# Inhibitory Mechanism of Carbon Steel Corrosion in Sea Water by an Aqueous Extract of Henna Leaves

Authors: V. Johnsirani; J. Sathiyabama; Susai Rajendran; A. Suriya Prabha
Journal: ISRN Corrosion
 (2012)

---

## Abstract

The inhibition efficiency (IE) of an aqueous extract of henna leaves in controlling corrosion of carbon steel in seawater has been evaluated by weight-loss method. The weight loss study reveals that the formulation consisting of 8 mL of  henna extract (HE) and 25 ppm of Zn2+ has 94% inhibition efficiency in controlling corrosion of carbon steel in sea water. Polarization study reveals that HE and Zn2+ system functions as mixed type inhibitor. AC impedance spectra reveal that protective film is formed on the metal surface. The nature of the metal surface has been analysed by FTIR spectra, SEM, and AFM analysis.


---

## Body

## 1. Introduction

Sea water is one of the most corroded and most abundant naturally occurring electrolytes. The corrosive behaviour of seawater is reflected by the fact that most of the common structural metals and alloys are attacked by this liquid or its surrounding environments. The sea water environments can be divided into five zones, namely, subsoil, continuously submerged, tidal, splash zone above high tidal, and atmospheric zone [1]. The corrosion behaviour of metals and alloys differs from one zone to another. In splash zone the stainless steels have usually satisfactory performance while the carbon and low alloy steels do not. Anderson and Ross had found that the austenitic grades performed much better than martensitic and ferritic grades [2]. The Ni, Cu, and P alloyed steels were found to be much more resistant than carbon steel in splash zone [3]. Also, it was found that Mn, P, and Al had measurable influence on corrosion rates of low carbon steels under tidal exposure. After 5-year exposure test, it was found that the rate of attack in splash zone was much higher than the atmospheric π and deep submerged zones [4].Metals and alloys are often exposed to the action of acids and alkalis in industrial processes thereby prompting their deterioration [5]. One of the most effective means of protecting metals and alloys surfaces from corrosion in acid and alkaline environments is the use of corrosion inhibitors [6]. Corrosion inhibitors are usually added to the acid/alkaline solution to reduce the metal loss. Recently, studies on the use of drugs have been reported by several researchers [7, 8]. Some of these corrosion inhibitors are, however, toxic to the environment. This has prompted the search for green corrosion inhibitors that are nontoxic and ecofriendly for metals and alloys in acidic and alkaline solutions [9]. These green corrosion inhibitors have been found to have centre for π-electrons and functional groups (such as –C=C–, –OR, –OH, –NR2, –NH2, and –SR) which provide electrons that facilitate the adsorption of the inhibitor on the metal/alloy surface.Before the 1960s, inorganic inhibitors such as zinc, chromate, polyphosphate, and nitrites were used as inhibitors. However, discharge of such material has become unacceptable due to environmental hazards [10]. The recent trend is towards environmental friendly inhibitors. Increasing awareness of health and ecological risk has drawn attention to finding more suitable inhibitors, which are nontoxic. Accordingly, greater research efforts have been directed towards formulating environmentally acceptable inhibitors. The extracts from plant leaves, roots, and seeds have been studied and found to be cheap, environmental friendly, and good corrosion inhibitors. The plant leave extracts studied include Opuntia extract [11], Carica papaya (CP) and Azadirachta indica (AI) [12], Vernonia amygdalina [13], Telfairia occidentalis [14], Phyllanthus amarus [15], fenugreek leaves [16], olive leaves [17], Euphorbia hirta, and Dialium guineense [18]. The roots studied include ginseng root [19], Beet root extract [20], Kopsia singaporensis extract [21], and Murraya koenighii [22]. The present work is undertaken.(1)
To evaluate the inhibition efficiency (IE) of henna extract (HE)-Zn2+ system in controlling corrosion of carbon steel immersed in sea water in the absence and presence of Zn2+ by weight-loss method.(2)
To study the mechanism of corrosion inhibition by polarization study and AC impedance spectra.(3)
To analyse the protective film by FTIR spectra, scanning electron microscope (SEM), and atomic force microscope.(4)
To propose the mechanism of corrosion inhibition based on the above results.


## 2. Experimental

### 2.1. Preparation of Plant Extract

The leaves of henna were dried and ground to powder and 10 gm of the powdered leaves weighed and boiled with double-distilled water, filtering the suspending impurities and making up to 100 mL. The extract was used as corrosion inhibitor in the present study.


### 2.2. Preparation of the Specimens

Carbon steel specimens (0.026% S, 0.06% P, 0.4% Mn, 0.1% C and rest iron) of the dimensions 1.0 × 4.0 × 0.2 cm were polished to a mirror finish, degreased with trichloroethylene, and used for the weight-loss method and surface examination studies.


### 2.3. Weight-Loss Method

#### 2.3.1. Determination of Surface Area of the Specimens

The length, breadth, and the thickness of carbon steel specimens and the radius of the holes were determined with the help of vernier calipers of high precision, and the surface areas of the specimens were calculated.


#### 2.3.2. Weighing the Specimens before and after Corrosion

All the weighing of the carbon steel specimens before and after corrosion was carried out using Shimadzu Balance-AY62.


#### 2.3.3. Determination of Corrosion Rate

The weighed specimens in triplicate were suspended by means of glass hooks in 100 mL beakers containing 100 mL various test solutions, and after 24 hours of immersion, the specimens were taken out, washed in running water, dried, and weighed. From the change in weights of the specimens, corrosion rates were calculated using the following relationship:(1)Corrosion rate(mm/year)=87.6W/DAT,
where W = weight-loss in milligrams, D = density of specimen g/cm3, A = area of specimen in square cm, and T = exposure time in hours.Corrosion inhibition efficiency (IE) was then calculated using the equation(2)IE=100[1-(W2W1)]%,
whereW1 = corrosion rate in the absence of inhibitor and W2 = corrosion rate in the presence of inhibitor.


### 2.4. Surface Examination Study

The carbon steel specimens were immersed in various test solutions for a period of one day. After one day, the specimens were taken out and dried. The nature of the film formed on the surface of metal specimen was analyzed by various surface analysis techniques.


### 2.5. Potentiodynamic Polarization

Polarization studies were carried out in a CHI electrochemical workstation with impedance model 660 A. It was provided with IR compensation facility. A three-electrode cell assembly was used. The working electrode was carbon steel. A SCE was the reference electrode. Platinum was the counter electrode. From polarization study, corrosion parameters such as corrosion potential (Ecorr), corrosion current (Icorr), Tafel slopes anodic = ba, and cathodic = bc were calculated, and linear polarization study (LPR) was done. The scan rate (V/S) was 0.01. Hold time at (Efcs) was zero and quiet time (s) was two.


