# Effect of Diacetyl Monoxime Thiosemicarbazone on the Corrosion of Aged 18 Ni 250 Grade Maraging Steel in Sulphuric Acid Solution

Authors: T. Poornima; Jagannath Nayak; A. Nityananda Shetty
Journal: Journal of Metallurgy
 (2012)

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## Abstract

The corrosion inhibition of the aged 18 Ni 250 grade maraging steel in 0.5 M sulphuric acid by diacetyl monoxime thiosemicarbazone (DAMTSC) at 303–323 K has been investigated by potentiodynamic polarization, EIS, and SEM techniques. Good inhibition efficiency of DAMTSC was revealed even at low concentrations, which increased with the increase in DAMTSC concentration and decreased with the increase in temperature. The activation energies,Ea, as well as other thermodynamic parameters (ΔGads0; ΔHads0; ΔGads0), were evaluated and discussed. The adsorption of DAMTSC on the aged maraging steel surface was found to obey the Langmuir adsorption isotherm model and shows mixed type inhibition behavior.


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## Body

## 1. Introduction

Corrosion of structural elements is a major issue for any industry because of the chemical environment of chemical processing. Maraging steels are special class of ultrahigh strength steels that differ from conventional steels in that they are hardened by a metallurgical reaction that does not involve carbon [1]. They derive high strength from age hardening of low carbon, Fe-Ni martensitic matrix [2]. The need for highly reliable substances of high strength and high ductility is gradually increasing with the development of aerospace industry. According to available literature, atmospheric exposure of 18 Ni maraging steel leads to corrosion in a uniform manner and the steel becomes completely rust covered [3]. Pit depths tend to be shallower than high strength steels [4]. Critical and passive current densities increase as the structure is varied from fully annealed to fully aged [5]. Maraging steels are found to be less susceptible to hydrogen embrittlement than common high strength steels owing to significantly low diffusion of hydrogen in them [6]. Several technical papers covering alloy design, material processing, thermo-mechanical treatments, welding, strengthening mechanisms, and so forth, have been published [7]. Search in the literature reveals very little work being reported on the corrosion behavior and on the use of inhibitors in controlling corrosion of maraging steel except some of our publications [8, 9]. Metals and alloys frequently come in contact with acid solutions during cleaning, pickling, descaling, acidising, and so forth. Materials used in acid environment should have good corrosion resistance. The present work is intended to study the corrosion inhibition of aged 18 Ni 250 grade maraging steel in 0.5 M sulphuric acid medium using DAMTSC as inhibitor.The majority of the well known inhibitors are organic compounds containing heteroatom, such as O, N, or S and multiple bonds or aromatic rings, which allow an adsorption on the metal surface through lone pair of electrons and/or pi electrons present in these molecules [10, 11]. Sulphur containing compounds are preferred for H2SO4 while those containing nitrogen act more effectively in HCl [12]. The inhibition efficiency should increase in the order O < N < S < P [13]. The molecules that contain both nitrogen and sulphur in their structures are of particular importance, since these provide an excellent inhibition compared with the compounds that contain only sulphur or nitrogen [14]. Thiosemicarbazones and their derivatives have continued to be subject of extensive investigation in chemistry and biology owing to their broad spectrum of antitumor [15], antimalarial [16], antiviral [17], antibacterial [18], antifungal [19], and many other applications including corrosion inhibition of metals [20–23].


## 2. Experimental Methods

### 2.1. Materials

The maraging steel samples (M 250 grade) in aged condition were taken from plates. The solution annealed and air cooled plates were subjected to aging treatment at 480 ± 5°C for 3 hours and air cooled. Percentage composition of 18 Ni 250 grade aged maraging steel samples is given in Table1. Cylindrical test coupons were cut from the plate and sealed with epoxy resin in such a way that, the area exposed to the medium is 0.503 cm2. These coupons were polished as per standard metallographic practice, belt grinding followed by polishing on emery papers, finally on polishing wheel using legated alumina to obtain mirror finish, degreased with acetone, washed with double distilled water, and dried before immersing in the corrosion medium. The inhibitor diacetyl monoxime thiosemicarbazone (DAMTSC) was synthesized as per the reported procedure [24] by reaction between diacetyl monoxime and thiosemicarbazide and its structure is given in Scheme 1.Table 1
Composition of the aged maraging steel specimen.
Element
Composition
C
0.015%
Ni
17–19%
Mo
4.6–5.2%
Co
7–8.5%
Si
0.1%
O
30 ppm
H
2.0 ppm
Ti
0.3–0.6%
Al
0.005–0.15%
Mn
0.1%
P
0.01%
S
0.01%
N
30 ppm
Fe
BalanceScheme 1


### 2.2. Medium

Standard 0.5 M sulphuric acid solution was prepared by diluting analar grade 98% sulphuric acid by using double distilled water. The solutions of inhibitor with 1.14 × 10−4 M, 2.85 × 10−4 M, 5.70 × 10−4 M and 8.55 × 10−4 M concentration were prepared in standard 0.5 M sulphuric acid. Experiments were carried out using calibrated thermostat at temperatures 303 K, 308 K, 313 K, 318 K, 323 K (±0.5 K).


### 2.3. Electrochemical Measurement

#### 2.3.1. Tafel Polarisation Studies

Electrochemical measurements were carried out by using an electrochemical work station, Auto Lab 30 and GPES software. Tafel plot measurements were carried out using conventional three electrode Pyrex glass cell with platinum counter electrode and saturated calomel electrode (SCE) as reference electrode. All the values of potential are referred to the SCE. Finely polished maraging steel specimens were exposed to corrosion medium of sulphuric acid in the presence and absence of inhibitor at different temperatures and allowed to establish a steady state open circuit potential. The potentiodynamic current-potential curves were recorded by polarizing the specimen to −250 mV cathodically and +250 mV anodically with respect to open circuit potential (OCP) at scan rate of 1 mV s−1.


#### 2.3.2. Electrochemical Impedance Spectroscopy Studies (EIS)

The corrosion behavior of the maraging steel specimen was also obtained from EIS technique using electrochemical work station, Auto Lab 30, and FRA software. In EIS technique a small amplitude ac signal of 10 mV and frequency spectrum from 100 kHz to 0.01 Hz was impressed at the OCP and impedance data were analyzed using Nyquist plots. The charge transfer resistance,Rct was extracted from the diameter of the semicircle in Nyquist plot.In all the above measurements, at least three similar results were considered and their average values are reported.


#### 2.3.3. Scanning Electron Microscopy (SEM) Analysis

The SEM images were recorded using JEOL JSM-6380 LA analytical scanning electron microscope.


## 2.1. Materials

The maraging steel samples (M 250 grade) in aged condition were taken from plates. The solution annealed and air cooled plates were subjected to aging treatment at 480 ± 5°C for 3 hours and air cooled. Percentage composition of 18 Ni 250 grade aged maraging steel samples is given in Table1. Cylindrical test coupons were cut from the plate and sealed with epoxy resin in such a way that, the area exposed to the medium is 0.503 cm2. These coupons were polished as per standard metallographic practice, belt grinding followed by polishing on emery papers, finally on polishing wheel using legated alumina to obtain mirror finish, degreased with acetone, washed with double distilled water, and dried before immersing in the corrosion medium. The inhibitor diacetyl monoxime thiosemicarbazone (DAMTSC) was synthesized as per the reported procedure [24] by reaction between diacetyl monoxime and thiosemicarbazide and its structure is given in Scheme 1.Table 1
Composition of the aged maraging steel specimen.
Element
Composition
C
0.015%
Ni
17–19%
Mo
4.6–5.2%
Co
7–8.5%
Si
0.1%
O
30 ppm
H
2.0 ppm
Ti
0.3–0.6%
Al
0.005–0.15%
Mn
0.1%
P
0.01%
S
0.01%
N
30 ppm
Fe
BalanceScheme 1


## 2.2. Medium

Standard 0.5 M sulphuric acid solution was prepared by diluting analar grade 98% sulphuric acid by using double distilled water. The solutions of inhibitor with 1.14 × 10−4 M, 2.85 × 10−4 M, 5.70 × 10−4 M and 8.55 × 10−4 M concentration were prepared in standard 0.5 M sulphuric acid. Experiments were carried out using calibrated thermostat at temperatures 303 K, 308 K, 313 K, 318 K, 323 K (±0.5 K).


