frequencies because of the solution resistance and the reaction at the3b2:?thyc=10metal/solution 3b2:?thycinterface, respectively [47–3b2:?show $132#51]. As the exposure period is increased, strengthening of the passive film on the SS plate may occur [52]. However, in case of Ti, there is a possibility of formation of the protective passive film due to transformation 3b2:?A3B2 tlsb -.1ptof unstable titanium oxides into stable oxides, while on 3b2:?thyc=10SS-sprayed3b2:?thyc samples, it is due to deposition of corrosion products on the coating defects in H2SO4-contaminated water solution [53]. The two semicircle loops of Ti and SS coatings were successfully distinguished from each other for this exposure 3b2:?A3B2 tlsb .1ptperiod. Therefore, the Ti coating had provided greater 3b2:?A3B2 tlsb -.1ptprotection than the SS coating. These two diffused semicircle loops on sprayed coatings are not clearly seen because of the reduced conductivity of the solution and scattered data.The dimensions of semicircle loops in the Nyquist plot of the Ti coating were bigger, indicating that the anodic surface area of the coating was decreased by the formation of the protective oxide film rather than SS coating. The SS plate had higher resistance to the H2SO4-contaminated solution owing to the formation of the protective passive film [42].The SS plate surface exhibited the protective passive film that is resistant to corrosion because the values of bothZ′real and −Z″imaginary axes are increased (Figure[tnq_nbsp] 5(a)). From the initial to the prolonged exposure, the SS plate showed higher resistance to corrosion which can be attributed to the formation of compact and uniform passive layers [42].3b2:?A3B2 tlsb .1ptOn the other hand, the SS and Ti coatings showed less resistance to corrosion than the SS plate because of the formation of surface 3b2:?thyc=10defects/cracks, 3b2:?thycwhich enhanced the corrosion rate due to penetration of aggressive solution. It 3b2:?A3B2 tlsb -.1ptcan be seen from Figure[tnq_nbsp] 5(a) that the SS and Ti coatings exhibit diffused semicircle loops separated by two small loops.The bigger loop shifted toward Z″imaginary because of the formation of the capacitive passive 3b2:?thyc=10film/corrosion 3b2:?thycproducts. The lower frequency loop shifted toward Z′real of the Nyquist plots (Figure[tnq_nbsp] 5(a)) because of the increased resistance to corrosion.3b2:?A3B2 tlsb -.1ptThe nature of corrosion 3b2:?thyc=10products/passive 3b2:?thycfilms plays[tnq_nbsp] a major role in controlling the corrosion of the sprayed samples at prolonged exposure [53]. In case of the SS plate and Ti coating, the passive film controls the corrosion of the samples. There is no role played by chemistry; rather, morphology controls the corrosion of samples.The impedance values measured at lowest frequency (0.01 Hz) in Figure[tnq_nbsp] 5(b) were found to be the highest than those of 1 h of exposure to acidic pH solution for all samples. The impedance values of both sprayed coatings exhibited almost identical characteristics, but those of the Ti coatings were higher. This result is attributed to the fact that the Ti coating is more resistant to corrosion in the H2SO4 solution at pH 4 after 312 h of exposure [45,[tnq_nbsp] 54–3b2:?show $132#56]. The Ti and SS coatings exhibited higher resistance at the highest studied frequency due to formation of passive films and deposition of corrosion products in 3b2:?thyc=10defects/pores, 3b2:?thycrespectively, and showed higher impedance.After 312 h of exposure, the corrosion of SS and Ti coatings in acidic solution was controlled by their respective corrosion products and passive film [53]. The impedance value of Ti coating was greater than that of the 3b2:?thyc=10SS-sprayed3b2:?thyc coating owing to the more stable and adherent passive oxide film formed on its surface after exposure to the solution. The SS plate had the highest impedance values compared to the sprayed coatings.The phase shiftθ (°)-log f (Hz) Bode plots of samples after 312 h of exposure to solution are shown in Figure[tnq_nbsp] 5(b). The scattered data shown in the middle frequency range are attributed due to the 3b2:?thyc=10defective/porous 3b2:?thycoxide film caused by the corrosion products of SS and Ti coatings.The shifting of the phase angle maxima toward −75° for the SS plate was attributed to the formation of the homogeneous passive film on the surface, which revealed the strengthening of the film in the solution. This result indicates that the passive3b2:?thyc=10film/corrosion 3b2:?thycproducts formed on the plate are surface resisted to the attack of corrosive ions [57].The impedance data were validated by Kramers–3b2:?show $132#Kronig (3b2:?thyc=10K-K)3b2:?thyc transformation by transforming the real axis into the imaginary axis and vice versa. The K-3b2:?show $132#K transformations are shown in Figure[tnq_nbsp] S2 and have been described elsewhere [58–3b2:?show $132#60]. These results confirm the agreement between the experimental data and K-3b2:?show $132#K transformations, which is accordance with the linear system theory.Brug’s formula has been widely used to extract effective capacitance values from CPE parameters for studies on double layers [61]. Brug et[tnq_nbsp] al. [62] have established the relationship between CPE parameters and effective capacitance (Ceff) associated with the CPE which can therefore be expressed as follows:3b2:?A3B2 tpb 1pt(1)Ceff=Q1nR1−n3b2:?A3B2 }go/3b2:?A3B2 :go3b2:?A3B2 show \/n,where Q is the CPE parameter such as nonideal 3b2:?thyc=10double-layer3b2:?thyc capacitance, R is a resistance caused by dissolution of the metal or alloy at the 3b2:?thyc=10metal/solution 3b2:?thycinterface in low frequency, and n is the CPE exponent (−1 <n < 1). When n is ∼1, 0.5, 0, and −1, the CPE is equivalent to a capacitor, the Warburg diffusion, a resistor, and an inductor, respectively.After fitting of EIS data to a suitable EEC, the electrochemical parameters are shown in Table[tnq_nbsp] 3b2:?show [?"tbl1",5,"cond1"]1. The Rs is very high for all systems due to low conductively of the solution. The Rs is gradually decreased with increasing exposure periods due to involvement of more ions after reaction of metals in acidic pH solution [53].Table 1
Electrochemical parameters of the SS plate, SS-sprayed coating, and Ti-sprayed coating extracted after fitting of EIS data to suitable EECs with different exposure periods in pH 4 solution.