### 2.6. AC Impedance Spectra

The instrument used for polarization study was used to record AC impedance spectra also. The cell set up was also the same. The real part (Z′) and imaginary part (Z′′) of the cell impedance were measured in ohms at various frequencies. Values of charge transfer resistance (Rt) and the double layer capacitance (Cdl) were calculated.


### 2.7. Fourier Transform Infrared Spectra

These spectra were recorded in a Perkin-Elmer-1600 spectrophotometer using KBr pellet. The FTIR spectrum of the protective film was recorded by carefully removing the film, mixing it with KBr and making the pellet.


### 2.8. Scanning Electron Microscopic Studies (SEM)

The carbon steel immersed in blank solution and in the inhibitor solution for a period of one day was removed, rinsed with double-distilled water, dried, and observed in a scanning electron microscope to examine the surface morphology. The surface morphology measurements of carbon steel were examined using JEOL MODEL6390 computer-controlled scanning electron microscope.


### 2.9. Atomic Force Microscopy Characterization (AFM)

The carbon steel specimens immersed in blank and in the inhibitor solution for a period of one day were removed, rinsed with double-distilled water, dried, and subjected to the surface examination. Atomic force microscopy (Veeco dinnova model) was used to observe the samples’ surface in tapping mode, using cantilever with linear tips. The scanning area in the images was 5μm × 5 μm, and the scan rate was 0.6 HZ/second.


## 2.1. Preparation of Plant Extract

The leaves of henna were dried and ground to powder and 10 gm of the powdered leaves weighed and boiled with double-distilled water, filtering the suspending impurities and making up to 100 mL. The extract was used as corrosion inhibitor in the present study.


## 2.2. Preparation of the Specimens

Carbon steel specimens (0.026% S, 0.06% P, 0.4% Mn, 0.1% C and rest iron) of the dimensions 1.0 × 4.0 × 0.2 cm were polished to a mirror finish, degreased with trichloroethylene, and used for the weight-loss method and surface examination studies.


## 2.3. Weight-Loss Method

### 2.3.1. Determination of Surface Area of the Specimens

The length, breadth, and the thickness of carbon steel specimens and the radius of the holes were determined with the help of vernier calipers of high precision, and the surface areas of the specimens were calculated.


### 2.3.2. Weighing the Specimens before and after Corrosion

All the weighing of the carbon steel specimens before and after corrosion was carried out using Shimadzu Balance-AY62.


### 2.3.3. Determination of Corrosion Rate

The weighed specimens in triplicate were suspended by means of glass hooks in 100 mL beakers containing 100 mL various test solutions, and after 24 hours of immersion, the specimens were taken out, washed in running water, dried, and weighed. From the change in weights of the specimens, corrosion rates were calculated using the following relationship:(1)Corrosion rate(mm/year)=87.6W/DAT,
where W = weight-loss in milligrams, D = density of specimen g/cm3, A = area of specimen in square cm, and T = exposure time in hours.Corrosion inhibition efficiency (IE) was then calculated using the equation(2)IE=100[1-(W2W1)]%,
whereW1 = corrosion rate in the absence of inhibitor and W2 = corrosion rate in the presence of inhibitor.


## 2.3.1. Determination of Surface Area of the Specimens

The length, breadth, and the thickness of carbon steel specimens and the radius of the holes were determined with the help of vernier calipers of high precision, and the surface areas of the specimens were calculated.


## 2.3.2. Weighing the Specimens before and after Corrosion

All the weighing of the carbon steel specimens before and after corrosion was carried out using Shimadzu Balance-AY62.


## 2.3.3. Determination of Corrosion Rate

The weighed specimens in triplicate were suspended by means of glass hooks in 100 mL beakers containing 100 mL various test solutions, and after 24 hours of immersion, the specimens were taken out, washed in running water, dried, and weighed. From the change in weights of the specimens, corrosion rates were calculated using the following relationship:(1)Corrosion rate(mm/year)=87.6W/DAT,
where W = weight-loss in milligrams, D = density of specimen g/cm3, A = area of specimen in square cm, and T = exposure time in hours.Corrosion inhibition efficiency (IE) was then calculated using the equation(2)IE=100[1-(W2W1)]%,
whereW1 = corrosion rate in the absence of inhibitor and W2 = corrosion rate in the presence of inhibitor.


## 2.4. Surface Examination Study

The carbon steel specimens were immersed in various test solutions for a period of one day. After one day, the specimens were taken out and dried. The nature of the film formed on the surface of metal specimen was analyzed by various surface analysis techniques.


## 2.5. Potentiodynamic Polarization

Polarization studies were carried out in a CHI electrochemical workstation with impedance model 660 A. It was provided with IR compensation facility. A three-electrode cell assembly was used. The working electrode was carbon steel. A SCE was the reference electrode. Platinum was the counter electrode. From polarization study, corrosion parameters such as corrosion potential (Ecorr), corrosion current (Icorr), Tafel slopes anodic = ba, and cathodic = bc were calculated, and linear polarization study (LPR) was done. The scan rate (V/S) was 0.01. Hold time at (Efcs) was zero and quiet time (s) was two.


## 2.6. AC Impedance Spectra

The instrument used for polarization study was used to record AC impedance spectra also. The cell set up was also the same. The real part (Z′) and imaginary part (Z′′) of the cell impedance were measured in ohms at various frequencies. Values of charge transfer resistance (Rt) and the double layer capacitance (Cdl) were calculated.


## 2.7. Fourier Transform Infrared Spectra

These spectra were recorded in a Perkin-Elmer-1600 spectrophotometer using KBr pellet. The FTIR spectrum of the protective film was recorded by carefully removing the film, mixing it with KBr and making the pellet.


## 2.8. Scanning Electron Microscopic Studies (SEM)

The carbon steel immersed in blank solution and in the inhibitor solution for a period of one day was removed, rinsed with double-distilled water, dried, and observed in a scanning electron microscope to examine the surface morphology. The surface morphology measurements of carbon steel were examined using JEOL MODEL6390 computer-controlled scanning electron microscope.


## 2.9. Atomic Force Microscopy Characterization (AFM)

The carbon steel specimens immersed in blank and in the inhibitor solution for a period of one day were removed, rinsed with double-distilled water, dried, and subjected to the surface examination. Atomic force microscopy (Veeco dinnova model) was used to observe the samples’ surface in tapping mode, using cantilever with linear tips. The scanning area in the images was 5μm × 5 μm, and the scan rate was 0.6 HZ/second.