## 2.3. Electrochemical Measurement

### 2.3.1. Tafel Polarisation Studies

Electrochemical measurements were carried out by using an electrochemical work station, Auto Lab 30 and GPES software. Tafel plot measurements were carried out using conventional three electrode Pyrex glass cell with platinum counter electrode and saturated calomel electrode (SCE) as reference electrode. All the values of potential are referred to the SCE. Finely polished maraging steel specimens were exposed to corrosion medium of sulphuric acid in the presence and absence of inhibitor at different temperatures and allowed to establish a steady state open circuit potential. The potentiodynamic current-potential curves were recorded by polarizing the specimen to −250 mV cathodically and +250 mV anodically with respect to open circuit potential (OCP) at scan rate of 1 mV s−1.


### 2.3.2. Electrochemical Impedance Spectroscopy Studies (EIS)

The corrosion behavior of the maraging steel specimen was also obtained from EIS technique using electrochemical work station, Auto Lab 30, and FRA software. In EIS technique a small amplitude ac signal of 10 mV and frequency spectrum from 100 kHz to 0.01 Hz was impressed at the OCP and impedance data were analyzed using Nyquist plots. The charge transfer resistance,Rct was extracted from the diameter of the semicircle in Nyquist plot.In all the above measurements, at least three similar results were considered and their average values are reported.


### 2.3.3. Scanning Electron Microscopy (SEM) Analysis

The SEM images were recorded using JEOL JSM-6380 LA analytical scanning electron microscope.


## 2.3.1. Tafel Polarisation Studies

Electrochemical measurements were carried out by using an electrochemical work station, Auto Lab 30 and GPES software. Tafel plot measurements were carried out using conventional three electrode Pyrex glass cell with platinum counter electrode and saturated calomel electrode (SCE) as reference electrode. All the values of potential are referred to the SCE. Finely polished maraging steel specimens were exposed to corrosion medium of sulphuric acid in the presence and absence of inhibitor at different temperatures and allowed to establish a steady state open circuit potential. The potentiodynamic current-potential curves were recorded by polarizing the specimen to −250 mV cathodically and +250 mV anodically with respect to open circuit potential (OCP) at scan rate of 1 mV s−1.


## 2.3.2. Electrochemical Impedance Spectroscopy Studies (EIS)

The corrosion behavior of the maraging steel specimen was also obtained from EIS technique using electrochemical work station, Auto Lab 30, and FRA software. In EIS technique a small amplitude ac signal of 10 mV and frequency spectrum from 100 kHz to 0.01 Hz was impressed at the OCP and impedance data were analyzed using Nyquist plots. The charge transfer resistance,Rct was extracted from the diameter of the semicircle in Nyquist plot.In all the above measurements, at least three similar results were considered and their average values are reported.


## 2.3.3. Scanning Electron Microscopy (SEM) Analysis

The SEM images were recorded using JEOL JSM-6380 LA analytical scanning electron microscope.


## 3. Results and Discussion

### 3.1. Tafel Polarization Measurement

The corrosion inhibition of aged maraging steel investigated in 0.5 M sulphuric acid containing various concentration of DAMTSC inhibitor at different temperatures using Tafel polarization technique and Tafel plots at 303 K are shown in Figure1. The valuable potentiodynamic polarization parameters including corrosion potential (Ecorr), corrosion current density (Icorr), anodic and cathodic slopes (ba and bc), and corrosion rate (C.R) were calculated from Tafel plots and are summarized in Table 2. The C.R is calculated using
(1)corrosionrate(mm/y)=3270(E.W)IcorrD,
where 3270 is a constant that defines the unit of corrosion rate, Icorr=corrosion current density in A/cm2, D=density of the corroding material, 8.23 g/cm3, E.W = equivalent weight of corroding material (atomic weight/oxidation number), 29.2 [25]. The percentage inhibition efficiency (IE%) was calculated from
(2)IE%=Icorr-Icorr(inh)Icorr×100,
where Icorr and Icorr(inh) are corrosion current densities in the absence and in the presence of inhibitor, respectively. The value of corrosion current density (Icorr) decreased effectively even with low concentration of DAMTSC and inhibition efficiency increases with increase inhibitor concentration. The presence of inhibitor does not shift the Ecorr remarkably, implying that DAMTSC acts as mixed type inhibitor affecting both anodic and cathodic reactions [23]. According to Li and others [26], if the displacement in corrosion potential is more than ±85 mV with respect to corrosion potential of the blank, the inhibitor can be considered as a cathodic or anodic type. But the maximum displacement in this study is less than 10 mV, which indicates that DAMTSC is a mixed type inhibitor. According to Cao [27] if the shift in Ecorr is negligible the inhibition is most probably caused by a geometric blocking effect of the adsorbed inhibitive species on the surface of corroding metal. The anodic and cathodic Tafel slopes remain almost unchanged upon addition of inhibitor. Thus the adsorbed inhibitor acts by simple blocking of active sites for anodic and cathodic processes, thereby reducing the surface area for corrosion without affecting the corrosion mechanism [28, 29]. The parallel cathodic Tafel curves suggest that the hydrogen evolution is activation controlled and the reduction mechanism is not affected by the presence of the inhibitors [30].Table 2
Results of  Tafel polarization studies on aged maraging steel in 0.5 M sulphuric acid with and without different concentrations of DAMTSC at various temperatures.
Temp (K)
Conc. of inhibitor        (×10−4) (M)
E
corr (V)
I
corr  × (10−4)   (A cm−2)
b
c (V dec−1)
b
a  (V dec−1)
C.R    in mm Y−1
IE (%)
303
0.00
−0.310
75.26
0.107
0.103
87.30
1.14
−0.310
27.18
0.103
0.102
31.53
63.9
2.85
−0.306
11.72
0.095
0.106
13.60
84.4
5.70
−0.306
9.57
0.106
0.113
11.10
87.3
8.55
−0.303
7.73
0.093
0.111
8.97
89.7
308
0.00
−0.312
85.43
0.122
0.127
99.10
1.14
−0.310
32.07
0.127
0.098
37.20
62.5
2.85
−0.309
21.64
0.098
0.112
25.10
74.7
5.70
−0.304
13.19
0.112
0.115
15.30
84.6
8.55
−0.307
10.99
0.115
0.118
12.75
87.1
313
0.00
−0.309
96.55
0.118
0.112
112.00
1.14
−0.309
37.67
0.112
0.086
43.70
61.0
2.85
−0.307
26.21
0.086
0.084
30.40
72.9
5.70
−0.308
17.76
0.084
0.106
20.60
81.6
8.55
−0.304
14.40
0.106
0.086
16.70
85.1
318
0.00
−0.312
106.90
0.086
0.117
124.00
1.14
−0.305
50.95
0.117
0.098
59.10
56.5
2.85
−0.305
36.81
0.098
0.095
42.70
68.6
5.70
−0.308
24.22
0.095
0.089
28.10
79.3
8.55
−0.301
19.05
0.089
0.098
22.10
83.8
323
0.00
−0.311
117.24
0.098
0.123
136.00
1.14
−0.303
58.97
0.123
0.095
68.40
49.7
2.85
−0.305
42.84
0.095
0.091
49.70
63.5
5.70
−0.303
30.09
0.091
0.088
34.90
74.3
8.55
−0.301
25.95
0.088
0.088
30.10
77.9Figure 1
Tafel polarization curves for the aged maraging steel in 0.5 M sulphuric acid containing different concentrations inhibitor.For anodic polarization curves of specimen with DAMTSC (in Figure2), it seems that at the working electrode potential, higher than −200 mV/SCE, the presence of inhibitor does not change the current versus potential characteristics. At this potential, the anodic current increases steeply and a flat region is observed. This potential can be defined as desorption potential [31]. The phenomenon may be due to the obvious metal dissolution, which leads to desorption of the inhibitor molecule from the electrode surface. In this case, the desorption rate of the inhibitor is higher than its adsorption rate, so the corrosion current increases more obviously with rising potential [32]. At still higher polarization potential the anodic current density change drastically, resulting in sharp increase in Tafel slope. The Tafel slope of the third portion is nearly equal to that in the absence of DAMTSC. Above desorption potential coverage of inhibitor decreases rapidly. Similar conclusion was reported by Qu et al. [33].Nyquist plots for the aged maraging steel specimen in 0.5 M sulphuric acid (a) containing different concentrations of inhibitor, (b) without inhibitor.
(a)
(b)