Time (h)
1
312
3b2:?show "^tbl_width"$01649v
Sample ID
SS plate
SS sprayed
Ti sprayed
SS plate
SS sprayed
Ti sprayed
R
s (kΩ·cm2)
6.77
5.20
8.64
5.02
5.37
8.45
Rp (kΩ·cm2)
166.00
16.02
48.19
22.82
20.15
21.07
CPE1
Q1 (1 × 10−6) (Ω−1·cm−2·sn)
2.7
62.0
7.3
29.6
41.6
37.2
n1
0.99
0.99
0.99
0.95
0.92
0.93
C
eff1 (µF·cm−2)
2.68
62.0
7.22
29.0
41.0
36.5
R
ct (kΩ·cm2)
11.87
9.15
10.97
350.02
11.56
46.33
CPE2
Q2 (1 × 10−6) (Ω−1·cm−2·sn)
91.6
252.3
105.8
20.9
156.1
38.5
n2
0.75
0.44
0.54
0.91
0.46
0.60
C
eff2 (µF·cm−2)
94.19
731.72
120.11
25.44
312.13
56.62
W (1 × 10−6) (Ω·cm2·s−0.5)
—
—
—
4.2
—
—TheRp and Rct values of samples are gradually decreased and increased, respectively, as exposure periods increased. The Rp is emphasizing due to resistance caused by inhomogeneity of the metal surface, and it is decreased due to corrosion. The Rct is increased for SS plate and Ti-sprayed coating due to protective nature of passive film while SS sprayed coating owing to deposition of corrosion products on surface. The corrosion products and passive oxide film[tnq_nbsp] increase their thickness as exposure periods were increased, resulting in high Rct than 1 h of exposure [63]. The capacitance of the 3b2:?thyc=10metal/coating 3b2:?thycsurface and passive 3b2:?thyc=10film/3b2:?show [?tjl=20mm]&tjlpc;[?tjl]corrosion 3b2:?thycproducts is derived as Ceff1 and Ceff2, respectively. The Ceff1 is dramatically increased as Rp is decreased with exposure periods, which indicates that the 3b2:?thyc=10metal/coating 3b2:?thycsurface started to corrode, but as the Rct is increased, Ceff2 is[tnq_nbsp] decreased. The Ceff2 result was attributed to that the surface[tnq_nbsp] became homogenized due to formation of the passive layer or corrosion products on the 3b2:?thyc=10metal/coating 3b2:?thycinterface after 312 h of exposure. However, it is found that the Ceff is[tnq_nbsp] greater for SS coating than the Ti coating and SS plate in all[tnq_nbsp] exposure periods. It indicates that 3b2:?A3B2 tlsb .1ptthe SS coating is more[tnq_nbsp] inhomogeneous and defective than other samples. The thickening of the oxide film was attributed to anodic oxidation and formation of the protective passive 3b2:?thyc=10film/corrosion 3b2:?thycproducts that reduced the penetration of aggressive ions [64]. The corrosion product itself caused resistance to corrosion due to uniform and adherent deposition.After 312 h of exposure, W was observed for the SS plate, possibly resulting from diffusion of the protective passive layer on the surface [65,[tnq_nbsp] 66]. As exposure periods are increased, the passive film strength also increased.After 1 h of exposure,Rp is found to be highest for all samples due to a barrier type of protection exhibited by the coatings. The NiO, Fe2O3, FeO, and Cr2O3 thin films are formed on the SS plate [42] which give the protection against corrosion. Initially, the metal or coating surface does not start to react with solution resulting in high Rp, but once proper reaction has occurred, the surfaces start to corrode. At the time of corrosion initiation, Rct will involve which causes resistance to penetration of the solution toward the metal surface. Therefore, Rp is decreased and Rct is increased as exposure periods are increased. The film formed on the surfaces was imperfect and rough [67,[tnq_nbsp] 68] after 1 h of exposure; thus, the dispersion coefficient (n) is less for CPE2. As the exposure periods were increased, the Rct values increased and CPE decreased for passive 3b2:?thyc=10layer/corrosion 3b2:?thycproducts of the samples. Rct is high for all samples due to deposition of corrosion products on 3b2:?thyc=10SS-sprayed3b2:?thyc coating and the protective passive layer on 3b2:?thyc=10Ti-sprayed3b2:?thyc coatings and the SS plate after 312 h of exposure.


### 3.4.2. Potentiodynamic Studies

3b2:?A3B2 tlsb -.1ptPotentiodynamic studies were carried out after 312 h of exposure, and results are shown in Figure[tnq_nbsp] 3b2:?show [?"gr6",5,"cond1"]6. The SS plate showed pitting and many breakdown potentials during anodic scanning. The breakdown potentials may be caused by oxidation of the metal surface due to impressed current which form a new phase or a metastable passive film, that altered the passive film properties [69]. Therefore, there is a chance that another oxide phase could form on the surface, which might be protective in nature.Figure 6
Potentiodynamic plots of the SS plate and sprayed coatings in pH 4 solution after 312 h of exposure (1 mV/s).3b2:?A3B2 tlsb -.1ptThe current density of the SS plate is lower than that of Ti and SS coatings during anodic scanning. The interesting observation is found in case of Ti and SS coatings that there is a gradual increase in anodic current density during anodic scanning. It may be due to corrosion or transformation of unstable oxide films of these samples, and whatever corrosion 3b2:?thyc=10products/passive 3b2:?thycfilm formed was deposited on the surface.The anodic and passive corrosion current of the Ti coating was lower than that of the SS coating, which means that, in this case, the former is more likely to form compact, protective, and adherent passive oxide films [70,[tnq_nbsp] 71].3b2:?A3B2 tlsb -.1ptThe passive film of 3b2:?thyc=10Ti-sprayed3b2:?thyc coating resisted the penetration of corrosive species of the solution; thus, the reducing corrosion rate is observed. During cathodic scanning, all samples exhibited hydrogen evolution reaction which dominated over the oxygen reduction reaction [72].The electrochemical parameters were extracted after fitting of potentiodynamic plots to the Tafel region using the Stern–3b2:?show $132#Geary equation3b2:?A3B2 tpb -1.2pt(2)Icorr=BRtotal.The Stern–3b2:?show $132#Geary constant (B) can be calculated by putting the values for corrosion current density (Icorr) and total polarization resistance (Rtotal) in (2). The extracted data on the corrosion potential (Ecorr), Icorr, Rtotal, B, and the corrosion rate of samples after 312 h of exposure to pH 4 solution are shown in Table[tnq_nbsp] 3b2:?show [?"tbl2",5,"cond1"]2.Table 2
Electrochemical parameters extracted after fitting of potentiodynamic plots to the Tafel region.