## 3. Results and Discussions

### 3.1. Weight-Loss Study

The physicochemical parameters of sea water are given in Table1.Table 1
Water analysis (Thondi sea water, Tamil Nadu, India).
Parameters
Result
Total dissolved salts (mg/L)
30539 ppm
Electrical conductivity (micro mhos/cm)
44910
pH
7.06
Total hardness (CaCO3 equivalent)
100 ppm
Calcium as Ca (mg/L)
19 ppm
Magnesium as Mg (mg/L)
12 ppm
Sodium as Na (mg/L)
0
Chloride as Cl (mg/L)
11400 ppm
Fluoride as F (mg/L)
0
Free ammonia as NH3 (mg/L)
0
Sulphate as SO4 (mg/L)
6708 ppmThe calculated inhibition efficiencies (IEs) of henna extract in controlling the corrosion of carbon steel immersed in sea water both in the absence and presence of zinc ion have been tabulated in Table2. The calculated values indicate the ability of henna extract to be a good corrosion inhibitor. The inhibition efficiency is found to be enhanced in the presence of zinc ion. The formulation consisting of 8 mL of HE and 25 ppm of Zn2+ offers 94% inhibition efficiency. This mixture of inhibitors shows better IE than the individual inhibitors [23].Table 2
The corrosion inhibition efficiencies and the corresponding corrosion rates (millimeter per year) of HE-Zn2+ system.
Inhibitor HE (mL)
Zn2+ (ppm)
0
25
50
IE%
CR (mn/y)
IE%
CR (mm/y)
IE%
CR (mm/y)
0
—
0.1576
16
0.1323
47
0.0835
2
28
0.1134
41
0.0929
64
0.0567
4
35
0.1024
47
0.0835
76
0.0378
6
47
0.0835
71
0.0457
88
0.0189
8
65
0.0551
94
0.0094
82
0.0283
10
47
0.0835
64
0.0567
75
0.0394


#### 3.1.1. Synergism Parameter (SI)

Synergism parameters are indications of synergistic effect existing between inhibitors [24, 25]. SI value is found to be greater than one indicating the synergistic effect existing between Zn2+ of concentrations 25 ppm and 50 ppm with various concentrations of HE. The results are given in Table 3.Table 3
Synergism    parameter (SI).
HE (mL)
θ
1
Zn2+  25 (ppm) (θ2)
HE-Zn2+  θ1+2′
S
I
Zn2+  50 (ppm) (θ2)
HE-Zn2+  θ1+2′
S
I
2
0.28
0.16
0.41
1.0250
0.47
0.64
1.06
4
0.35
0.16
0.47
1.0301
0.47
0.76
1.4354
6
0.47
0.16
0.71
1.5351
0.47
0.88
2.3408
8
0.65
0.16
0.94
4.9
0.47
0.82
1.0305
10
0.47
0.16
0.64
1.2366
0.47
0.75
1.1236In Table3 synergism parameters were calculated using the relation
(3)SI=1-θ1+21-θ1+2′,
where
(4)θ1+2=(θ1+θ2)-(θ1×θ2),
where θ1 = surface coverage of inhibitor (HE) and θ2 = surface coverage of inhibitor (Zn2+) and where
(5)θ1+2′=combinedsurfacecoverageofinhibitors(HE)and(Zn2+)surfacecoverage=IE%100.


#### 3.1.2. Analysis of Variance (F-Test)

F-test is used to investigate if the synergistic effect existing between two inhibitors is statistically significant or not [26, 27]. If F-value is above 5.32 for 1, 8 degrees of freedom, the synergistic effect proves to be statistically significant. If it is below 5.32 for 1, 8 degrees of freedom indicating that it is statistically insignificant at a 0.05 level of significance. The results are given in Table 4.Table 4
Analysis of variance (F-Test ).
Zn2+ (ppm)
Source of variance
Sum of squares
Degrees of freedom
Mean square
F
Level of significance ofF
25
Between
180
1
180
0.56
P < 0.05
Within
2560
8
320
50
Between
532
1
532
3.82
P < 0.05
Within
1115
8
139The obtainedF-value 0.56 for 25 ppm Zn2+ is not statistically significant, since it is less than the critical F-value 5.32 for 1, 8 degrees of freedom at 0.05 level of significance. Therefore it is concluded that the influence of 25 ppm Zn2+ on the inhibition efficiencies of various concentrations of HE is not statistically significant. The obtained F-value 3.82 for 50 ppm Zn2+ is not statistically significant, since it is less than the critical F-value 5.32 for 1, 8 degrees of freedom at 0.05 level of significance. Therefore it is concluded that the influence of 50 ppm Zn2+ on the inhibition efficiencies of various concentrations of HE is not statistically significant.


### 3.2. Analysis of Polarization Curves

Polarization study has been used to detect the formation of protective film on the metal surface. When a protective film is formed on the metal surface, the linear polarization resistance (LPR) increases and the corrosion current (Icorr) decreases. The potentiodynamic polarization curves of carbon steel immersed in various test solutions are shown in Figure 1. The corrosion parameters, namely, corrosion potential (Ecorr), Tafel slopes (bc = cathodic; ba = anodic), linear polarization resistance (LPR), and corrosion current (Icorr) are given in Table 5. When carbon steel is immersed in sea water, the corrosion potential is −816 mV versus SCE. The formulation consisting of 8 mL of HE solution and 25 ppm of Zn2+ shifts the corrosion potential to −808 mV versus SCE. The corrosion potential shift is very small. This suggests that the HE-Zn2+ formulation functions as a mixed inhibitor controlling the anodic reaction and cathodic reaction, to the same extent. The corrosion current value and LPR value for sea water are 6.354×10-6A/cm2 and 6.500×103 ohm cm2. For the formulation of HE (8 mL) and Zn2+ (25 ppm), the corrosion current value is decreased to 5.234×10-6 A/cm2, and the LPR value is increased to 7.752×103 ohm cm2. This indicates that a protective film is formed on the metal surface. When a protective film is formed on the metal surface, LPR value increases and corrosion current value decreases [28].Table 5
Potentiodynamic polarization curves of carbon steel immersed in various test solution.
System
E
corr
mV versus SCE
b
c mV/decade
b
a mV/decade
LPR ohm cm2
I
corr
A/cm2
Sea water
−816
157
239
6.500 × 103
6.354 × 10−6
Sea water + (8 mL) HE + (25 ppm) Zn2+
−808
156
231
7.752 × 103
5.234 × 10−6Figure 1
Polarization curves of carbon steel immersed in various test solutions. (a) Sea water and (b) sea water + HE 8 mL +Zn2+ 25 ppm.