### 3.2. Electrochemical Impedance Spectroscopy

The results of potentiodynamic polarization experiments were compared with the results of impedance measurements, since EIS has emerged as probably the most powerful technique currently available for identifying corrosion reaction mechanisms. In order to get more information about the corrosion inhibition of aged maraging steel specimens in 0.5 M sulphuric acid containing different concentrations of DAMTSC, EIS measurements were carried out at different temperatures and they are displayed as Nyquists plots. The Nyquist plots obtained for aged samples of maraging steel specimens in 0.5 M sulphuric acid with and without various concentrations of DAMTSC are as shown in Figure2.It is clear from the figures that the shapes of the impedance plots for inhibited specimen are different from those of uninhibited one. The Nyquists plots of aged maraging steel in 0.5 M H2SO4 without inhibitor are characterized by two time constants with a depressed capacitive semicircle at high frequency region (HF) followed by an inductive loop at low frequency (LF) region. These are not perfect semicircles, because the Nyquist plots obtained in the real system represent a general behavior where the double layer at the metal solution interface does not behave as an ideal capacitor [34]. The depressed capacitive loop with its centre below the real axis often refers to the frequency dispersion of interfacial impedance which has been attributed to the roughness, inhomogeneity of the solid surfaces, and adsorption of inhibitors [10]. The HF capacitive loop was attributed to charge transfer of the corrosion process and time constant of the electric double layer [35]. The LF inductive loop can be attributed to relaxation process obtained by adsorbed sulphate ions and protons [36]. It may also be attributed to the redissolution of the passivated surface at low frequencies [37]. In the present case, the low frequency inductive loop in inhibitor free acid medium can be attributed to surface dissolution process at low frequencies. Based on the shape of Nyquist plots we have decided to use equivalent circuit as given in Figure 3(a) which has been previously used to model iron/acid interface [38]. It consists of solution resistance (Rs), charge transfer resistance (Rct), and constant phase element (CPE). The surface roughness, degree of polycrystallinity, and also anion adsorption lead to capacitance dispersion at solid electrodes and hence the real iron/acid interface system deviates from ideal capacitive behavior, which can be empirically represented by CPE [39]. The point of intersection between inductive loop and the real axis represents (Rs+Rct). Rs represents solution resistance due to the ohmic resistances of corrosion product films and the solution enclosed between the working electrode and the reference electrode. Rct represents the charge transfer resistance whose value is a measure of electron transfer across the surface and is inversely proportional to corrosion rate [40].Equivalent circuit used to fit experimental EIS data for the corrosion of aged maraging steel specimen in sulphuric acid (a) without inhibitor, (b) with inhibitor.
(a)
(b)The Nyquists plots of aged maraging steel in 0.5 M H2SO4 containing various concentrations of DAMTSC consists of two semicircles at high and middle frequency range and an inductive loop at low frequency range. These plots are analysed by an equivalent circuit, which is represented in Figure 3(b). The smaller HF capacitive loop suggested that some sort of film exists on this alloy in the studied solution which can be attributed to film formation by adsorbed inhibitor while larger middle frequency capacitive loop can be attributed to charge transfer reaction [41–44]. The resistance values corresponding to high frequency and middle frequency semicircles are represented as Rf and Rct, respectively, in equivalent circuit Figure 3(b). Both Rf and Rct increase with increase in inhibitor concentration. The sum of Rf and Rct are taken for calculation of inhibition efficiency. The capacitance of film (Cf) was represented by CPE1 and double layer capacitance (Cdl) was represented as CPE2. All these values are shown in Table 3. The low frequency inductive loop in inhibited acid solution might be attributed to the surface dissolution, indicating that the alloy still dissolved by the direct charge transfer at the inhibitor adsorbed steel surface [45]. The fact that this inductive loop cannot be observed at maximum concentration of DAMTSC supports this view. The study at higher temperature reveals single semicircle at low concentrations of inhibitor and appearance of two semicircles only at high concentration of inhibitor, this can be attributed to the fact that formation of inhibitor film is favorable only at higher inhibitor concentration above 303 K.Table 3
EIS data of aged maraging steel in 0.5 M sulphuric acid with and without different concentrations of DAMTSC at various temperatures.
Temp. (K)
Conc. of inhibitor(×10−4) (M)
R
f Ohm·cm2
R
ct Ohm·cm2
R
total Ohm·cm2
C
f (10−6) (F)
C
dl (10−6) (F)
IE (%)
303
0.00
2.8
2.80
14490
1.14
1.17
5.66
6.83
874
9920
59.0
2.85
5.37
9.38
14.75
799
9762
81.0
5.70
6.12
13.5
19.62
653
1590
85.7
8.55
7.89
17.8
25.70
509
1163
89.1
308
0.00
2.30
2.30
87620
1.14
5.10
5.10
10112
54.9
2.85
8.87
8.87
7566
74.2
5.70
5.6
11.2
16.8
954
6347
86.3
8.55
7.19
11.6
18.7
765
4850
87.7
313
0.00
1.6
1.60
113500
1.14
4.32
3.32
23724
51.8
2.85
5.75
5.75
20880
72.2
5.70
3.43
7.34
10.8
2417
11340
85.2
8.55
4.2
7.9
12.1
1450
10800
86.7
318
0.00
1.4
1.4
226570
1.14
2.7
2.7
31326
48.1
2.85
4.8
4.8
27667
70.8
5.70
3.25
5.05
8.3
2625
23471
83.1
8.55
3.40
5.3
8.7
1562
20125
83.9
323
0.00
1.3
1.3
541000
1.14
2.3
2.3
33540
43.5
2.85
3.5
3.5
24500
62.9
5.70
1.44
4.16
5.6
2966
20760
76.8
8.55
2.20
4.90
7.1
1695
15600
81.7The impedance of CPE is expressed as(3)ZCPE=1Y0(jw)n,
where the amplitude Y0 and n are frequency independent, and ω is the angular frequency for which -Z′′ reaches its maximum values, n is dependent on the surface morphology: -1≤n≤1. The HF loops have depressed semicircular appearance, with 0.5≤n≤1, which is often referred to as frequency dispersion as a result of the non homogeneity or roughness [46].The double layer capacitancesCdl, for a circuit including CPE were calculated from the following equation [47]:
(4)Cdl=Y0(wmax)n-1,
where wmax=2πfmax, and fmax is the frequency at which the imaginary component of the impedance is maximal. According to the expression of the double layer capacitance presented in the Helmholtz model [48],
(5)Cdl=εε0dS,
where d is the thickness of the film, S is the surface area of the electrode, ε0 is the permittivity of air, and ε is the local dielectric constant. The value of Cdl decreases due to adsorption of inhibitor molecules, which displaces water molecules originally adsorbed on the mild steel surface and decreases the active surface area. The values of double layer capacitance decreases with increase in inhibitor concentration indicating that inhibitor molecules function by adsorption at the metal/solution interface, leading to protective film on the alloy surface, and decreasing the extent of dissolution reaction [49].The corrosion current densityIcorr can be calculated using charge transfer resistance Rct, together with Stern-Geary equation [40]:
(6)Icorr=babc2.303RctA(ba+bc).
The charge transfer resistance Rct and double layer capacitance Cdl were determined by the analysis of Nyquist plot and their values are given in Table 3. The inhibition efficiency in different concentrations of DAMTSC was calculated from the charge transfer resistance according to
(7)IE%=(Rct-Rct0Rct)×100,
where Rct and Rct0 represent the charge transfer resistance in the presence and absence of inhibitor. The values of Rct increases with increase in inhibitor concentration and the results indicate that charge transfer process mainly controls the corrosion process. The inhibition efficiency values obtained by the electron impedance method are in good agreement with the ones obtained by Tafel’s method.