Sample ID
Electrochemical parameters
E
corr (V) versus Ag/AgCl
I
corr (µA·cm−2)
R
total (kΩ·cm2)
B (V)
Corrosion rate (µm·y−1)
3b2:?show "^tbl_width"$01649v
SS plate
0.138
0.382
379.860
0.15
5.132
SS sprayed
−0.594
19.803
33.792
0.67
266.043
Ti sprayed
−0.403
1.913
68.464
0.13
42.703TheEcorr of the SS plate and SS and Ti coatings are 0.138, −0.594, and −0.403 V versus 3b2:?thyc=10Ag/AgCl, 3b2:?thycrespectively. The SS plate exhibited nobler Ecorr than the Ti coating followed by the SS coating.The nobler potential of the SS plate is due to formation of the3b2:?thyc=10Cr-enriched3b2:?thyc oxide film, whereas others exhibited the active potential. The active Ecorr of SS- and 3b2:?thyc=10Ti-sprayed3b2:?thyc coatings compared to the SS plate is attributed to the presence of defects on the coating surface.Lai et[tnq_nbsp] al. observed that when SS was exposed to H2SO4-contaminated water solution, it formed NiO, Fe2O3, FeO, and Cr2O3 thin films which were protective in nature and noble [42]. The active potential of SS coating was due to the presence of defective or porous 3b2:?thyc=10oxide/corrosion 3b2:?thycfilms that made the sample more susceptible to corrosion and exhibit the mixed potential [73].The studied pH solution was acidic and led to the[tnq_nbsp] deterioration of the samples. During exposure, the formed corrosion products deposited on the sample surface. The corrosion products blocked the 3b2:?thyc=10defects/pores 3b2:?thycof the samples and resisted the penetration of the solution [74,[tnq_nbsp] 75].The iron oxides were more active and therefore exhibited the active potential. The Ti coating exhibited a nobler potential than SS coating because it had only fine and elongated cracks (Figure[tnq_nbsp] 2(c)), which stifled the aggressive species of the solution from reaching the base metal. In contrast, the SS coating contains many connected pores and valley morphology where the acidic solution can accumulate and induce crevice corrosion.These results indicate that the passive film formed on the3b2:?thyc=10Ti-sprayed3b2:?thyc coating after exposure to pH 4 solution is protective, nonporous, compact, and resistant to the penetration of aggressive ions in the solution. The SS coating has porous and nonprotective corrosion 3b2:?thyc=10products/iron 3b2:?thycoxides.TheRtotal values of the SS plate, 3b2:?thyc=10SS-sprayed3b2:?thyc coating, and 3b2:?thyc=10Ti-sprayed3b2:?thyc coating are 379.860, 33.792, and 68.464 kΩ·cm2, respectively. The higher Rtotal value of Ti coating compared to the SS coating suggests that it can be used as a coating to protect the materials in H2SO4-contaminated water solution, even at low pH. The B values were calculated by using (2), and it was found that the SS plate and 3b2:?thyc=10Ti-sprayed3b2:?thyc coating were identical and less active, while 3b2:?thyc=10SS-sprayed3b2:?thyc coating showed 0.67 V which is more pronounced to corrosion [76]. The B value of the SS plate and 3b2:?thyc=10Ti-sprayed3b2:?thyc coating is under 3b2:?A3B2 tlsb -.1ptthe active control, while the 3b2:?thyc=10SS-sprayed3b2:?thyc surface exhibits active dissolution values which influence the corrosion phenomena. The calculated Icorr value of 3b2:?thyc=10SS-sprayed3b2:?thyc samples reveals the 3b2:?A3B2 tlsb .1ptactiveness of coating, while the SS plate and 3b2:?thyc=10Ti-sprayed3b2:?thyc coating control the corrosion process in acidic solution at longer duration of exposure.The corrosion rate (µm·y−1) was calculated by the following equation [77]:3b2:?A3B2 tpb .7pt(3)Corrosion  rateµm⋅y−1=3.27×Icorr×EWd.The corrosion rate in (3) is expressed in micrometres per year (µ3b2:?thyc=10m/year) 3b2:?thycand Icorr in µA·cm−2. The Icorr was obtained by dividing the total surface area of the working electrode under the corrosion current (µA). EW represents the equivalent weight (g·mol−1), and d is the density (g·cm−3).The corrosion rate of the SS coating is 266.043µm·y−1 and is greater than that of the SS plate and Ti coating by 51. 84 and 6.23 times, respectively. This result indicates that the SS is not an effective coating material for deposition by the arc thermal spray process in pH 4 solution and long duration of exposure.The corrosion rate data of the SS coating revealed that it totally3b2:?thyc=10dissolved/corroded 3b2:?thycdown to the base substrate. The initial coating thickness was 200 µm, while the corrosion rate was 266.043 µm·y−1. Thus, it may be reported that the Ti coating was effective in protecting the surface than the SS coating. The Ti can be used as a coating material to protect the waste water reservoir and extend its service life.


### 3.4.3. Characterization of Corrosion Products after Potentiodynamic Studies in pH 4 by Different Techniques

3b2:?A3B2 tlsb .1ptThe morphology of corrosion products was examined by SEM, and results are shown in Figure[tnq_nbsp] 3b2:?show [?"gr7",5,"cond1"]7. On the SS plate surface, the passive film was adherent, uniform, and regularly deposited, thus preventing the penetration of solution 3b2:?A3B2 tlsb -.1pt(Figure[tnq_nbsp] 7(a)). The edges of the surface show few cracks caused 3b2:?A3B2 tlsb .1ptby the destructive potentiodynamic experiment, and the passive film prevented the cracking. After potentiodynamic studies, the SS plate surface did not show any other type of corrosion products/rust.3b2:?tex type="figure" layout="twocols" n="2,3"Figure 7
SEM images of corrosion products formed on the (a) SS plate, (b) SS-sprayed coating, and (c) Ti-sprayed coating after potentiodynamic studies in pH 4 solution.
(a)
(b)
(b)3b2:?A3B2 tlsb -.1ptThe SS coating exhibited different sizes of corrosion product morphology with micropore formation (Figure[tnq_nbsp] 7(b)). 3b2:?A3B2 tlsb .2ptThe 3b2:?thyc=10net-like3b2:?thyc microstructure of corrosion products is attributed to the presence of porous iron oxides. Through the net and thread morphologies, the acidic solution easily penetrated the substrate and formed corrosion products.The morphology of corrosion products formed on the Ti coating was totally different from that on the SS plate and sprayed coating. The passive films formed on the3b2:?thyc=10Ti-sprayed3b2:?thyc surface exhibit microcracks, plate, and globular morphology (Figure[tnq_nbsp] 7(c)). The globular particles block the micro- and macrocracks on the top surface. Therefore, enhanced corrosion resistance was observed after 312 h of exposure than on 3b2:?thyc=10SS-sprayed3b2:?thyc coating.Passive oxide films of Ti coating contain3b2:?thyc=10plate-like3b2:?thyc microstructures that were uniformly deposited on the surface. Similar morphologies were not observed in the corrosion products of the SS plate and sprayed coating.The phases present in the corrosion products of all samples after potentiodynamic studies were studied by XRD. The identification of phases in corrosion products is shown in Figure[tnq_nbsp] 3b2:?show [?"gr8",5,"cond1"]8. The SS plate exhibits the presence of tetrataenite (FeNi) and Fe. It is reported that FeNi is unstable and can deteriorate into other forms, if it exposes for long term to 3b2:?thyc=10low-temperature3b2:?thyc environments [78].Figure 8
XRD of the SS plate and sprayed coatings after potentiodynamic studies in pH 4 solution (0.5°/min).3b2:?A3B2 tlsb -.1ptThe presence of lepidocrocite (γ-FeOOH) in the corrosion products of 3b2:?thyc=10SS-sprayed3b2:?thyc coatings confirmed that this coating was susceptible to corrosion in acidic solution. However, Ti coating exhibits composite oxides along with Ti and TiO. Therefore, the improved corrosion resistance of 3b2:?thyc=10Ti-sprayed3b2:?thyc coating is observed by formation of TiO2 (rutile and anatase), and this observation corroborates with EIS and potentiodynamic results. The passive oxides of Ti such as TiO2 and Ti3O5 have formed. The TiO2 is thermodynamically more stable than others [79]. Therefore, the 3b2:?thyc=10Ti-sprayed3b2:?thyc coating is attributed to improved corrosion resistance properties of coating in H2SO4 solution. The transformation of Ti into TiO2 in the H2SO4 environment is well documented elsewhere [80,[tnq_nbsp] 81]. TiO3 and some amount of TiO (Figure[tnq_nbsp] 3) may be transformed into TiO2 and Ti3O5 due to a strong oxidizing ability of H2SO4 solution. Thus, corrosion 3b2:?thyc=10products/passive 3b2:?thycfilm of 3b2:?thyc=10Ti-sprayed3b2:?thyc coating exhibits some peaks of TiO (Figure[tnq_nbsp] 8). Therefore, corrosion is observed after 312 h of exposure to H2SO4 solution. Once proper transformation 3b2:?A3B2 tlsb -.1ptof Ti and TiO into the stable form occurred, then the corrosion rate would be completely suppressed.