### 3.3. Analysis of AC Impedance Spectra

AC impedance spectra have been studied to detect the formation of film on the metal surface. If a protective film is formed, the charge transfer resistance increases and double-layer capacitance value decreases [29, 30]. The AC impedance spectra of carbon steel immersed in various solutions are shown in Figure 2. The AC impedance parameters, namely, charge transfer resistance (Rt) and double-layer capacitance (Cdl), (derived from Nyquist plot) are given in Table 6.Table 6
AC impedance parameters of carbon steel immersed in various test solutions.
System
R
t ohm cm2
C
dl F/cm2
Impedance valuelog (z/ohm)
Sea water
101.10
5.0445 × 10−8
2.060
HE (8 mL) + Zn2+ 25 ppm
123.68
4.1195 × 10−8
2.132Figure 2
AC impedance spectra of carbon steel immersed in various test solutions (Nyquist plots). (a) Sea water and (b) sea water + HE 8 mL +Zn2+ 25 ppm.The AC impedance parameters, namely, charge transfer resistance (Rt) and double-layer capacitance (Cdl), are given in Table 6. When carbon steel is immersed in sea water, Rt value is 101.10 ohm cm2 and Cdl value is 5.0445×10-8 F/cm2. When HE and Zn2+ are added to sea water, Rt value increases from 101.10 ohm cm2 to 123.68 ohm cm2 and the Cdl decreases from 5.0445×10-8 F/cm2 to 4.1195×10-8 F/cm2. This suggests that a protective film is formed on the surface of the metal. This is further supported by the increase in impedance value [log (z/ohm)] from 2.060 to 2.132 (Figure 3). This accounts for the very high IE of HE-Zn2+ system.Figure 3
AC impedance spectra of carbon steel immersed in various test solutions (impedance-Bode plots). (a) Sea water and (b) sea water + HE 8 mL +Zn2+ 25 ppm.In electrochemical studies, instantaneous corrosion rate is measured. But in weight-loss method, corrosion rate is measured after a longer period in the present study after one day. After a longer period, the protective film becomes more compact and stable. This accounts for the discrepancy in IE, obtained from AC impedance spectra and from weight-loss method (Figure4).Figure 4
AC impedance spectra of carbon steel immersed in various test solutions. (Phase-Bode plots). (a) Sea water, (b) sea water + HE 8 mL +Zn2+ 25 ppm.


### 3.4. Analysis of FTIR Spectra

The active principle in an aqueous extract of henna extract is Lawsone. The red-orange colour of the extract is due to Lawsone (see Scheme1).Scheme 1A few drops of an aqueous extract of henna were dried on a glass plate. A solid mass was obtained. Its FTIR spectrum of pure HE is shown in Figure5(a). The phenolic O–H stretch appeared at 3414 cm−1. The C=O stretching frequency appeared at 1632 cm−1. Thus Lawsone was characterized by IR spectroscopy [31](a) FTIR spectrum of pure henna extract evaporated to dryness and (b) FTIR spectrum of film formed on metal surface after immersion in sea water containing 8 mL of HE-25 ppmZn2+.
(a)
(b)The FTIR spectrum of the protective film formed on the metal after immersion in the sea water for one day containing 8 mL of HE and 25 ppm ofZn2+ is shown in Figure 5(b). It is found that the phenolic O–H stretch has shifted from 3414 cm−1 to 3415 cm−1. The C=O stretching frequency has shifted from 1632 cm−1 to 1613 cm−1. It was inferred that Lawsone has coordinate with Fe2+ through the phenolic oxygen, aromatic ring, and C=O group of the p-benzo quinone resulting in the formation of Fe2+-Lawsone complex on the anodic sites of the metal surface. The band at 1362 cm−1 due to the Zn(OH)2 formed on the cathodic sites of the metal surface. Thus FTIR spectral study leads to the conclusion that the protective film consists of Fe2+-Lawsone complex and Zn(OH)2 [32–35].


### 3.5. SEM Analysis of Metal Surface

The SEM images of magnification (×500) of carbon steel specimens immersed in sea water for one day in the absence and presence of inhibitor system are shown in Figure6 image (b) and image (c), respectively.SEM micrographs of; (a) polished carbon steel (control)—Magnification ×500, (b) carbon steel immersed in sea water—magnification ×500, (c) carbon steel immersed in sea water containing 8ml of HE and 25 ppm ofZn2+—magnification—×500.
(a)
(b)
(c)The SEM micrographs of polished carbon steel surface (control) in Figure6 image (a) show the smooth surface of the metal. This shows the absence of any corrosion products formed on the metal surface. The SEM micrographs of carbon steel surface immersed in sea water in Figure 6 image (b) show the roughness of the metal surface which indicates the corrosion of carbon steel in sea water. Figure 6 image (c) indicates that in presence of 8 mL of HE-Zn2+ (25 ppm) mixture in sea water, the surface coverage increases which in turn results in the formation of insoluble complex on the surface of the metal (HE-Zn2+ inhibitor complex), and the surface is covered by a thin layer of inhibitors which control the dissolution of carbon steel. Such results have been earlier [36].


### 3.6. Atomic Force Microscopy Characterization

AFM is a powerful technique to investigate the surface morphology at nano- to microscale and has become a new choice to study the influence of inhibitor on the generation and the progress of the corrosion at the metal/solution interface [36–38]. The three-dimensional (3D) AFM morphologies and the AFM cross-sectional profile for polished carbon steel surface (reference sample), carbon steel surface immersed in sea water (blank sample), and carbon steel surface immersed in sea water containing the formulation of 8 mL of HE and 25 ppm of Zn2+ are shown in Figure 7 images (a, d, g), (b, e, h), (c, f, i), respectively.Two-dimensional AFM images of the surface of (a) polished carbon steel (control); (b) carbon steel immersed in sea water (blank); (c) carbon steel immersed in sea water containing HE(8 mL) +Zn2+ (25 ppm). Three-dimensional AFM images of the surface of (d) polished carbon steel (control); (e) carbon steel immersed in sea water (blank); (f) carbon steel immersed in sea water containing HE(8 mL) + Zn2+ (25 ppm). AFM cross-sectional images of the surface of (g) polished carbon steel (control); (h) carbon steel immersed in sea water (blank); (i) carbon steel immersed in sea water containing HE(8 mL) + Zn2+ (25 ppm).
(a)
(b)
(c)
(d)
(e)
(f)
(g)
(h)
(i)