### 3.3. Effect of Temperature

The effect of temperature on the inhibited acid-metal reaction is highly complex because many changes occur on the metal surface, such as rapid etching and desorption of the inhibitor and the inhibitor itself, and in some cases may undergo decomposition and/or rearrangement [10]. However it facilitates the calculation of many thermodynamic functions for the inhibition and/or the adsorption processes which contribute in determining the type of adsorption of the studied inhibitors. In the present study, with increase in solution temperature, corrosion potential (Ecorr), anodic Tafel slope (ba), and cathodic Tafel slope (bc) values are not affected much. This indicates that increase in temperature does not change the mechanism of corrosion reaction. But Icorr and hence the corrosion rate of the specimen increases with increase in temperature for both blank and inhibited solutions. This may be attributed to the fact that the hydrogen evolution overpotential decreases with increase in temperature that leads to increase in cathodic reaction rate [50]. The inhibition efficiency decreases with increase in temperature which indicates desorption of inhibitor molecules [51]. This is suggestive of physisorption [52]. But this decrease is small at higher concentration of inhibitor.The apparent activation energy (Ea) for the corrosion process in the presence and absence of inhibitor can be calculated using Arrhenius law equation [10],
(8)ln(C.R)=B-(EaRT),
where B is a constant which depends on the metal type and R is the universal gas constant. The plot of ln(corrosionrate) versus reciprocal of absolute temperature 1/T gives straight line whose slope=-Ea/R gives activation energy for the corrosion process. The Arrhenius plots for the specimen are shown in Figure 4.Figure 4
Arrhenious plots for the dissolution of aged maraging steel in 0.5 M sulphuric acid containing different concentrations of inhibitor.The entropy and enthalpy of activation for the dissolution of alloy,ΔHa and ΔSa, were calculated from transition state theory equation [10]
(9)C.R=(RTNh)exp(ΔSaR)exp(-ΔHaRT),
where h is Plank’s constant, N is Avagadro’s number. A plot of ln(corrosionrate/T) versus 1/T gives straight line with slope=-ΔHa/R and intercept=ln(R/Nh)+ΔSa/R. The calculated values of Ea, ΔHa, and ΔSa  are given in Table 4. The plot of ln(corrosionrate/T) versus 1/T for aged samples of maraging steel is shown in Figure 5.Table 4
Activation parameters for the corrosion of aged maraging steel in 0.5 M sulphuric acid with and without different concentrations of DAMTSC.
Conc. of inhibitor(×10−4) (M)
E
a kJ mol−1
Δ
H
a kJ mol−1
Δ
S
a J mol−1 K−1
0
18.1
15.5
−156.7
1.14
32.7
30.1
−117.3
2.85
47.3
44.6
−77.7
5.70
48.4
45.8
−75.7
8.55
51.1
48.4
−62.5Figure 5
ln
(
corrosion
rate
/
T
) versus 1/T for aged maraging steel in 0.5 M sulphuric acid containing different concentrations of inhibitor.The effect of chemically stable surface active inhibitors is to increase the energy of activation and to decrease the surface area available for corrosion [53]. The value of activation energy (Ea) in 0.5 M sulphuric acid solution containing inhibitor is greater than that without inhibitor. The extent of increase is proportional to the inhibitor concentration, indicating that the energy barrier for the corrosion reaction increases with increase in DAMTSC concentration. The increase in apparent activation energy Ea may be interpreted as due to physical adsorption [54] of the inhibitor, which results in increase in surface coverage with the increase in the concentration of the inhibitor. The decrease in the inhibition efficiency of DAMTSC with increase in temperature can be attributed to an appreciable decrease in the adsorption of the inhibitor on the metal surface with increase in temperature and a corresponding increase in corrosion rate due to the fact that greater area of metal is exposed to the acid [55].The values of entropy of activation in the absence and presence of inhibitor are large and negative. This implies that the activated complex in the rate determining step represents an association rather than dissociation step, resulting in a decrease in randomness on going from reactants to activated complex [56, 57]. The entropy of activation values are more for inhibited solutions than that for the uninhibited solutions. This suggested that an increase in randomness occurred on going from reactants to the activated complex. This might be the results of the adsorption of organic inhibitor molecules from the acidic solution which could be regarded as a quasi-substitution process between the organic compound in the aqueous phase and water molecules at electrode surface [58, 59]. In this situation, the adsorption of organic inhibitor is accompanied by desorption of water molecules from the surface. Thus the increasing in entropy of activation was attributed to the increase in solvent entropy [60].


### 3.4. Adsorption Isotherm

Organic corrosion inhibitors are known to decrease metal dissolution via adsorption on the metal/corrodent interface to form a protective film which separates the metal surface from the corrosive medium. The adsorption route is usually regarded as a substitution process between the organic inhibitor in the aqueous solution[Inh(sol)] and water molecules adsorbed at the metal surface [H2O(ads)] as follows [61]:
(10)Inh(sol)+χH2O(ads)⟷Inh(ads)+χH2O(sol),
where χ represents the number of water molecules replaced by one molecule of adsorbed inhibitor. The adsorption bond strength is dependent on the composition of the metal, corrodent, inhibitor structure, concentration, and orientation as well as temperature. Basic information on the interaction between the inhibitor and alloy surface can be provided by adsorption isotherm. In order to obtain isotherm, the linear relation between surface coverage (θ) value and Cinh must be found. The surface coverage θ is given by
(11)θ=IE%100,
where IE% is percentage inhibition efficiency as calculated using (2).Attempts were made to fit the degree of surface coverage (θ) values to various isotherms including Langmuir, Temkin, Frumkin, and Flory-Huggins isotherms. By far the best fit is obtained with the modified Langmuir adsorption isotherm. Langmuir isotherm for monolayer chemisorption is given by
(12)Cinhθ=1Kads+Cinh,
where Cinh is the concentration of inhibitor, Kads is the equilibrium constant for adsorption process, and θ is the degree of the surface coverage which is calculated using (11).This model has also been used for other inhibitor systems [61]. The plot of Cinh/θ versus Cinh gives a straight line with intercept 1/Kads as shown in Figure 6.Figure 6
Langmuir adsorption isotherm of DAMTSC on aged maraging steel in 0.5 M sulphuric acid at different temperatures.The values of standard free energy of adsorption are related toKads by the relation (13).
(13)Kads=155.5e(-ΔGads0/RT),
where the value 55.5 is the concentration of water in solution in M (mol/L), R is the universal gas constant and T is absolute temperature [61]. The thermodynamic data obtained from adsorption isotherm are tabulated in Table 5. The correlation coefficient (R2) was used to choose the isotherm that best fit experimental data [10]. The linear regression coefficients are close to unity and the slopes of straight lines are nearly unity, suggesting that the adsorption of DAMTSC obeys Langmuir’s adsorption isotherm and there is negligible interaction between the adsorbed molecules [26]. The high values of Kads for the studied inhibitor indicate strong adsorption of inhibitor on the alloy surface. Negative values of ΔGads0 are characteristic feature of strong spontaneous adsorption for the studied compounds, which also reflect the high values of inhibition. The negative ΔGads0 values calculated from (13) are consistent with the spontaneity of the adsorption process and the stability of the adsorbed layer on the maraging steel surface. Generally, the standard free energy values of −20 kJ mol−1 or less negative are associated with an electrostatic interaction between charged molecules and charged metal surface, resulting in physisorption and those of −40 kJ mol−1 or more negative involve charge sharing or transfer from the inhibitor molecules to the metal surface to form a coordinate covalent bond, resulting in chemisorption [62]. The ΔGads0 values obtained for the studied inhibitor on the alloy surface in 0.5 M sulphuric acid are −33 to −35 kJ/mol. These values predict both physical and chemical adsorption of inhibitor [63]. A plot of ΔGads0 versus T was used to calculate heat of adsorption ΔHads0 and the standard adsorption entropy ΔSads0 according to the following thermodynamic basic equation [10]:
(14)ΔGads0=ΔHads0-TΔSads0.
The thermodynamic data obtained for DAMTSC using Langmuir adsorption isotherm are depicted in Table 5.Table 5
Thermodynamic parameters for the adsorption of DAMTSC on aged maraging steel surface in 0.5 M sulphuric acid.
Temperature(°C)
K (mol−1 L)
-
Δ
G
ads
0 kJ mol−1
R
2
Slope
Δ
H
ads
0 kJ mol−1
Δ
S
ads
0 J mol−1 K−1
303
20227
35.1
0.9997
1.05
308
14931
34.3
0.9997
1.07
−62.25
90
313
14391
34.2
0.9997
1.09
318
11246
33.6
0.9995
1.09
323
9476.9
33.2
0.9997
1.14The values of thermodynamic parameters for the adsorption of inhibitors can provide valuable information about the mechanism of corrosion inhibition. While an endothermic adsorption process (ΔHads0>0) is attributed unequivocally to chemisorption [64], an exothermic adsorption process (ΔHads0<0) may involve either physisorption or chemisorption or a mixture of both the processes. In an exothermic process, physisorption is distinguished from chemisorption by considering the absolute value of adsorption enthalpy. Typically, the enthalpy of a physisorption process is below 41.86 kJ mol−1, while that of a chemisorption process approaches 100 kJ mol−1 [65]. In the present case, the calculated value of ΔHads0 is −69.014 kJ mol−1, which is an intermediate case [63], probably involving both physisorption and chemisorption. The ΔSads0 value is large and negative, indicating that decrease in disordering takes place on going from reactant to alloy adsorbed species. This can be attributed to the fact that adsorption is an exothermic process and is always accompanied by decrease in entropy.