## 3.4.1. EIS Studies

3b2:?A3B2 tlsb -.11ptThe samples were immersed in pH 4 solution for different periods of exposure. EIS was carried out to study their corrosion characteristics. These results are shown in Figures[tnq_nbsp] 3b2:?show [?"gr4",5,"cond1"]4 and 3b2:?show [?"gr5",5,"cond1"]5. The electrical equivalent circuit (EEC) is shown in the corresponding Nyquist plots. The EEC of the SS plate exposed to the pH 4 solution for 1 h is shown in Figure[tnq_nbsp] 4(a). In the Nyquist plot of the SS plate, the sample after 1 h of exposure is differentiated by two depressed semicircle loops such as one at high while another at the lower studied frequency. For more clarity of plots at high frequency, the Nyquist result of samples is shown in Figure[tnq_nbsp] S1 (supplementary figure). These results can be explained either by the heterogeneity of the solid surface or by the dispersion of some physical properties. The interface of the surface cannot be considered as an ideal capacitor due to heterogeneity of the surface, and it may involve a 3b2:?thyc=10constant-phase3b2:?thyc element (CPE) as a substitute of the ideal capacitor. The first EEC consists of the[tnq_nbsp] solution resistance (Rs), polarization resistance (Rp), and CPE1 due to the metal surface and nonideal 3b2:?thyc=10double-layer3b2:?thyc capacitance behaviour [34–3b2:?show $132#36]. The Rp and CPE1 are parallel to each other. However, the second EEC contains the charge transfer resistance (Rct) and CPE2. The formation of Rct may be due to formation of the protective passive layer on the SS plate surface in acidic pH solution after 1 h of exposure. These two EECs are connected in series to each other.3b2:?tex type="figure" layout="twocols" n="2"Figure 4
Impedance spectra: (a) Nyquist and (b) Bode |Z| and (θ) of the SS plate and sprayed coatings in pH 4 solution after 1 h of exposure (100 kHz to 0.01 Hz).
bottom
3b2:?A3B2 tpt 4pt(a)
(b)3b2:?tex type="figure" layout="twocols" n="2"Figure 5
Impedance spectra: (a) Nyquist and (b) Bode |Z| and (θ) of the SS plate and sprayed coatings in pH 4 solution after 312 h of exposure (100 kHz to 0.01 Hz).
bottom
3b2:?A3B2 tpt -2pt(a)
(b)The EECs for the SS- and3b2:?thyc=10Ti-sprayed3b2:?thyc coating systems are somewhat different from the SS plate, and they are inserted in Figure[tnq_nbsp] 4(a). The different EECs for these coatings may be attributed to the inherent property of the arc thermal spray process, where coatings suffer from surface defects. Due to reaction on the metal surface, Rct participated owing to initiation of the corrosion process in acidic pH solution. The reaction on the metal surface caused by Rct led to the formation of a 3b2:?thyc=10passive/oxide 3b2:?thyclayer on the metal surface, which increased the resistance and reduced the corrosion reaction. The CPE1 due to a nonideal behaviour of the coating surface and Rp are parallel to each other, while another EEC is connected in series to Rp which contains CPE2 and Rct [15]. The CPE2 and Rct are parallel to each other.The Nyquist plots reveal the real characteristics of the samples after 1 h of exposure (Figure[tnq_nbsp] 4(a)). The samples were exposed to the solution for 1 h to stabilize the potential; thereafter, EIS measurements were performed.Figure[tnq_nbsp] 4(a) shows the two semicircle loops in the Nyquist plots exhibited by coated samples. The SS- and 3b2:?thyc=10Ti-sprayed3b2:?thyc coatings show zigzag and scattered plots which might be due to low conductivity of electrolytes, deposition of defective coating, formation of a defective passive film, and the presence of more resistive elements such as Ti in 3b2:?thyc=10Ti-sprayed3b2:?thyc coating while Cr and Ni in 3b2:?thyc=10SS-sprayed3b2:?thyc samples [37–3b2:?show $132#40]. These results are attributed to the fact that both sprayed samples exhibit capacitive properties due to the presence of defects. However, the Ti coating imposed a resistance greater than the SS coating due to the formation of a 3b2:?thyc=10passive/oxide 3b2:?thycfilm with capacitive behaviour which enabled surface resistance to penetrate the ions of the acidic solution.In the sulfuric acid solution, the Ti surface tends to form defective passive films with high resistance. Similar results have been observed by Baron et[tnq_nbsp] al. on TiAlV and TiAlNb alloys in Tyrode’s solution [41].The dimensions of the semicircle loop of SS plate samples clearly show high capacitive property that enables the surface to resist the penetration of the solution. The capacitive property of the passive film on the SS plate was attributed to the formation of3b2:?thyc=10Cr-enriched3b2:?thyc oxide and NiO in the H2SO4-contaminated water solution [42].The dimensions of the semicircle loops of SS- and3b2:?thyc=10Ti-sprayed3b2:?thyc coatings were less than those of the SS plate because the sprayed samples were more susceptible to corrosion, owing to the formation of 3b2:?thyc=10defects/pores 3b2:?thycon the coating surface in the solution after 1 h of exposure.The impedance at low frequency (0.01 Hz) and the phaseθ (°) of 3b2:?A3B2 tlsb -0.15ptBode plots were plotted against log |Z| (Ω·cm2) ver3b2:?show $132#sus log f (Hz) and θ (°) versus log f (Hz) in Figure[tnq_nbsp] 4(b), re3b2:?show $132#spec3b2:?show $132#tively.[tnq_nbsp] The impedance values of the SS plate sample were greater than[tnq_nbsp] those of the SS and Ti coatings. The SS coating exhibited lower impedance values because of the presence of more 3b2:?thyc=10defects/pores 3b2:?thycat locations where the chances of penetration by the acidic solution are high, and this initiated the deterioration of the coating.3b2:?A3B2 tlsb -.28ptFrom the log |Z| (Ω·cm2)-log f (Hz) Bode plots (Figure[tnq_nbsp] 4(b)), 3b2:?A3B2 tlsb -.1ptit can be seen that, at high frequency (100 kHz), resistance was moreover identical to that observed at low frequency, that is, 0.01 Hz, which might be attributed to the low conductivity of acidic pH solution.In this study, the solution was prepared by adding a few drops of 0.1 M·H2SO4 to distilled water. The solution conductivity was very low, which caused the resistance in total impedance. The conductivity of the solution is an important parameter that must be considered in electrochemical studies. However, the Ti coating exhibited higher impedance than the SS coating due to formation of a thick and protective passive oxide film. The low impedance values of the sprayed samples are due to the presence of more defects or less interfacial resistance between the splats of coating than on the SS plate sample.3b2:?A3B2 tlsb -.1ptThe surface finish and coating microstructure play an important role to determine the corrosion resistance