#### 3.6.1. Root Mean Square Roughness, Average and Roughness and Peak-to-Valley Value

AFM image analysis is performed to obtain the average roughness,Ra (the average deviation of all points roughness profile from a mean line over the evaluation length), root-mean-square roughness, Rq (the average of the measured height deviations taken within the evolution length and measured from the mean line), and the maximum peak-to-valley (p-v) height values (largest single peak-to-valley height in five adjoining sampling heights) [36]. Table 7 is a summary of (Rq), (Ra), (P-V) value for carbon steel surface immersed in different environment.Table 7
AFM data for carbon steel surface immersed in inhibited and uninhibited environment.
Samples
RMS (Rq) roughness (nm)
Average (Ra) roughness (nm)
Maximum peak-to-valley height (nm)
(
1
) Polished carbon steel (Control)
4.33
3.41
35.28
(
2
) Carbon steel immersed in sea water (blank)
17.10
13.58
92.28
(
3
) Carbon steel immersed in sea water + HE (8 mL)  + Zn2+ 25 (ppm)
11.8
7.65
27.47Figure7 (a, d, g) displays the surface topography of uncorroded metal surface. The values of Rq, Ra, and p-v height for the polished carbon steel surface (reference sample) are 4.3 nm, 3.4 nm, and 35.28 nm, respectively. The slight roughness observed on the polished carbon steel surface is due to atmospheric corrosion.Figure7 (b, e, h) displays the corroded metal surface with few pits in the absence of the inhibitor immersed in sea water. The (Rq), (Ra), (p-v) height values for the carbon steel surface are 17.10 nm, 13.58 nm, and 92.28 nm, respectively.These data suggest that carbon steel surface immersed in sea water has a greater surface roughness than the polished metal surface, which shows that the unprotected carbon steel surface is rougher and is due to the corrosion of the carbon steel in sea water environment.Figure7 (c, f, i) displays the surface after immersion in sea water containing 8 mL of Zn2+. The (Rq), (Ra) (p-v) height values for the carbon steel surface are 11.84 nm, 7.65 nm, and 27.47 nm, respectively. The (Rq), (Ra) (p-v) height values are considerably less in the inhibited environment compared to the uninhibited environment. These parameters confirm that the surface is smoother. The smoothness of the surface is due to the formation of a compact protective film of Fe2+-HE complex and Zn(oH)2 on the metal surface thereby inhibiting the corrosion of carbon steel [36].


### 3.7. Mechanism of Corrosion Inhibition

Weight-loss method reveals that the formulation consisting of 8 mL of HE and 25 ppm ofZn2+ offers 94% IE to carbon steel immersed in sea water. A synergistic effect exists between HE and Zn2+. Polarization study reveals that HE-Zn2+ system functions as a mixed inhibitor. AC impedance spectra reveal that a protective film is formed on the metal surface. FTIR spectra reveal that the protective film consists of Fe2+-Lawsone complex and Zn(OH)2.In order to explain the Previous facts in a holistic way, the following mechanism of corrosion inhibition is proposed. When the formulation consisting of sea water, henna extract, andZn2+ is prepared, there is formation of Zn2+-Lawsone complex in solution.(i)
When carbon steel is immersed in the solution, theZn2+-Lawsone complex diffuses from the bulk of the solution towards the metal surface.(ii)
On the metal surface,Zn2+-Lawsone complex is converted into Fe2+-Lawsone complex. Zn2+ is released Zn2+(6)lawsone+Fe2+→Fe2+-Lowsone+Zn2+.(iii)
The releasedZn2+ combines with OH− to form Zn(OH)2 on the cathodic sites:
(7)Zn2++2OH-→Zn(OH)2↓.(iv)
Thus the protective film consists ofFe2+-Lawsone complex and Zn(OH)2. This accounts for the synergistic effect.


## 3.1. Weight-Loss Study

The physicochemical parameters of sea water are given in Table1.Table 1
Water analysis (Thondi sea water, Tamil Nadu, India).
Parameters
Result
Total dissolved salts (mg/L)
30539 ppm
Electrical conductivity (micro mhos/cm)
44910
pH
7.06
Total hardness (CaCO3 equivalent)
100 ppm
Calcium as Ca (mg/L)
19 ppm
Magnesium as Mg (mg/L)
12 ppm
Sodium as Na (mg/L)
0
Chloride as Cl (mg/L)
11400 ppm
Fluoride as F (mg/L)
0
Free ammonia as NH3 (mg/L)
0
Sulphate as SO4 (mg/L)
6708 ppmThe calculated inhibition efficiencies (IEs) of henna extract in controlling the corrosion of carbon steel immersed in sea water both in the absence and presence of zinc ion have been tabulated in Table2. The calculated values indicate the ability of henna extract to be a good corrosion inhibitor. The inhibition efficiency is found to be enhanced in the presence of zinc ion. The formulation consisting of 8 mL of HE and 25 ppm of Zn2+ offers 94% inhibition efficiency. This mixture of inhibitors shows better IE than the individual inhibitors [23].Table 2
The corrosion inhibition efficiencies and the corresponding corrosion rates (millimeter per year) of HE-Zn2+ system.
Inhibitor HE (mL)
Zn2+ (ppm)
0
25
50
IE%
CR (mn/y)
IE%
CR (mm/y)
IE%
CR (mm/y)
0
—
0.1576
16
0.1323
47
0.0835
2
28
0.1134
41
0.0929
64
0.0567
4
35
0.1024
47
0.0835
76
0.0378
6
47
0.0835
71
0.0457
88
0.0189
8
65
0.0551
94
0.0094
82
0.0283
10
47
0.0835
64
0.0567
75
0.0394


### 3.1.1. Synergism Parameter (SI)

Synergism parameters are indications of synergistic effect existing between inhibitors [24, 25]. SI value is found to be greater than one indicating the synergistic effect existing between Zn2+ of concentrations 25 ppm and 50 ppm with various concentrations of HE. The results are given in Table 3.Table 3
Synergism    parameter (SI).
HE (mL)
θ
1
Zn2+  25 (ppm) (θ2)
HE-Zn2+  θ1+2′
S
I
Zn2+  50 (ppm) (θ2)
HE-Zn2+  θ1+2′
S
I
2
0.28
0.16
0.41
1.0250
0.47
0.64
1.06
4
0.35
0.16
0.47
1.0301
0.47
0.76
1.4354
6
0.47
0.16
0.71
1.5351
0.47
0.88
2.3408
8
0.65
0.16
0.94
4.9
0.47
0.82
1.0305
10
0.47
0.16
0.64
1.2366
0.47
0.75
1.1236In Table3 synergism parameters were calculated using the relation
(3)SI=1-θ1+21-θ1+2′,
where
(4)θ1+2=(θ1+θ2)-(θ1×θ2),
where θ1 = surface coverage of inhibitor (HE) and θ2 = surface coverage of inhibitor (Zn2+) and where
(5)θ1+2′=combinedsurfacecoverageofinhibitors(HE)and(Zn2+)surfacecoverage=IE%100.