### 3.5. Mechanism of Corrosion Inhibition

In order to predict the type of adsorption, the corrosion mechanism of iron must be known. It was suggested that the anodic dissolution of iron in sulphuric acid solutions follow the consecutive or BDD mechanism [66] as reported by [41]. Iron electro dissolution in acidic sulphate solution depends primarily on the adsorbed intermediate FeOHads according to the mechanism reproduced below:
(15)FeH2Oads⟺FeOHads+H++e(16)FeOHads→FeOH++e(17)FeOH++H+⟺Fe2++2e
The following stages are possible for the hydrogen evolution reaction on iron [67]:
(18)Fe+H++e-→FeH(19)FeH+FeH→2Fe+H2(20)FeH+H++e-→Fe+H2
The corrosion rate of iron in H2SO4 solutions is controlled by both hydrogen evolution reaction and iron dissolution.An anodic oxidation mechanism in the presence of inhibitor may be similar to the mechanism discussed by McCafferty et al. [68–70] as reported by [71]. Consider
(21)Fe+H2O⟺Fe·H2Oads(22)Fe·H2Oads+Y⟺FeOH-ads+H++Y(23)Fe·H2Oads+Y⟺FeYads+H2O(24)FeOHads-→FeOHads+e(ratedeterminingstep)(25)FeYads⟺FeYads++e(26)FeOHads+FeYads+⟺FeOH++FeYads(27)FeOH++H+⟺Fe2++H2O
where Y is inhibitor molecule.According to the above mechanism, displacement of some adsorbed water molecules on the metal surface by inhibitor species to yield the adsorbed intermediate FeYads in step (23) reduces the amount of the species FeOHads- available for the rate determining step. The inhibitor used may form dominating adsorbed intermediate which results in inhibitive effects. This conclusion is in line with those of Obot and Obi-Egbedi [72], and Shoesmith [73]. Aged maraging steel results from precipitation of intermetallics. Since these intermetallics have different composition, their electrochemical behavior is expected to be different compared to matrix [74]. Also there will be strain fields around these coherent precipitates as a result of lattice mismatch between the precipitate and the matrix due to the difference in the crystal structure and lattice parameters. These strain fields in combination with the galvanic effect due to the composition difference leads to the enhanced corrosion of aged maraging steel in acid medium. Considering the inhomogeneous nature, the surface of alloy is generally characterized by multiple adsorption sites having activation energies and heats of adsorption. Inhibitor molecules may thus be adsorbed more readily at surface active sites having suitable adsorption enthalpies.The adsorption mechanism for a given inhibitor depends on factors such as, the nature of metal, the corrosive medium, the pH, and the concentration of the inhibitor as well as the functional groups present in its molecule, since different groups are adsorbed to different extents. For instance S-containing substances have been shown to preferentially chemisorb on the surface of iron in acidic medium, whereas N-containing substances tend to favor physisorption [75]. It is reported that protective value of sulphur containing compounds is superior to that of nitrogen containing compounds. This may be due to greater polarizability of sulphur atom and the presence of two electron pairs available for coordination [22].The inhibition effect of DAMTSC in sulphuric acid solution can be explained as follows: DAMTSC might be protonated in the acid solution as follows:(28)DAMTSC+xH+→[DAMTSCHx]x+
Thus in aqueous acidic solution, DAMTSC exists partly in the form of protonated species and partly as neutral molecules. Generally two modes of adsorption could be considered. Taking into consideration of many reported results Hackerman et al. [76] concluded that, the surface charge of iron in sulphuric acid solution at the free corrosion potential is positive. Therefore, the adsorption of protonated species would be through electrostatic attraction between the positively charged molecule and the negatively charged sulphate ions adsorbed on the metal surface via physisorption. In the study of the effect of inorganic anions and organic compounds on corrosion inhibition of mild steel in various acids, Li et al. [77] reported the degree of adsorption of anions was in the order I->Br->Cl->SO42->ClO4-. The sulphate ions with low degree of adsorption could not cover the metal surface fully and therefore there is always possibility of the neutral inhibitor molecules occupying the vacant adsorption sites on the metal surface via the chemisorption mode of protection of corroding surface [78]. The possible bonding sites of the neutral inhibitor are the free electron pairs on sulphur and nitrogen atoms, as well as oxygen, which could interact with vacant d orbital of iron [78].


### 3.6. Scanning Electron Microscope Studies (SEM)

The scanning electron microscope images were recorded to establish the interaction of acid medium with the metal surface. The surface morphology of the aged samples was examined by SEM immediately after the sample is subjected to corrosion tests in H2SO4 medium in the absence and in the presence of inhibitor. The SEM image of corroded aged maraging steel sample in Figure 7(a) shows degradation of alloy in the absence of inhibitor. The attack by H2SO4 is seen to be more at grain boundary since these regions are highly susceptible to corrosion. In aged samples the intermetallic precipitation at grain boundary may be responsible for the higher rate of corrosion. Figure 7(b) represents SEM image of aged maraging steel after the corrosion tests in a medium of sulphuric acid containing DMBTSC. The image clearly shows the adsorbed layer of inhibitor molecules on the alloy surface thus protecting the metal from corrosion.SEM image of the aged maraging steel after immersion in (a) 0.5 M sulphuric acid without inhibitor, (b) 0.5 M sulphuric acid containing DAMTSC.
(a)
(b)


## 3.1. Tafel Polarization Measurement

The corrosion inhibition of aged maraging steel investigated in 0.5 M sulphuric acid containing various concentration of DAMTSC inhibitor at different temperatures using Tafel polarization technique and Tafel plots at 303 K are shown in Figure1. The valuable potentiodynamic polarization parameters including corrosion potential (Ecorr), corrosion current density (Icorr), anodic and cathodic slopes (ba and bc), and corrosion rate (C.R) were calculated from Tafel plots and are summarized in Table 2. The C.R is calculated using
(1)corrosionrate(mm/y)=3270(E.W)IcorrD,
where 3270 is a constant that defines the unit of corrosion rate, Icorr=corrosion current density in A/cm2, D=density of the corroding material, 8.23 g/cm3, E.W = equivalent weight of corroding material (atomic weight/oxidation number), 29.2 [25]. The percentage inhibition efficiency (IE%) was calculated from
(2)IE%=Icorr-Icorr(inh)Icorr×100,
where Icorr and Icorr(inh) are corrosion current densities in the absence and in the presence of inhibitor, respectively. The value of corrosion current density (Icorr) decreased effectively even with low concentration of DAMTSC and inhibition efficiency increases with increase inhibitor concentration. The presence of inhibitor does not shift the Ecorr remarkably, implying that DAMTSC acts as mixed type inhibitor affecting both anodic and cathodic reactions [23]. According to Li and others [26], if the displacement in corrosion potential is more than ±85 mV with respect to corrosion potential of the blank, the inhibitor can be considered as a cathodic or anodic type. But the maximum displacement in this study is less than 10 mV, which indicates that DAMTSC is a mixed type inhibitor. According to Cao [27] if the shift in Ecorr is negligible the inhibition is most probably caused by a geometric blocking effect of the adsorbed inhibitive species on the surface of corroding metal. The anodic and cathodic Tafel slopes remain almost unchanged upon addition of inhibitor. Thus the adsorbed inhibitor acts by simple blocking of active sites for anodic and cathodic processes, thereby reducing the surface area for corrosion without affecting the corrosion mechanism [28, 29]. The parallel cathodic Tafel curves suggest that the hydrogen evolution is activation controlled and the reduction mechanism is not affected by the presence of the inhibitors [30].Table 2
Results of  Tafel polarization studies on aged maraging steel in 0.5 M sulphuric acid with and without different concentrations of DAMTSC at various temperatures.
Temp (K)
Conc. of inhibitor        (×10−4) (M)
E
corr (V)
I
corr  × (10−4)   (A cm−2)
b
c (V dec−1)
b
a  (V dec−1)
C.R    in mm Y−1
IE (%)
303
0.00
−0.310
75.26
0.107
0.103
87.30
1.14
−0.310
27.18
0.103
0.102
31.53
63.9
2.85
−0.306
11.72
0.095
0.106
13.60
84.4
5.70
−0.306
9.57
0.106
0.113
11.10
87.3
8.55
−0.303
7.73
0.093
0.111
8.97
89.7
308
0.00
−0.312
85.43
0.122
0.127
99.10
1.14
−0.310
32.07
0.127
0.098
37.20
62.5
2.85
−0.309
21.64
0.098
0.112
25.10
74.7
5.70
−0.304
13.19
0.112
0.115
15.30
84.6
8.55
−0.307
10.99
0.115
0.118
12.75
87.1
313
0.00
−0.309
96.55
0.118
0.112
112.00
1.14
−0.309
37.67
0.112
0.086
43.70
61.0
2.85
−0.307
26.21
0.086
0.084
30.40
72.9
5.70
−0.308
17.76
0.084
0.106
20.60
81.6
8.55
−0.304
14.40
0.106
0.086
16.70
85.1
318
0.00
−0.312
106.90
0.086
0.117
124.00
1.14
−0.305
50.95
0.117
0.098
59.10
56.5
2.85
−0.305
36.81
0.098
0.095
42.70
68.6
5.70
−0.308
24.22
0.095
0.089
28.10
79.3
8.55
−0.301
19.05
0.089
0.098
22.10
83.8
323
0.00
−0.311
117.24
0.098
0.123
136.00
1.14
−0.303
58.97
0.123
0.095
68.40
49.7
2.85
−0.305
42.84
0.095
0.091
49.70
63.5
5.70
−0.303
30.09
0.091
0.088
34.90
74.3
8.55
−0.301
25.95
0.088
0.088
30.10
77.9Figure 1
Tafel polarization curves for the aged maraging steel in 0.5 M sulphuric acid containing different concentrations inhibitor.For anodic polarization curves of specimen with DAMTSC (in Figure2), it seems that at the working electrode potential, higher than −200 mV/SCE, the presence of inhibitor does not change the current versus potential characteristics. At this potential, the anodic current increases steeply and a flat region is observed. This potential can be defined as desorption potential [31]. The phenomenon may be due to the obvious metal dissolution, which leads to desorption of the inhibitor molecule from the electrode surface. In this case, the desorption rate of the inhibitor is higher than its adsorption rate, so the corrosion current increases more obviously with rising potential [32]. At still higher polarization potential the anodic current density change drastically, resulting in sharp increase in Tafel slope. The Tafel slope of the third portion is nearly equal to that in the absence of DAMTSC. Above desorption potential coverage of inhibitor decreases rapidly. Similar conclusion was reported by Qu et al. [33].Nyquist plots for the aged maraging steel specimen in 0.5 M sulphuric acid (a) containing different concentrations of inhibitor, (b) without inhibitor.
(a)
(b)