properties of materials in the solution. The pH 4 solution is very aggressive and causes localized or pitting corrosion of the oxide films formed during exposure [43–3b2:?show $132#45].3b2:?A3B2 tlsb -.1ptThe defective parts of coatings can function as an anode, while the remaining acts as a cathode, resulting in the formation of microgalvanic cells on the surface. The presence of microgalvanic cells enhances the corrosion rate of materials; thus, there is a chance of getting low impedance. Such observations are found in SS- and 3b2:?thyc=10Ti-sprayed3b2:?thyc coatings. In view of the above, it can be observed that the SS coating exhibited 3b2:?thyc=10valley-type3b2:?thyc deposits (Figure[tnq_nbsp] 2(b)) where the acidic solution could 3b2:?thyc=10stagnant/deposit 3b2:?thycand cause localized and crevice corrosion. During the initial period of exposure, both sprayed coatings had defects that resulted in lower impedance values than the SS plate surface.3b2:?A3B2 tlsb .1ptThe SS plate shows a −40° shift of the phase angle maxima at the lower studied frequency and reveals high resistance to corrosion in the pH 4 solution (Figure[tnq_nbsp] 4(b)). On the other hand, Ti and SS coatings exhibited −1° and −2° shifts, respectively, which indicate their susceptibility to corrosion during the initial period of exposure [37]. In the middle frequency range, the samples exhibited scattering which might be due to the capacitive response of the defective passive film that was formed during exposure of the samples to H2SO4 solution [38–3b2:?show $132#40].The shifting of maxima at the higher studied frequency (100 kHz) is due to the deposition of corrosion products on the3b2:?thyc=10SS-sprayed3b2:?thyc sample, whereas on the 3b2:?thyc=10Ti-coated3b2:?thyc sample, it is due to formation of the resistive passive film. It can be seen that the Ti coating exhibited approximately −57° shift followed by the SS coating at −38°, while the SS plate had the lowest shift at −23°. These results indicate that the 3b2:?thyc=10Ti-sprayed3b2:?thyc surface formed a protective passive film owing to reaction at the 3b2:?thyc=10coating/solution 3b2:?thycinterface. Thus, the Ti coating exhibited higher resistance to the acidic solution.As the exposure periods were extended, the increased dimensions of semicircle loops in the Nyquist plots showed increased corrosion resistance [46]. The bigger loops in the Nyquist plots reveal high resistance to corrosion in any environment. Such results can be seen from Figure[tnq_nbsp] 5(a) after 312 h of exposure to pH 4 solution. The EEC for the SS plate after 312 h of exposure is inserted in Figure[tnq_nbsp] 5(a). The Warburg impedance (W) is caused by diffusion of the protective passive layer on the SS plate surface in the H2SO4-contaminated solution. Rct and W are parallel to the CPE2[tnq_nbsp] [16].At longer periods of exposure (312 h), many parameters are involved owing to the complex reaction process on the3b2:?thyc=10metal/solution 3b2:?thycinterface.All samples exhibited two loops in the Nyquist plots, one at higher and another at lower frequencies. The loops formed at higher and lower frequencies because of the solution resistance and the reaction at the3b2:?thyc=10metal/solution 3b2:?thycinterface, respectively [47–3b2:?show $132#51]. As the exposure period is increased, strengthening of the passive film on the SS plate may occur [52]. However, in case of Ti, there is a possibility of formation of the protective passive film due to transformation 3b2:?A3B2 tlsb -.1ptof unstable titanium oxides into stable oxides, while on 3b2:?thyc=10SS-sprayed3b2:?thyc samples, it is due to deposition of corrosion products on the coating defects in H2SO4-contaminated water solution [53]. The two semicircle loops of Ti and SS coatings were successfully distinguished from each other for this exposure 3b2:?A3B2 tlsb .1ptperiod. Therefore, the Ti coating had provided greater 3b2:?A3B2 tlsb -.1ptprotection than the SS coating. These two diffused semicircle loops on sprayed coatings are not clearly seen because of the reduced conductivity of the solution and scattered data.The dimensions of semicircle loops in the Nyquist plot of the Ti coating were bigger, indicating that the anodic surface area of the coating was decreased by the formation of the protective oxide film rather than SS coating. The SS plate had higher resistance to the H2SO4-contaminated solution owing to the formation of the protective passive film [42].The SS plate surface exhibited the protective passive film that is resistant to corrosion because the values of bothZ′real and −Z″imaginary axes are increased (Figure[tnq_nbsp] 5(a)). From the initial to the prolonged exposure, the SS plate showed higher resistance to corrosion which can be attributed to the formation of compact and uniform passive layers [42].3b2:?A3B2 tlsb .1ptOn the other hand, the SS and Ti coatings showed less resistance to corrosion than the SS plate because of the formation of surface 3b2:?thyc=10defects/cracks, 3b2:?thycwhich enhanced the corrosion rate due to penetration of aggressive solution. It 3b2:?A3B2 tlsb -.1ptcan be seen from Figure[tnq_nbsp] 5(a) that the SS and Ti coatings exhibit diffused semicircle loops separated by two small loops.The bigger loop shifted toward Z″imaginary because of the formation of the capacitive passive 3b2:?thyc=10film/corrosion 3b2:?thycproducts. The lower frequency loop shifted toward Z′real of the Nyquist plots (Figure[tnq_nbsp] 5(a)) because of the increased resistance to corrosion.3b2:?A3B2 tlsb -.1ptThe nature of corrosion 3b2:?thyc=10products/passive 3b2:?thycfilms plays[tnq_nbsp] a major role in controlling the corrosion of the sprayed samples at prolonged exposure [53]. In case of the SS plate and Ti coating, the passive film controls the corrosion of the samples. There is no role played by chemistry; rather, morphology controls the corrosion of samples.The impedance values measured at lowest frequency (0.01 Hz) in Figure[tnq_nbsp] 5(b) were found to be the highest than those of 1 h of exposure to acidic pH solution for all samples. The impedance values of both sprayed coatings exhibited almost identical characteristics, but those of the Ti coatings were higher. This result is attributed to the fact that the Ti coating is more resistant to corrosion in the H2SO4 solution at pH 4 after 312 h of exposure [45,[tnq_nbsp] 54–3b2:?show $132#56]. The Ti and SS coatings exhibited higher resistance at the highest studied frequency due to formation of passive films and deposition of corrosion products in 3b2:?thyc=10defects/pores, 3b2:?thycrespectively, and showed higher impedance.After 312 h of exposure, the corrosion