# Corrosion and Inhibition Effects of Mild Steel in Hydrochloric Acid Solutions Containing Organophosphonic Acid

Authors: Manish Gupta; Jyotsna Mishra; K. S. Pitre
Journal: International Journal of Corrosion
 (2013)

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## Abstract

A study has been made on the mechanism of corrosion of mild steel and the effect of nitrilo trimethylene phosphonic (NTMP) acid as a corrosion inhibitor in acidic medium, that is, 10% HC1 using the weight loss method and electrochemical techniques, that is, potentiodynamic and galvanostatic polarization measurements. Although corrosion is a long-time process, but it takes place at a faster rate in the beginning which goes on decreasing with due course of time. The above-mentioned methods of corrosion rate determination furnish an average value for a long-time interval. Looking at the versatility and minimum detection limit of the voltammetric method, the authors have developed a new voltammetric method for the determination of corrosion rate at short-time intervals. The results of corrosion of mild steel in 10% HC1 solution with and without NTMP inhibitor at short-time intervals have been reported. The corrosion inhibition efficiency of NTMP is 93% after 24 h.


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## Body

## 1. Introduction

Mild steel is a major material of construction. It is extensively used in chemical and allied industries for handling alkalis, acids, and salt solutions [1]. Hydrochloric (HC1) acid is the solvent most often employed for chemical cleaning. It attacks a wide range of scales [2]. Nitrogen compounds constitute the largest class of inhibitors for hydrochloric acid [3] solution. During the past decade a number of polymers and phosphonates have been used in different inhibitor compositions in aqueous and acid solutions [4]. They form stable complexes and some times act as detergent also. The role of inhibitor is the prevention of the adsorption of aggressive anions and reduction of the dissolution rate of the passivating oxide. In the present paper we have studied the action and effectiveness of nitrilo trimethylene phosphonic acid (NTMP) as inhibitor for corrosion of carbon steel in (10% HCl) acid solution.Most of the methods are proposed in the field of corrosion rate determination, but they furnish an average value for a long-time interval [5]. Looking at the sensitivity and minimum detection limits [6, 7] of polarographic techniques, that is, direct current polarography (DCP), differential pulse polarography (DPP), and voltammetric, that is, differential pulse anodic stripping voltammetric (DPASV) method has been used to determine the corrosion rates and inhibition efficiency of NTMP of mild steel in 10% HC1 solution at short time intervals. Significantly it has also been possible to determine the corrosion rates simultaneously with respect to Fe(II) and Fe(III) which is not possible using other methods prevalent in the field. Their results have been discussed in the paper.


## 2. Experimental

### 2.1. Chemicals and Reagents

All the chemicals used were of anal R/BDH grade. The inhibitor (NTMP, 99.9% pure) was synthesized at the Central Research Institute For Chemistry, Budapest, Hungery. Experiments were carried out in 10% HC1 solution. Mild Steel specimens50*20*4 mm size having composition (C-0.23%, P-0.05%, S-0.055%) were used in experiments. These specimen were polished following the usual procedure [8]. All the measurements were carried out at room temperature 30°C.


### 2.2. Gravimetric Measurements

After abrasion with 600 grit papers all samples were degreased with acetone, pickled in 15% HC1 solution, washed under running tap water, rinsed in distilled water, dried with acetone, and weighed. The variation of solution corrosiveness and corrosion rate as a function of time was investigated using the planned interval test technique [8]. Three weighed specimens were introduced into the solution. These were removed after various exposure times, giving damage factors referred to as A1 for 22 h, At for 3 h, and At+1 for 25 h. A fourth specimen (B) was introduced into the cell for the last period for 3 h. The values of A1, B (measured values) and Ac (calculated from A1, and At+1) relate to the same testing period, but under different conditions. This technique is schematically illustrated in Figure 1.Figure 1
Procedure of “planned interval test” technique identical specimen all placed in the some corrosive fluid imposed conditions of the test kept constant for entire timeAt+1. The terms Al,   At,  At+1, and B represent corrosive damage experienced by each specimen, Ac is calculated by substracting At from At+1.After each experiment, the specimens were cleaned by washing in water and acetone then weighed; from these measurements the corrosion rates were calculated. The methods of evaluating the results and combinations of situations are summarized in Table1.Table 1
Results of planned interval test (PIT) on mild steel in 10% HCl solution.
S. no.
Period
Without inhibitor
With inhibitor
% inhibition
Concentration(milligram × 102)
Corrosion rate(milligram centimeter−2 hour−1  × 102)
Concentration(milligram ×  102)
Corrosion rate(milligram centimeter−2 hour−1  × 102)
1
A
1 (3 h)
18.96
21.2
5.21
5.82
72
2
A
t (22 h)
31.05
4.70
6.32
0.96
79
3
A
t
+
1 (22 + 3 h)
34.58
4.64
6.40
0.85
82
4
B (3 h)
3.33
3.72
0.50
0.50
96
5
A
C (3 h)
3.53
3.94
0.08
0.08
97