## 3.2. Electrochemical Impedance Spectroscopy

The results of potentiodynamic polarization experiments were compared with the results of impedance measurements, since EIS has emerged as probably the most powerful technique currently available for identifying corrosion reaction mechanisms. In order to get more information about the corrosion inhibition of aged maraging steel specimens in 0.5 M sulphuric acid containing different concentrations of DAMTSC, EIS measurements were carried out at different temperatures and they are displayed as Nyquists plots. The Nyquist plots obtained for aged samples of maraging steel specimens in 0.5 M sulphuric acid with and without various concentrations of DAMTSC are as shown in Figure2.It is clear from the figures that the shapes of the impedance plots for inhibited specimen are different from those of uninhibited one. The Nyquists plots of aged maraging steel in 0.5 M H2SO4 without inhibitor are characterized by two time constants with a depressed capacitive semicircle at high frequency region (HF) followed by an inductive loop at low frequency (LF) region. These are not perfect semicircles, because the Nyquist plots obtained in the real system represent a general behavior where the double layer at the metal solution interface does not behave as an ideal capacitor [34]. The depressed capacitive loop with its centre below the real axis often refers to the frequency dispersion of interfacial impedance which has been attributed to the roughness, inhomogeneity of the solid surfaces, and adsorption of inhibitors [10]. The HF capacitive loop was attributed to charge transfer of the corrosion process and time constant of the electric double layer [35]. The LF inductive loop can be attributed to relaxation process obtained by adsorbed sulphate ions and protons [36]. It may also be attributed to the redissolution of the passivated surface at low frequencies [37]. In the present case, the low frequency inductive loop in inhibitor free acid medium can be attributed to surface dissolution process at low frequencies. Based on the shape of Nyquist plots we have decided to use equivalent circuit as given in Figure 3(a) which has been previously used to model iron/acid interface [38]. It consists of solution resistance (Rs), charge transfer resistance (Rct), and constant phase element (CPE). The surface roughness, degree of polycrystallinity, and also anion adsorption lead to capacitance dispersion at solid electrodes and hence the real iron/acid interface system deviates from ideal capacitive behavior, which can be empirically represented by CPE [39]. The point of intersection between inductive loop and the real axis represents (Rs+Rct). Rs represents solution resistance due to the ohmic resistances of corrosion product films and the solution enclosed between the working electrode and the reference electrode. Rct represents the charge transfer resistance whose value is a measure of electron transfer across the surface and is inversely proportional to corrosion rate [40].Equivalent circuit used to fit experimental EIS data for the corrosion of aged maraging steel specimen in sulphuric acid (a) without inhibitor, (b) with inhibitor.
(a)
(b)The Nyquists plots of aged maraging steel in 0.5 M H2SO4 containing various concentrations of DAMTSC consists of two semicircles at high and middle frequency range and an inductive loop at low frequency range. These plots are analysed by an equivalent circuit, which is represented in Figure 3(b). The smaller HF capacitive loop suggested that some sort of film exists on this alloy in the studied solution which can be attributed to film formation by adsorbed inhibitor while larger middle frequency capacitive loop can be attributed to charge transfer reaction [41–44]. The resistance values corresponding to high frequency and middle frequency semicircles are represented as Rf and Rct, respectively, in equivalent circuit Figure 3(b). Both Rf and Rct increase with increase in inhibitor concentration. The sum of Rf and Rct are taken for calculation of inhibition efficiency. The capacitance of film (Cf) was represented by CPE1 and double layer capacitance (Cdl) was represented as CPE2. All these values are shown in Table 3. The low frequency inductive loop in inhibited acid solution might be attributed to the surface dissolution, indicating that the alloy still dissolved by the direct charge transfer at the inhibitor adsorbed steel surface [45]. The fact that this inductive loop cannot be observed at maximum concentration of DAMTSC supports this view. The study at higher temperature reveals single semicircle at low concentrations of inhibitor and appearance of two semicircles only at high concentration of inhibitor, this can be attributed to the fact that formation of inhibitor film is favorable only at higher inhibitor concentration above 303 K.Table 3
EIS data of aged maraging steel in 0.5 M sulphuric acid with and without different concentrations of DAMTSC at various temperatures.
Temp. (K)
Conc. of inhibitor(×10−4) (M)
R
f Ohm·cm2
R
ct Ohm·cm2
R
total Ohm·cm2
C
f (10−6) (F)
C
dl (10−6) (F)
IE (%)
303
0.00
2.8
2.80
14490
1.14
1.17
5.66
6.83
874
9920
59.0
2.85
5.37
9.38
14.75
799
9762
81.0
5.70
6.12
13.5
19.62
653
1590
85.7
8.55
7.89
17.8
25.70
509
1163
89.1
308
0.00
2.30
2.30
87620
1.14
5.10
5.10
10112
54.9
2.85
8.87
8.87
7566
74.2
5.70
5.6
11.2
16.8
954
6347
86.3
8.55
7.19
11.6
18.7
765
4850
87.7
313
0.00
1.6
1.60
113500
1.14
4.32
3.32
23724
51.8
2.85
5.75
5.75
20880
72.2
5.70
3.43
7.34
10.8
2417
11340
85.2
8.55
4.2
7.9
12.1
1450
10800
86.7
318
0.00
1.4
1.4
226570
1.14
2.7
2.7
31326
48.1
2.85
4.8
4.8
27667
70.8
5.70
3.25
5.05
8.3
2625
23471
83.1
8.55
3.40
5.3
8.7
1562
20125
83.9
323
0.00
1.3
1.3
541000
1.14
2.3
2.3
33540
43.5
2.85
3.5
3.5
24500
62.9
5.70
1.44
4.16
5.6
2966
20760
76.8
8.55
2.20
4.90
7.1
1695
15600
81.7The impedance of CPE is expressed as(3)ZCPE=1Y0(jw)n,
where the amplitude Y0 and n are frequency independent, and ω is the angular frequency for which -Z′′ reaches its maximum values, n is dependent on the surface morphology: -1≤n≤1. The HF loops have depressed semicircular appearance, with 0.5≤n≤1, which is often referred to as frequency dispersion as a result of the non homogeneity or roughness [46].The double layer capacitancesCdl, for a circuit including CPE were calculated from the following equation [47]:
(4)Cdl=Y0(wmax)n-1,
where wmax=2πfmax, and fmax is the frequency at which the imaginary component of the impedance is maximal. According to the expression of the double layer capacitance presented in the Helmholtz model [48],
(5)Cdl=εε0dS,
where d is the thickness of the film, S is the surface area of the electrode, ε0 is the permittivity of air, and ε is the local dielectric constant. The value of Cdl decreases due to adsorption of inhibitor molecules, which displaces water molecules originally adsorbed on the mild steel surface and decreases the active surface area. The values of double layer capacitance decreases with increase in inhibitor concentration indicating that inhibitor molecules function by adsorption at the metal/solution interface, leading to protective film on the alloy surface, and decreasing the extent of dissolution reaction [49].The corrosion current densityIcorr can be calculated using charge transfer resistance Rct, together with Stern-Geary equation [40]:
(6)Icorr=babc2.303RctA(ba+bc).
The charge transfer resistance Rct and double layer capacitance Cdl were determined by the analysis of Nyquist plot and their values are given in Table 3. The inhibition efficiency in different concentrations of DAMTSC was calculated from the charge transfer resistance according to
(7)IE%=(Rct-Rct0Rct)×100,
where Rct and Rct0 represent the charge transfer resistance in the presence and absence of inhibitor. The values of Rct increases with increase in inhibitor concentration and the results indicate that charge transfer process mainly controls the corrosion process. The inhibition efficiency values obtained by the electron impedance method are in good agreement with the ones obtained by Tafel’s method.