### 3.1.2. Analysis of Variance (F-Test)

F-test is used to investigate if the synergistic effect existing between two inhibitors is statistically significant or not [26, 27]. If F-value is above 5.32 for 1, 8 degrees of freedom, the synergistic effect proves to be statistically significant. If it is below 5.32 for 1, 8 degrees of freedom indicating that it is statistically insignificant at a 0.05 level of significance. The results are given in Table 4.Table 4
Analysis of variance (F-Test ).
Zn2+ (ppm)
Source of variance
Sum of squares
Degrees of freedom
Mean square
F
Level of significance ofF
25
Between
180
1
180
0.56
P < 0.05
Within
2560
8
320
50
Between
532
1
532
3.82
P < 0.05
Within
1115
8
139The obtainedF-value 0.56 for 25 ppm Zn2+ is not statistically significant, since it is less than the critical F-value 5.32 for 1, 8 degrees of freedom at 0.05 level of significance. Therefore it is concluded that the influence of 25 ppm Zn2+ on the inhibition efficiencies of various concentrations of HE is not statistically significant. The obtained F-value 3.82 for 50 ppm Zn2+ is not statistically significant, since it is less than the critical F-value 5.32 for 1, 8 degrees of freedom at 0.05 level of significance. Therefore it is concluded that the influence of 50 ppm Zn2+ on the inhibition efficiencies of various concentrations of HE is not statistically significant.


## 3.1.1. Synergism Parameter (SI)

Synergism parameters are indications of synergistic effect existing between inhibitors [24, 25]. SI value is found to be greater than one indicating the synergistic effect existing between Zn2+ of concentrations 25 ppm and 50 ppm with various concentrations of HE. The results are given in Table 3.Table 3
Synergism    parameter (SI).
HE (mL)
θ
1
Zn2+  25 (ppm) (θ2)
HE-Zn2+  θ1+2′
S
I
Zn2+  50 (ppm) (θ2)
HE-Zn2+  θ1+2′
S
I
2
0.28
0.16
0.41
1.0250
0.47
0.64
1.06
4
0.35
0.16
0.47
1.0301
0.47
0.76
1.4354
6
0.47
0.16
0.71
1.5351
0.47
0.88
2.3408
8
0.65
0.16
0.94
4.9
0.47
0.82
1.0305
10
0.47
0.16
0.64
1.2366
0.47
0.75
1.1236In Table3 synergism parameters were calculated using the relation
(3)SI=1-θ1+21-θ1+2′,
where
(4)θ1+2=(θ1+θ2)-(θ1×θ2),
where θ1 = surface coverage of inhibitor (HE) and θ2 = surface coverage of inhibitor (Zn2+) and where
(5)θ1+2′=combinedsurfacecoverageofinhibitors(HE)and(Zn2+)surfacecoverage=IE%100.


## 3.1.2. Analysis of Variance (F-Test)

F-test is used to investigate if the synergistic effect existing between two inhibitors is statistically significant or not [26, 27]. If F-value is above 5.32 for 1, 8 degrees of freedom, the synergistic effect proves to be statistically significant. If it is below 5.32 for 1, 8 degrees of freedom indicating that it is statistically insignificant at a 0.05 level of significance. The results are given in Table 4.Table 4
Analysis of variance (F-Test ).
Zn2+ (ppm)
Source of variance
Sum of squares
Degrees of freedom
Mean square
F
Level of significance ofF
25
Between
180
1
180
0.56
P < 0.05
Within
2560
8
320
50
Between
532
1
532
3.82
P < 0.05
Within
1115
8
139The obtainedF-value 0.56 for 25 ppm Zn2+ is not statistically significant, since it is less than the critical F-value 5.32 for 1, 8 degrees of freedom at 0.05 level of significance. Therefore it is concluded that the influence of 25 ppm Zn2+ on the inhibition efficiencies of various concentrations of HE is not statistically significant. The obtained F-value 3.82 for 50 ppm Zn2+ is not statistically significant, since it is less than the critical F-value 5.32 for 1, 8 degrees of freedom at 0.05 level of significance. Therefore it is concluded that the influence of 50 ppm Zn2+ on the inhibition efficiencies of various concentrations of HE is not statistically significant.


## 3.2. Analysis of Polarization Curves

Polarization study has been used to detect the formation of protective film on the metal surface. When a protective film is formed on the metal surface, the linear polarization resistance (LPR) increases and the corrosion current (Icorr) decreases. The potentiodynamic polarization curves of carbon steel immersed in various test solutions are shown in Figure 1. The corrosion parameters, namely, corrosion potential (Ecorr), Tafel slopes (bc = cathodic; ba = anodic), linear polarization resistance (LPR), and corrosion current (Icorr) are given in Table 5. When carbon steel is immersed in sea water, the corrosion potential is −816 mV versus SCE. The formulation consisting of 8 mL of HE solution and 25 ppm of Zn2+ shifts the corrosion potential to −808 mV versus SCE. The corrosion potential shift is very small. This suggests that the HE-Zn2+ formulation functions as a mixed inhibitor controlling the anodic reaction and cathodic reaction, to the same extent. The corrosion current value and LPR value for sea water are 6.354×10-6A/cm2 and 6.500×103 ohm cm2. For the formulation of HE (8 mL) and Zn2+ (25 ppm), the corrosion current value is decreased to 5.234×10-6 A/cm2, and the LPR value is increased to 7.752×103 ohm cm2. This indicates that a protective film is formed on the metal surface. When a protective film is formed on the metal surface, LPR value increases and corrosion current value decreases [28].Table 5
Potentiodynamic polarization curves of carbon steel immersed in various test solution.
System
E
corr
mV versus SCE
b
c mV/decade
b
a mV/decade
LPR ohm cm2
I
corr
A/cm2
Sea water
−816
157
239
6.500 × 103
6.354 × 10−6
Sea water + (8 mL) HE + (25 ppm) Zn2+
−808
156
231
7.752 × 103
5.234 × 10−6Figure 1
Polarization curves of carbon steel immersed in various test solutions. (a) Sea water and (b) sea water + HE 8 mL +Zn2+ 25 ppm.