of SS and Ti coatings in acidic solution was controlled by their respective corrosion products and passive film [53]. The impedance value of Ti coating was greater than that of the 3b2:?thyc=10SS-sprayed3b2:?thyc coating owing to the more stable and adherent passive oxide film formed on its surface after exposure to the solution. The SS plate had the highest impedance values compared to the sprayed coatings.The phase shiftθ (°)-log f (Hz) Bode plots of samples after 312 h of exposure to solution are shown in Figure[tnq_nbsp] 5(b). The scattered data shown in the middle frequency range are attributed due to the 3b2:?thyc=10defective/porous 3b2:?thycoxide film caused by the corrosion products of SS and Ti coatings.The shifting of the phase angle maxima toward −75° for the SS plate was attributed to the formation of the homogeneous passive film on the surface, which revealed the strengthening of the film in the solution. This result indicates that the passive3b2:?thyc=10film/corrosion 3b2:?thycproducts formed on the plate are surface resisted to the attack of corrosive ions [57].The impedance data were validated by Kramers–3b2:?show $132#Kronig (3b2:?thyc=10K-K)3b2:?thyc transformation by transforming the real axis into the imaginary axis and vice versa. The K-3b2:?show $132#K transformations are shown in Figure[tnq_nbsp] S2 and have been described elsewhere [58–3b2:?show $132#60]. These results confirm the agreement between the experimental data and K-3b2:?show $132#K transformations, which is accordance with the linear system theory.Brug’s formula has been widely used to extract effective capacitance values from CPE parameters for studies on double layers [61]. Brug et[tnq_nbsp] al. [62] have established the relationship between CPE parameters and effective capacitance (Ceff) associated with the CPE which can therefore be expressed as follows:3b2:?A3B2 tpb 1pt(1)Ceff=Q1nR1−n3b2:?A3B2 }go/3b2:?A3B2 :go3b2:?A3B2 show \/n,where Q is the CPE parameter such as nonideal 3b2:?thyc=10double-layer3b2:?thyc capacitance, R is a resistance caused by dissolution of the metal or alloy at the 3b2:?thyc=10metal/solution 3b2:?thycinterface in low frequency, and n is the CPE exponent (−1 <n < 1). When n is ∼1, 0.5, 0, and −1, the CPE is equivalent to a capacitor, the Warburg diffusion, a resistor, and an inductor, respectively.After fitting of EIS data to a suitable EEC, the electrochemical parameters are shown in Table[tnq_nbsp] 3b2:?show [?"tbl1",5,"cond1"]1. The Rs is very high for all systems due to low conductively of the solution. The Rs is gradually decreased with increasing exposure periods due to involvement of more ions after reaction of metals in acidic pH solution [53].Table 1
Electrochemical parameters of the SS plate, SS-sprayed coating, and Ti-sprayed coating extracted after fitting of EIS data to suitable EECs with different exposure periods in pH 4 solution.
Time (h)
1
312
3b2:?show "^tbl_width"$01649v
Sample ID
SS plate
SS sprayed
Ti sprayed
SS plate
SS sprayed
Ti sprayed
R
s (kΩ·cm2)
6.77
5.20
8.64
5.02
5.37
8.45
Rp (kΩ·cm2)
166.00
16.02
48.19
22.82
20.15
21.07
CPE1
Q1 (1 × 10−6) (Ω−1·cm−2·sn)
2.7
62.0
7.3
29.6
41.6
37.2
n1
0.99
0.99
0.99
0.95
0.92
0.93
C
eff1 (µF·cm−2)
2.68
62.0
7.22
29.0
41.0
36.5
R
ct (kΩ·cm2)
11.87
9.15
10.97
350.02
11.56
46.33
CPE2
Q2 (1 × 10−6) (Ω−1·cm−2·sn)
91.6
252.3
105.8
20.9
156.1
38.5
n2
0.75
0.44
0.54
0.91
0.46
0.60
C
eff2 (µF·cm−2)
94.19
731.72
120.11
25.44
312.13
56.62
W (1 × 10−6) (Ω·cm2·s−0.5)
—
—
—
4.2
—
—TheRp and Rct values of samples are gradually decreased and increased, respectively, as exposure periods increased. The Rp is emphasizing due to resistance caused by inhomogeneity of the metal surface, and it is decreased due to corrosion. The Rct is increased for SS plate and Ti-sprayed coating due to protective nature of passive film while SS sprayed coating owing to deposition of corrosion products on surface. The corrosion products and passive oxide film[tnq_nbsp] increase their thickness as exposure periods were increased, resulting in high Rct than 1 h of exposure [63]. The capacitance of the 3b2:?thyc=10metal/coating 3b2:?thycsurface and passive 3b2:?thyc=10film/3b2:?show [?tjl=20mm]&tjlpc;[?tjl]corrosion 3b2:?thycproducts is derived as Ceff1 and Ceff2, respectively. The Ceff1 is dramatically increased as Rp is decreased with exposure periods, which indicates that the 3b2:?thyc=10metal/coating 3b2:?thycsurface started to corrode, but as the Rct is increased, Ceff2 is[tnq_nbsp] decreased. The Ceff2 result was attributed to that the surface[tnq_nbsp] became homogenized due to formation of the passive layer or corrosion products on the 3b2:?thyc=10metal/coating 3b2:?thycinterface after 312 h of exposure. However, it is found that the Ceff is[tnq_nbsp] greater for SS coating than the Ti coating and SS plate in all[tnq_nbsp] exposure periods. It indicates that 3b2:?A3B2 tlsb .1ptthe SS coating is more[tnq_nbsp] inhomogeneous and defective than other samples. The thickening of the oxide film was attributed to anodic oxidation and formation of the protective passive 3b2:?thyc=10film/corrosion 3b2:?thycproducts that reduced the penetration of aggressive ions [64]. The corrosion product itself caused resistance to corrosion due to uniform and adherent deposition.After 312 h of exposure, W was observed for the SS plate, possibly resulting from diffusion of the protective passive layer on the surface [65,[tnq_nbsp] 66]. As exposure periods are increased, the passive film strength also increased.After 1 h of exposure,Rp is found to be highest for all samples due to a barrier type of protection exhibited by the coatings. The NiO, Fe2O3, FeO, and Cr2O3 thin films are formed on the SS plate [42] which give the protection against corrosion. Initially, the metal or coating surface does not start to react with solution resulting in high Rp, but once proper reaction has occurred, the surfaces start to corrode. At the time of corrosion initiation, Rct will involve which causes resistance to penetration of the solution toward the metal surface. Therefore, Rp is decreased and Rct is increased as exposure periods are increased. The film formed on the surfaces was imperfect and rough [67,[tnq_nbsp] 68] after 1 h of exposure; thus, the dispersion coefficient (n) is less for CPE2. As the exposure periods were increased, the Rct values increased and CPE decreased for passive 3b2:?thyc=10layer/corrosion 3b2:?thycproducts of the samples. Rct is high for all samples due to deposition of corrosion products on 3b2:?thyc=10SS-sprayed3b2:?thyc coating and the protective passive layer on 3b2:?thyc=10Ti-sprayed3b2:?thyc coatings and the SS plate after 312 h of exposure.