## 3.3. Effect of Temperature

The effect of temperature on the inhibited acid-metal reaction is highly complex because many changes occur on the metal surface, such as rapid etching and desorption of the inhibitor and the inhibitor itself, and in some cases may undergo decomposition and/or rearrangement [10]. However it facilitates the calculation of many thermodynamic functions for the inhibition and/or the adsorption processes which contribute in determining the type of adsorption of the studied inhibitors. In the present study, with increase in solution temperature, corrosion potential (Ecorr), anodic Tafel slope (ba), and cathodic Tafel slope (bc) values are not affected much. This indicates that increase in temperature does not change the mechanism of corrosion reaction. But Icorr and hence the corrosion rate of the specimen increases with increase in temperature for both blank and inhibited solutions. This may be attributed to the fact that the hydrogen evolution overpotential decreases with increase in temperature that leads to increase in cathodic reaction rate [50]. The inhibition efficiency decreases with increase in temperature which indicates desorption of inhibitor molecules [51]. This is suggestive of physisorption [52]. But this decrease is small at higher concentration of inhibitor.The apparent activation energy (Ea) for the corrosion process in the presence and absence of inhibitor can be calculated using Arrhenius law equation [10],
(8)ln(C.R)=B-(EaRT),
where B is a constant which depends on the metal type and R is the universal gas constant. The plot of ln(corrosionrate) versus reciprocal of absolute temperature 1/T gives straight line whose slope=-Ea/R gives activation energy for the corrosion process. The Arrhenius plots for the specimen are shown in Figure 4.Figure 4
Arrhenious plots for the dissolution of aged maraging steel in 0.5 M sulphuric acid containing different concentrations of inhibitor.The entropy and enthalpy of activation for the dissolution of alloy,ΔHa and ΔSa, were calculated from transition state theory equation [10]
(9)C.R=(RTNh)exp(ΔSaR)exp(-ΔHaRT),
where h is Plank’s constant, N is Avagadro’s number. A plot of ln(corrosionrate/T) versus 1/T gives straight line with slope=-ΔHa/R and intercept=ln(R/Nh)+ΔSa/R. The calculated values of Ea, ΔHa, and ΔSa  are given in Table 4. The plot of ln(corrosionrate/T) versus 1/T for aged samples of maraging steel is shown in Figure 5.Table 4
Activation parameters for the corrosion of aged maraging steel in 0.5 M sulphuric acid with and without different concentrations of DAMTSC.
Conc. of inhibitor(×10−4) (M)
E
a kJ mol−1
Δ
H
a kJ mol−1
Δ
S
a J mol−1 K−1
0
18.1
15.5
−156.7
1.14
32.7
30.1
−117.3
2.85
47.3
44.6
−77.7
5.70
48.4
45.8
−75.7
8.55
51.1
48.4
−62.5Figure 5
ln
(
corrosion
rate
/
T
) versus 1/T for aged maraging steel in 0.5 M sulphuric acid containing different concentrations of inhibitor.The effect of chemically stable surface active inhibitors is to increase the energy of activation and to decrease the surface area available for corrosion [53]. The value of activation energy (Ea) in 0.5 M sulphuric acid solution containing inhibitor is greater than that without inhibitor. The extent of increase is proportional to the inhibitor concentration, indicating that the energy barrier for the corrosion reaction increases with increase in DAMTSC concentration. The increase in apparent activation energy Ea may be interpreted as due to physical adsorption [54] of the inhibitor, which results in increase in surface coverage with the increase in the concentration of the inhibitor. The decrease in the inhibition efficiency of DAMTSC with increase in temperature can be attributed to an appreciable decrease in the adsorption of the inhibitor on the metal surface with increase in temperature and a corresponding increase in corrosion rate due to the fact that greater area of metal is exposed to the acid [55].The values of entropy of activation in the absence and presence of inhibitor are large and negative. This implies that the activated complex in the rate determining step represents an association rather than dissociation step, resulting in a decrease in randomness on going from reactants to activated complex [56, 57]. The entropy of activation values are more for inhibited solutions than that for the uninhibited solutions. This suggested that an increase in randomness occurred on going from reactants to the activated complex. This might be the results of the adsorption of organic inhibitor molecules from the acidic solution which could be regarded as a quasi-substitution process between the organic compound in the aqueous phase and water molecules at electrode surface [58, 59]. In this situation, the adsorption of organic inhibitor is accompanied by desorption of water molecules from the surface. Thus the increasing in entropy of activation was attributed to the increase in solvent entropy [60].


## 3.4. Adsorption Isotherm

Organic corrosion inhibitors are known to decrease metal dissolution via adsorption on the metal/corrodent interface to form a protective film which separates the metal surface from the corrosive medium. The adsorption route is usually regarded as a substitution process between the organic inhibitor in the aqueous solution[Inh(sol)] and water molecules adsorbed at the metal surface [H2O(ads)] as follows [61]:
(10)Inh(sol)+χH2O(ads)⟷Inh(ads)+χH2O(sol),
where χ represents the number of water molecules replaced by one molecule of adsorbed inhibitor. The adsorption bond strength is dependent on the composition of the metal, corrodent, inhibitor structure, concentration, and orientation as well as temperature. Basic information on the interaction between the inhibitor and alloy surface can be provided by adsorption isotherm. In order to obtain isotherm, the linear relation between surface coverage (θ) value and Cinh must be found. The surface coverage θ is given by
(11)θ=IE%100,
where IE% is percentage inhibition efficiency as calculated using (2).Attempts were made to fit the degree of surface coverage (θ) values to various isotherms including Langmuir, Temkin, Frumkin, and Flory-Huggins isotherms. By far the best fit is obtained with the modified Langmuir adsorption isotherm. Langmuir isotherm for monolayer chemisorption is given by
(12)Cinhθ=1Kads+Cinh,
where Cinh is the concentration of inhibitor, Kads is the equilibrium constant for adsorption process, and θ is the degree of the surface coverage which is calculated using (11).This model has also been used for other inhibitor systems [61]. The plot of Cinh/θ versus Cinh gives a straight line with intercept 1/Kads as shown in Figure 6.Figure 6
Langmuir adsorption isotherm of DAMTSC on aged maraging steel in 0.5 M sulphuric acid at different temperatures.The values of standard free energy of adsorption are related toKads by the relation (13).
(13)Kads=155.5e(-ΔGads0/RT),
where the value 55.5 is the concentration of water in solution in M (mol/L), R is the universal gas constant and T is absolute temperature [61]. The thermodynamic data obtained from adsorption isotherm are tabulated in Table 5. The correlation coefficient (R2) was used to choose the isotherm that best fit experimental data [10]. The linear regression coefficients are close to unity and the slopes of straight lines are nearly unity, suggesting that the adsorption of DAMTSC obeys Langmuir’s adsorption isotherm and there is negligible interaction between the adsorbed molecules [26]. The high values of Kads for the studied inhibitor indicate strong adsorption of inhibitor on the alloy surface. Negative values of ΔGads0 are characteristic feature of strong spontaneous adsorption for the studied compounds, which also reflect the high values of inhibition. The negative ΔGads0 values calculated from (13) are consistent with the spontaneity of the adsorption process and the stability of the adsorbed layer on the maraging steel surface. Generally, the standard free energy values of −20 kJ mol−1 or less negative are associated with an electrostatic interaction between charged molecules and charged metal surface, resulting in physisorption and those of −40 kJ mol−1 or more negative involve charge sharing or transfer from the inhibitor molecules to the metal surface to form a coordinate covalent bond, resulting in chemisorption [62]. The ΔGads0 values obtained for the studied inhibitor on the alloy surface in 0.5 M sulphuric acid are −33 to −35 kJ/mol. These values predict both physical and chemical adsorption of inhibitor [63]. A plot of ΔGads0 versus T was used to calculate heat of adsorption ΔHads0 and the standard adsorption entropy ΔSads0 according to the following thermodynamic basic equation [10]:
(14)ΔGads0=ΔHads0-TΔSads0.
The thermodynamic data obtained for DAMTSC using Langmuir adsorption isotherm are depicted in Table 5.Table 5
Thermodynamic parameters for the adsorption of DAMTSC on aged maraging steel surface in 0.5 M sulphuric acid.
Temperature(°C)
K (mol−1 L)
-
Δ
G
ads
0 kJ mol−1
R
2
Slope
Δ
H
ads
0 kJ mol−1
Δ
S
ads
0 J mol−1 K−1
303
20227
35.1
0.9997
1.05
308
14931
34.3
0.9997
1.07
−62.25
90
313
14391
34.2
0.9997
1.09
318
11246
33.6
0.9995
1.09
323
9476.9
33.2
0.9997
1.14The values of thermodynamic parameters for the adsorption of inhibitors can provide valuable information about the mechanism of corrosion inhibition. While an endothermic adsorption process (ΔHads0>0) is attributed unequivocally to chemisorption [64], an exothermic adsorption process (ΔHads0<0) may involve either physisorption or chemisorption or a mixture of both the processes. In an exothermic process, physisorption is distinguished from chemisorption by considering the absolute value of adsorption enthalpy. Typically, the enthalpy of a physisorption process is below 41.86 kJ mol−1, while that of a chemisorption process approaches 100 kJ mol−1 [65]. In the present case, the calculated value of ΔHads0 is −69.014 kJ mol−1, which is an intermediate case [63], probably involving both physisorption and chemisorption. The ΔSads0 value is large and negative, indicating that decrease in disordering takes place on going from reactant to alloy adsorbed species. This can be attributed to the fact that adsorption is an exothermic process and is always accompanied by decrease in entropy.