## 3.3. Analysis of AC Impedance Spectra

AC impedance spectra have been studied to detect the formation of film on the metal surface. If a protective film is formed, the charge transfer resistance increases and double-layer capacitance value decreases [29, 30]. The AC impedance spectra of carbon steel immersed in various solutions are shown in Figure 2. The AC impedance parameters, namely, charge transfer resistance (Rt) and double-layer capacitance (Cdl), (derived from Nyquist plot) are given in Table 6.Table 6
AC impedance parameters of carbon steel immersed in various test solutions.
System
R
t ohm cm2
C
dl F/cm2
Impedance valuelog (z/ohm)
Sea water
101.10
5.0445 × 10−8
2.060
HE (8 mL) + Zn2+ 25 ppm
123.68
4.1195 × 10−8
2.132Figure 2
AC impedance spectra of carbon steel immersed in various test solutions (Nyquist plots). (a) Sea water and (b) sea water + HE 8 mL +Zn2+ 25 ppm.The AC impedance parameters, namely, charge transfer resistance (Rt) and double-layer capacitance (Cdl), are given in Table 6. When carbon steel is immersed in sea water, Rt value is 101.10 ohm cm2 and Cdl value is 5.0445×10-8 F/cm2. When HE and Zn2+ are added to sea water, Rt value increases from 101.10 ohm cm2 to 123.68 ohm cm2 and the Cdl decreases from 5.0445×10-8 F/cm2 to 4.1195×10-8 F/cm2. This suggests that a protective film is formed on the surface of the metal. This is further supported by the increase in impedance value [log (z/ohm)] from 2.060 to 2.132 (Figure 3). This accounts for the very high IE of HE-Zn2+ system.Figure 3
AC impedance spectra of carbon steel immersed in various test solutions (impedance-Bode plots). (a) Sea water and (b) sea water + HE 8 mL +Zn2+ 25 ppm.In electrochemical studies, instantaneous corrosion rate is measured. But in weight-loss method, corrosion rate is measured after a longer period in the present study after one day. After a longer period, the protective film becomes more compact and stable. This accounts for the discrepancy in IE, obtained from AC impedance spectra and from weight-loss method (Figure4).Figure 4
AC impedance spectra of carbon steel immersed in various test solutions. (Phase-Bode plots). (a) Sea water, (b) sea water + HE 8 mL +Zn2+ 25 ppm.


## 3.4. Analysis of FTIR Spectra

The active principle in an aqueous extract of henna extract is Lawsone. The red-orange colour of the extract is due to Lawsone (see Scheme1).Scheme 1A few drops of an aqueous extract of henna were dried on a glass plate. A solid mass was obtained. Its FTIR spectrum of pure HE is shown in Figure5(a). The phenolic O–H stretch appeared at 3414 cm−1. The C=O stretching frequency appeared at 1632 cm−1. Thus Lawsone was characterized by IR spectroscopy [31](a) FTIR spectrum of pure henna extract evaporated to dryness and (b) FTIR spectrum of film formed on metal surface after immersion in sea water containing 8 mL of HE-25 ppmZn2+.
(a)
(b)The FTIR spectrum of the protective film formed on the metal after immersion in the sea water for one day containing 8 mL of HE and 25 ppm ofZn2+ is shown in Figure 5(b). It is found that the phenolic O–H stretch has shifted from 3414 cm−1 to 3415 cm−1. The C=O stretching frequency has shifted from 1632 cm−1 to 1613 cm−1. It was inferred that Lawsone has coordinate with Fe2+ through the phenolic oxygen, aromatic ring, and C=O group of the p-benzo quinone resulting in the formation of Fe2+-Lawsone complex on the anodic sites of the metal surface. The band at 1362 cm−1 due to the Zn(OH)2 formed on the cathodic sites of the metal surface. Thus FTIR spectral study leads to the conclusion that the protective film consists of Fe2+-Lawsone complex and Zn(OH)2 [32–35].


## 3.5. SEM Analysis of Metal Surface

The SEM images of magnification (×500) of carbon steel specimens immersed in sea water for one day in the absence and presence of inhibitor system are shown in Figure6 image (b) and image (c), respectively.SEM micrographs of; (a) polished carbon steel (control)—Magnification ×500, (b) carbon steel immersed in sea water—magnification ×500, (c) carbon steel immersed in sea water containing 8ml of HE and 25 ppm ofZn2+—magnification—×500.
(a)
(b)
(c)The SEM micrographs of polished carbon steel surface (control) in Figure6 image (a) show the smooth surface of the metal. This shows the absence of any corrosion products formed on the metal surface. The SEM micrographs of carbon steel surface immersed in sea water in Figure 6 image (b) show the roughness of the metal surface which indicates the corrosion of carbon steel in sea water. Figure 6 image (c) indicates that in presence of 8 mL of HE-Zn2+ (25 ppm) mixture in sea water, the surface coverage increases which in turn results in the formation of insoluble complex on the surface of the metal (HE-Zn2+ inhibitor complex), and the surface is covered by a thin layer of inhibitors which control the dissolution of carbon steel. Such results have been earlier [36].


## 3.6. Atomic Force Microscopy Characterization

AFM is a powerful technique to investigate the surface morphology at nano- to microscale and has become a new choice to study the influence of inhibitor on the generation and the progress of the corrosion at the metal/solution interface [36–38]. The three-dimensional (3D) AFM morphologies and the AFM cross-sectional profile for polished carbon steel surface (reference sample), carbon steel surface immersed in sea water (blank sample), and carbon steel surface immersed in sea water containing the formulation of 8 mL of HE and 25 ppm of Zn2+ are shown in Figure 7 images (a, d, g), (b, e, h), (c, f, i), respectively.Two-dimensional AFM images of the surface of (a) polished carbon steel (control); (b) carbon steel immersed in sea water (blank); (c) carbon steel immersed in sea water containing HE(8 mL) +Zn2+ (25 ppm). Three-dimensional AFM images of the surface of (d) polished carbon steel (control); (e) carbon steel immersed in sea water (blank); (f) carbon steel immersed in sea water containing HE(8 mL) + Zn2+ (25 ppm). AFM cross-sectional images of the surface of (g) polished carbon steel (control); (h) carbon steel immersed in sea water (blank); (i) carbon steel immersed in sea water containing HE(8 mL) + Zn2+ (25 ppm).
(a)
(b)
(c)
(d)
(e)
(f)
(g)
(h)
(i)