## 3.4.2. Potentiodynamic Studies

3b2:?A3B2 tlsb -.1ptPotentiodynamic studies were carried out after 312 h of exposure, and results are shown in Figure[tnq_nbsp] 3b2:?show [?"gr6",5,"cond1"]6. The SS plate showed pitting and many breakdown potentials during anodic scanning. The breakdown potentials may be caused by oxidation of the metal surface due to impressed current which form a new phase or a metastable passive film, that altered the passive film properties [69]. Therefore, there is a chance that another oxide phase could form on the surface, which might be protective in nature.Figure 6
Potentiodynamic plots of the SS plate and sprayed coatings in pH 4 solution after 312 h of exposure (1 mV/s).3b2:?A3B2 tlsb -.1ptThe current density of the SS plate is lower than that of Ti and SS coatings during anodic scanning. The interesting observation is found in case of Ti and SS coatings that there is a gradual increase in anodic current density during anodic scanning. It may be due to corrosion or transformation of unstable oxide films of these samples, and whatever corrosion 3b2:?thyc=10products/passive 3b2:?thycfilm formed was deposited on the surface.The anodic and passive corrosion current of the Ti coating was lower than that of the SS coating, which means that, in this case, the former is more likely to form compact, protective, and adherent passive oxide films [70,[tnq_nbsp] 71].3b2:?A3B2 tlsb -.1ptThe passive film of 3b2:?thyc=10Ti-sprayed3b2:?thyc coating resisted the penetration of corrosive species of the solution; thus, the reducing corrosion rate is observed. During cathodic scanning, all samples exhibited hydrogen evolution reaction which dominated over the oxygen reduction reaction [72].The electrochemical parameters were extracted after fitting of potentiodynamic plots to the Tafel region using the Stern–3b2:?show $132#Geary equation3b2:?A3B2 tpb -1.2pt(2)Icorr=BRtotal.The Stern–3b2:?show $132#Geary constant (B) can be calculated by putting the values for corrosion current density (Icorr) and total polarization resistance (Rtotal) in (2). The extracted data on the corrosion potential (Ecorr), Icorr, Rtotal, B, and the corrosion rate of samples after 312 h of exposure to pH 4 solution are shown in Table[tnq_nbsp] 3b2:?show [?"tbl2",5,"cond1"]2.Table 2
Electrochemical parameters extracted after fitting of potentiodynamic plots to the Tafel region.
Sample ID
Electrochemical parameters
E
corr (V) versus Ag/AgCl
I
corr (µA·cm−2)
R
total (kΩ·cm2)
B (V)
Corrosion rate (µm·y−1)
3b2:?show "^tbl_width"$01649v
SS plate
0.138
0.382
379.860
0.15
5.132
SS sprayed
−0.594
19.803
33.792
0.67
266.043
Ti sprayed
−0.403
1.913
68.464
0.13
42.703TheEcorr of the SS plate and SS and Ti coatings are 0.138, −0.594, and −0.403 V versus 3b2:?thyc=10Ag/AgCl, 3b2:?thycrespectively. The SS plate exhibited nobler Ecorr than the Ti coating followed by the SS coating.The nobler potential of the SS plate is due to formation of the3b2:?thyc=10Cr-enriched3b2:?thyc oxide film, whereas others exhibited the active potential. The active Ecorr of SS- and 3b2:?thyc=10Ti-sprayed3b2:?thyc coatings compared to the SS plate is attributed to the presence of defects on the coating surface.Lai et[tnq_nbsp] al. observed that when SS was exposed to H2SO4-contaminated water solution, it formed NiO, Fe2O3, FeO, and Cr2O3 thin films which were protective in nature and noble [42]. The active potential of SS coating was due to the presence of defective or porous 3b2:?thyc=10oxide/corrosion 3b2:?thycfilms that made the sample more susceptible to corrosion and exhibit the mixed potential [73].The studied pH solution was acidic and led to the[tnq_nbsp] deterioration of the samples. During exposure, the formed corrosion products deposited on the sample surface. The corrosion products blocked the 3b2:?thyc=10defects/pores 3b2:?thycof the samples and resisted the penetration of the solution [74,[tnq_nbsp] 75].The iron oxides were more active and therefore exhibited the active potential. The Ti coating exhibited a nobler potential than SS coating because it had only fine and elongated cracks (Figure[tnq_nbsp] 2(c)), which stifled the aggressive species of the solution from reaching the base metal. In contrast, the SS coating contains many connected pores and valley morphology where the acidic solution can accumulate and induce crevice corrosion.These results indicate that the passive film formed on the3b2:?thyc=10Ti-sprayed3b2:?thyc coating after exposure to pH 4 solution is protective, nonporous, compact, and resistant to the penetration of aggressive ions in the solution. The SS coating has porous and nonprotective corrosion 3b2:?thyc=10products/iron 3b2:?thycoxides.TheRtotal values of the SS plate, 3b2:?thyc=10SS-sprayed3b2:?thyc coating, and 3b2:?thyc=10Ti-sprayed3b2:?thyc coating are 379.860, 33.792, and 68.464 kΩ·cm2, respectively. The higher Rtotal value of Ti coating compared to the SS coating suggests that it can be used as a coating to protect the materials in H2SO4-contaminated water solution, even at low pH. The B values were calculated by using (2), and it was found that the SS plate and 3b2:?thyc=10Ti-sprayed3b2:?thyc coating were identical and less active, while 3b2:?thyc=10SS-sprayed3b2:?thyc coating showed 0.67 V which is more pronounced to corrosion [76]. The B value of the SS plate and 3b2:?thyc=10Ti-sprayed3b2:?thyc coating is under 3b2:?A3B2 tlsb -.1ptthe active control, while the 3b2:?thyc=10SS-sprayed3b2:?thyc surface exhibits active dissolution values which influence the corrosion phenomena. The calculated Icorr value of 3b2:?thyc=10SS-sprayed3b2:?thyc samples reveals the 3b2:?A3B2 tlsb .1ptactiveness of coating, while the SS plate and 3b2:?thyc=10Ti-sprayed3b2:?thyc coating control the corrosion process in acidic solution at longer duration of exposure.The corrosion rate (µm·y−1) was calculated by the following equation [77]:3b2:?A3B2 tpb .7pt(3)Corrosion  rateµm⋅y−1=3.27×Icorr×EWd.The corrosion rate in (3) is expressed in micrometres per year (µ3b2:?thyc=10m/year) 3b2:?thycand Icorr in µA·cm−2. The Icorr was obtained by dividing the total surface area of the working electrode under the corrosion current (µA). EW represents the equivalent weight (g·mol−1), and d is the density (g·cm−3).The corrosion rate of the SS coating is 266.043µm·y−1 and is greater than that of the SS plate and Ti coating by 51. 84 and 6.23 times, respectively. This result indicates that the SS is not an effective coating material for deposition by the arc thermal spray process in pH 4 solution and long duration of exposure.The corrosion rate data of the SS coating revealed that it totally3b2:?thyc=10dissolved/corroded 3b2:?thycdown to the base substrate. The initial coating thickness was 200 µm, while the corrosion rate was 266.043 µm·y−1. Thus, it may be reported that the Ti coating was effective in protecting the surface than the SS coating. The Ti can be used as a coating material to protect the waste water reservoir and extend its service life.