## 3.5. Mechanism of Corrosion Inhibition

In order to predict the type of adsorption, the corrosion mechanism of iron must be known. It was suggested that the anodic dissolution of iron in sulphuric acid solutions follow the consecutive or BDD mechanism [66] as reported by [41]. Iron electro dissolution in acidic sulphate solution depends primarily on the adsorbed intermediate FeOHads according to the mechanism reproduced below:
(15)FeH2Oads⟺FeOHads+H++e(16)FeOHads→FeOH++e(17)FeOH++H+⟺Fe2++2e
The following stages are possible for the hydrogen evolution reaction on iron [67]:
(18)Fe+H++e-→FeH(19)FeH+FeH→2Fe+H2(20)FeH+H++e-→Fe+H2
The corrosion rate of iron in H2SO4 solutions is controlled by both hydrogen evolution reaction and iron dissolution.An anodic oxidation mechanism in the presence of inhibitor may be similar to the mechanism discussed by McCafferty et al. [68–70] as reported by [71]. Consider
(21)Fe+H2O⟺Fe·H2Oads(22)Fe·H2Oads+Y⟺FeOH-ads+H++Y(23)Fe·H2Oads+Y⟺FeYads+H2O(24)FeOHads-→FeOHads+e(ratedeterminingstep)(25)FeYads⟺FeYads++e(26)FeOHads+FeYads+⟺FeOH++FeYads(27)FeOH++H+⟺Fe2++H2O
where Y is inhibitor molecule.According to the above mechanism, displacement of some adsorbed water molecules on the metal surface by inhibitor species to yield the adsorbed intermediate FeYads in step (23) reduces the amount of the species FeOHads- available for the rate determining step. The inhibitor used may form dominating adsorbed intermediate which results in inhibitive effects. This conclusion is in line with those of Obot and Obi-Egbedi [72], and Shoesmith [73]. Aged maraging steel results from precipitation of intermetallics. Since these intermetallics have different composition, their electrochemical behavior is expected to be different compared to matrix [74]. Also there will be strain fields around these coherent precipitates as a result of lattice mismatch between the precipitate and the matrix due to the difference in the crystal structure and lattice parameters. These strain fields in combination with the galvanic effect due to the composition difference leads to the enhanced corrosion of aged maraging steel in acid medium. Considering the inhomogeneous nature, the surface of alloy is generally characterized by multiple adsorption sites having activation energies and heats of adsorption. Inhibitor molecules may thus be adsorbed more readily at surface active sites having suitable adsorption enthalpies.The adsorption mechanism for a given inhibitor depends on factors such as, the nature of metal, the corrosive medium, the pH, and the concentration of the inhibitor as well as the functional groups present in its molecule, since different groups are adsorbed to different extents. For instance S-containing substances have been shown to preferentially chemisorb on the surface of iron in acidic medium, whereas N-containing substances tend to favor physisorption [75]. It is reported that protective value of sulphur containing compounds is superior to that of nitrogen containing compounds. This may be due to greater polarizability of sulphur atom and the presence of two electron pairs available for coordination [22].The inhibition effect of DAMTSC in sulphuric acid solution can be explained as follows: DAMTSC might be protonated in the acid solution as follows:(28)DAMTSC+xH+→[DAMTSCHx]x+
Thus in aqueous acidic solution, DAMTSC exists partly in the form of protonated species and partly as neutral molecules. Generally two modes of adsorption could be considered. Taking into consideration of many reported results Hackerman et al. [76] concluded that, the surface charge of iron in sulphuric acid solution at the free corrosion potential is positive. Therefore, the adsorption of protonated species would be through electrostatic attraction between the positively charged molecule and the negatively charged sulphate ions adsorbed on the metal surface via physisorption. In the study of the effect of inorganic anions and organic compounds on corrosion inhibition of mild steel in various acids, Li et al. [77] reported the degree of adsorption of anions was in the order I->Br->Cl->SO42->ClO4-. The sulphate ions with low degree of adsorption could not cover the metal surface fully and therefore there is always possibility of the neutral inhibitor molecules occupying the vacant adsorption sites on the metal surface via the chemisorption mode of protection of corroding surface [78]. The possible bonding sites of the neutral inhibitor are the free electron pairs on sulphur and nitrogen atoms, as well as oxygen, which could interact with vacant d orbital of iron [78].


## 3.6. Scanning Electron Microscope Studies (SEM)

The scanning electron microscope images were recorded to establish the interaction of acid medium with the metal surface. The surface morphology of the aged samples was examined by SEM immediately after the sample is subjected to corrosion tests in H2SO4 medium in the absence and in the presence of inhibitor. The SEM image of corroded aged maraging steel sample in Figure 7(a) shows degradation of alloy in the absence of inhibitor. The attack by H2SO4 is seen to be more at grain boundary since these regions are highly susceptible to corrosion. In aged samples the intermetallic precipitation at grain boundary may be responsible for the higher rate of corrosion. Figure 7(b) represents SEM image of aged maraging steel after the corrosion tests in a medium of sulphuric acid containing DMBTSC. The image clearly shows the adsorbed layer of inhibitor molecules on the alloy surface thus protecting the metal from corrosion.SEM image of the aged maraging steel after immersion in (a) 0.5 M sulphuric acid without inhibitor, (b) 0.5 M sulphuric acid containing DAMTSC.
(a)
(b)


## 4. Conclusions

Potentiodynamic polarization and electrochemical impedance methods were used to evaluate the ability of DAMTSC to inhibit corrosion of aged maraging steel in 0.5 M sulphuric acid. The principle conclusions are as follows.(1)
The corrosion of aged maraging steel in 0.5 M sulphuric acid is significantly reduced by the addition of DAMTSC even at low concentration.(2)
The inhibition efficiency increases with increase in inhibitor concentration.(3)
The inhibition efficiency decreases with increase in temperature of medium.(4)
DAMTSC acts as mixed type inhibitor, affecting both anodic and cathodic reactions.(5)
The adsorption of DAMTSC on aged maraging steel surface obeys Langmuir’s adsorption isotherm model.(6)
The negative value ofΔGads0 obtained indicates that DAMTSC adsorbed spontaneously on the aged maraging steel. The adsorption process is exothermic and accompanied by decrease in entropy.(7)
TheΔGads0 and ΔHads0 values predict both physisorption and chemisorption of inhibitor on alloy surface.(8)
SEM images revealed protection of alloy surface in sulphuric acid medium by the adsorption of DAMTSC.(9)
The inhibition efficiencies obtained from potentiodynamic polarization and EIS techniques are in reasonably good agreement.



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*Source: 723687-2012-12-17.xml*