### 3.6.1. Root Mean Square Roughness, Average and Roughness and Peak-to-Valley Value

AFM image analysis is performed to obtain the average roughness,Ra (the average deviation of all points roughness profile from a mean line over the evaluation length), root-mean-square roughness, Rq (the average of the measured height deviations taken within the evolution length and measured from the mean line), and the maximum peak-to-valley (p-v) height values (largest single peak-to-valley height in five adjoining sampling heights) [36]. Table 7 is a summary of (Rq), (Ra), (P-V) value for carbon steel surface immersed in different environment.Table 7
AFM data for carbon steel surface immersed in inhibited and uninhibited environment.
Samples
RMS (Rq) roughness (nm)
Average (Ra) roughness (nm)
Maximum peak-to-valley height (nm)
(
1
) Polished carbon steel (Control)
4.33
3.41
35.28
(
2
) Carbon steel immersed in sea water (blank)
17.10
13.58
92.28
(
3
) Carbon steel immersed in sea water + HE (8 mL)  + Zn2+ 25 (ppm)
11.8
7.65
27.47Figure7 (a, d, g) displays the surface topography of uncorroded metal surface. The values of Rq, Ra, and p-v height for the polished carbon steel surface (reference sample) are 4.3 nm, 3.4 nm, and 35.28 nm, respectively. The slight roughness observed on the polished carbon steel surface is due to atmospheric corrosion.Figure7 (b, e, h) displays the corroded metal surface with few pits in the absence of the inhibitor immersed in sea water. The (Rq), (Ra), (p-v) height values for the carbon steel surface are 17.10 nm, 13.58 nm, and 92.28 nm, respectively.These data suggest that carbon steel surface immersed in sea water has a greater surface roughness than the polished metal surface, which shows that the unprotected carbon steel surface is rougher and is due to the corrosion of the carbon steel in sea water environment.Figure7 (c, f, i) displays the surface after immersion in sea water containing 8 mL of Zn2+. The (Rq), (Ra) (p-v) height values for the carbon steel surface are 11.84 nm, 7.65 nm, and 27.47 nm, respectively. The (Rq), (Ra) (p-v) height values are considerably less in the inhibited environment compared to the uninhibited environment. These parameters confirm that the surface is smoother. The smoothness of the surface is due to the formation of a compact protective film of Fe2+-HE complex and Zn(oH)2 on the metal surface thereby inhibiting the corrosion of carbon steel [36].


## 3.6.1. Root Mean Square Roughness, Average and Roughness and Peak-to-Valley Value

AFM image analysis is performed to obtain the average roughness,Ra (the average deviation of all points roughness profile from a mean line over the evaluation length), root-mean-square roughness, Rq (the average of the measured height deviations taken within the evolution length and measured from the mean line), and the maximum peak-to-valley (p-v) height values (largest single peak-to-valley height in five adjoining sampling heights) [36]. Table 7 is a summary of (Rq), (Ra), (P-V) value for carbon steel surface immersed in different environment.Table 7
AFM data for carbon steel surface immersed in inhibited and uninhibited environment.
Samples
RMS (Rq) roughness (nm)
Average (Ra) roughness (nm)
Maximum peak-to-valley height (nm)
(
1
) Polished carbon steel (Control)
4.33
3.41
35.28
(
2
) Carbon steel immersed in sea water (blank)
17.10
13.58
92.28
(
3
) Carbon steel immersed in sea water + HE (8 mL)  + Zn2+ 25 (ppm)
11.8
7.65
27.47Figure7 (a, d, g) displays the surface topography of uncorroded metal surface. The values of Rq, Ra, and p-v height for the polished carbon steel surface (reference sample) are 4.3 nm, 3.4 nm, and 35.28 nm, respectively. The slight roughness observed on the polished carbon steel surface is due to atmospheric corrosion.Figure7 (b, e, h) displays the corroded metal surface with few pits in the absence of the inhibitor immersed in sea water. The (Rq), (Ra), (p-v) height values for the carbon steel surface are 17.10 nm, 13.58 nm, and 92.28 nm, respectively.These data suggest that carbon steel surface immersed in sea water has a greater surface roughness than the polished metal surface, which shows that the unprotected carbon steel surface is rougher and is due to the corrosion of the carbon steel in sea water environment.Figure7 (c, f, i) displays the surface after immersion in sea water containing 8 mL of Zn2+. The (Rq), (Ra) (p-v) height values for the carbon steel surface are 11.84 nm, 7.65 nm, and 27.47 nm, respectively. The (Rq), (Ra) (p-v) height values are considerably less in the inhibited environment compared to the uninhibited environment. These parameters confirm that the surface is smoother. The smoothness of the surface is due to the formation of a compact protective film of Fe2+-HE complex and Zn(oH)2 on the metal surface thereby inhibiting the corrosion of carbon steel [36].


## 3.7. Mechanism of Corrosion Inhibition

Weight-loss method reveals that the formulation consisting of 8 mL of HE and 25 ppm ofZn2+ offers 94% IE to carbon steel immersed in sea water. A synergistic effect exists between HE and Zn2+. Polarization study reveals that HE-Zn2+ system functions as a mixed inhibitor. AC impedance spectra reveal that a protective film is formed on the metal surface. FTIR spectra reveal that the protective film consists of Fe2+-Lawsone complex and Zn(OH)2.In order to explain the Previous facts in a holistic way, the following mechanism of corrosion inhibition is proposed. When the formulation consisting of sea water, henna extract, andZn2+ is prepared, there is formation of Zn2+-Lawsone complex in solution.(i)
When carbon steel is immersed in the solution, theZn2+-Lawsone complex diffuses from the bulk of the solution towards the metal surface.(ii)
On the metal surface,Zn2+-Lawsone complex is converted into Fe2+-Lawsone complex. Zn2+ is released Zn2+(6)lawsone+Fe2+→Fe2+-Lowsone+Zn2+.(iii)
The releasedZn2+ combines with OH− to form Zn(OH)2 on the cathodic sites:
(7)Zn2++2OH-→Zn(OH)2↓.(iv)
Thus the protective film consists ofFe2+-Lawsone complex and Zn(OH)2. This accounts for the synergistic effect.


## 4. Conclusions

The present study leads to the following conclusions.(1)
The formulation consisting of 8 mL HE and 25 ppmZn2+ has 94% inhibition efficiency to carbon steel immersed in sea water.(2)
Polarization study reveals that HE-Zn2+ system functions as a mixed inhibitor.(3)
AC impedance spectra reveal that a protective film is formed on the metal surface.(4)
FTIR spectra reveal that the protective film consists ofFe2+-Lawsone complex and Zn(OH)2.



---

*Source: 574321-2012-12-16.xml*