## 3.4.3. Characterization of Corrosion Products after Potentiodynamic Studies in pH 4 by Different Techniques

3b2:?A3B2 tlsb .1ptThe morphology of corrosion products was examined by SEM, and results are shown in Figure[tnq_nbsp] 3b2:?show [?"gr7",5,"cond1"]7. On the SS plate surface, the passive film was adherent, uniform, and regularly deposited, thus preventing the penetration of solution 3b2:?A3B2 tlsb -.1pt(Figure[tnq_nbsp] 7(a)). The edges of the surface show few cracks caused 3b2:?A3B2 tlsb .1ptby the destructive potentiodynamic experiment, and the passive film prevented the cracking. After potentiodynamic studies, the SS plate surface did not show any other type of corrosion products/rust.3b2:?tex type="figure" layout="twocols" n="2,3"Figure 7
SEM images of corrosion products formed on the (a) SS plate, (b) SS-sprayed coating, and (c) Ti-sprayed coating after potentiodynamic studies in pH 4 solution.
(a)
(b)
(b)3b2:?A3B2 tlsb -.1ptThe SS coating exhibited different sizes of corrosion product morphology with micropore formation (Figure[tnq_nbsp] 7(b)). 3b2:?A3B2 tlsb .2ptThe 3b2:?thyc=10net-like3b2:?thyc microstructure of corrosion products is attributed to the presence of porous iron oxides. Through the net and thread morphologies, the acidic solution easily penetrated the substrate and formed corrosion products.The morphology of corrosion products formed on the Ti coating was totally different from that on the SS plate and sprayed coating. The passive films formed on the3b2:?thyc=10Ti-sprayed3b2:?thyc surface exhibit microcracks, plate, and globular morphology (Figure[tnq_nbsp] 7(c)). The globular particles block the micro- and macrocracks on the top surface. Therefore, enhanced corrosion resistance was observed after 312 h of exposure than on 3b2:?thyc=10SS-sprayed3b2:?thyc coating.Passive oxide films of Ti coating contain3b2:?thyc=10plate-like3b2:?thyc microstructures that were uniformly deposited on the surface. Similar morphologies were not observed in the corrosion products of the SS plate and sprayed coating.The phases present in the corrosion products of all samples after potentiodynamic studies were studied by XRD. The identification of phases in corrosion products is shown in Figure[tnq_nbsp] 3b2:?show [?"gr8",5,"cond1"]8. The SS plate exhibits the presence of tetrataenite (FeNi) and Fe. It is reported that FeNi is unstable and can deteriorate into other forms, if it exposes for long term to 3b2:?thyc=10low-temperature3b2:?thyc environments [78].Figure 8
XRD of the SS plate and sprayed coatings after potentiodynamic studies in pH 4 solution (0.5°/min).3b2:?A3B2 tlsb -.1ptThe presence of lepidocrocite (γ-FeOOH) in the corrosion products of 3b2:?thyc=10SS-sprayed3b2:?thyc coatings confirmed that this coating was susceptible to corrosion in acidic solution. However, Ti coating exhibits composite oxides along with Ti and TiO. Therefore, the improved corrosion resistance of 3b2:?thyc=10Ti-sprayed3b2:?thyc coating is observed by formation of TiO2 (rutile and anatase), and this observation corroborates with EIS and potentiodynamic results. The passive oxides of Ti such as TiO2 and Ti3O5 have formed. The TiO2 is thermodynamically more stable than others [79]. Therefore, the 3b2:?thyc=10Ti-sprayed3b2:?thyc coating is attributed to improved corrosion resistance properties of coating in H2SO4 solution. The transformation of Ti into TiO2 in the H2SO4 environment is well documented elsewhere [80,[tnq_nbsp] 81]. TiO3 and some amount of TiO (Figure[tnq_nbsp] 3) may be transformed into TiO2 and Ti3O5 due to a strong oxidizing ability of H2SO4 solution. Thus, corrosion 3b2:?thyc=10products/passive 3b2:?thycfilm of 3b2:?thyc=10Ti-sprayed3b2:?thyc coating exhibits some peaks of TiO (Figure[tnq_nbsp] 8). Therefore, corrosion is observed after 312 h of exposure to H2SO4 solution. Once proper transformation 3b2:?A3B2 tlsb -.1ptof Ti and TiO into the stable form occurred, then the corrosion rate would be completely suppressed.


## 4. Conclusions

From the above results and discussion, the following can be concluded:(1)
The EIS and potentiodynamic studies revealed the protective properties of Ti coating due to formation of the protective oxide film at longer duration of exposure to acidic solution.(2)
The improved corrosion resistance properties of3b2:?thyc=10Ti-sprayed3b2:?thyc coating than 3b2:?thyc=10SS-sprayed3b2:?thyc coating after 312 h of exposure to acidic solution is attributed to transformation of unstable oxides into stable, protective, and adherent TiO2 (rutile and anatase) which is a thermodynamically stable oxide.(3)
3b2:?A3B2 tlsb -.1ptExamination of the corrosion product morphology by SEM revealed the compact, globular, and crystalline corrosion 3b2:?thyc=10products/oxide 3b2:?thycfilms on the Ti sample, while the SS sample formed defective and 3b2:?thyc=10microcrack-bearing3b2:?thyc corrosion products.(4)
The SS plate showed uniform,3b2:?thyc=10crack-free3b2:?thyc passive films, with no trace of corrosion products after 312 h of exposure to acidic solution.



---

*Source: 4050175-2018-04-23.xml*