### 2.3. Potentiodynamic Polarization Measurement

The specimen was molded into epoxy resin in order to cover the sides of the rod, while the circular cross section area of the cylinder was exposed to the solution. The specimen was polished and washed with distilled water before the experiment. The potential scan path was 50 mV/5 min. The observed current was plotted against applied potential.


### 2.4. Galvanostatic Polarization Measurement

The galvanostatic polarization measurements were also made in 10% HC1 solution on 10 cm2 cylindrical surface of the working electrode. Saturated calomel electrode (SCE) was used as a reference electrode. An AJCO Electronics (model VT S5016) attached to a multiplex galvanometer was used for the galvanostatic studies. The polarization currents were measured by using a cycle of 10 s and changing potential by 10 mV. Both anodic and cathodic cycles were used alternately. The corrosion current density was plotted against the applied potential.


### 2.5. Voltammetric and Polarographic Measurements

An Elico Pulse Polarograph (model CL-90) was used for these studies. A cell consisting of three electrodes, namely, saturated calomel electrode as a reference, a coiled platinum wire as an auxiliary, and a dropping mercury electrode (for DCP and DPP)/glassy carbon fiber electrode (for DPASV) as a working electrode, was used. The test specimens were polished as discussed earlier, and one such specimen was suspended in 10% HCl solution at room temperature (25°C). Nitrogen gas was bubbled throughout the solution through the experiment to avoid the oxidation of dissolved Fe(II) to Fe(III). A definite aliquot of the solution was withdrawn from the test solution at different intervals (5, 10, 20 min, 1, 2, and 24 h), and polarograms and voltammograms were recorded in deaerated 0.1 M ammonium tartrate + 0.001% gelatin at pH9.0±0.1. The pH of the test solution was adjusted using ammonia solution. A similar experiment was performed using 170 mg/L NTMP in 10% HC1 solution.


## 2.1. Chemicals and Reagents

All the chemicals used were of anal R/BDH grade. The inhibitor (NTMP, 99.9% pure) was synthesized at the Central Research Institute For Chemistry, Budapest, Hungery. Experiments were carried out in 10% HC1 solution. Mild Steel specimens50*20*4 mm size having composition (C-0.23%, P-0.05%, S-0.055%) were used in experiments. These specimen were polished following the usual procedure [8]. All the measurements were carried out at room temperature 30°C.


## 2.2. Gravimetric Measurements

After abrasion with 600 grit papers all samples were degreased with acetone, pickled in 15% HC1 solution, washed under running tap water, rinsed in distilled water, dried with acetone, and weighed. The variation of solution corrosiveness and corrosion rate as a function of time was investigated using the planned interval test technique [8]. Three weighed specimens were introduced into the solution. These were removed after various exposure times, giving damage factors referred to as A1 for 22 h, At for 3 h, and At+1 for 25 h. A fourth specimen (B) was introduced into the cell for the last period for 3 h. The values of A1, B (measured values) and Ac (calculated from A1, and At+1) relate to the same testing period, but under different conditions. This technique is schematically illustrated in Figure 1.Figure 1
Procedure of “planned interval test” technique identical specimen all placed in the some corrosive fluid imposed conditions of the test kept constant for entire timeAt+1. The terms Al,   At,  At+1, and B represent corrosive damage experienced by each specimen, Ac is calculated by substracting At from At+1.After each experiment, the specimens were cleaned by washing in water and acetone then weighed; from these measurements the corrosion rates were calculated. The methods of evaluating the results and combinations of situations are summarized in Table1.Table 1
Results of planned interval test (PIT) on mild steel in 10% HCl solution.
S. no.
Period
Without inhibitor
With inhibitor
% inhibition
Concentration(milligram × 102)
Corrosion rate(milligram centimeter−2 hour−1  × 102)
Concentration(milligram ×  102)
Corrosion rate(milligram centimeter−2 hour−1  × 102)
1
A
1 (3 h)
18.96
21.2
5.21
5.82
72
2
A
t (22 h)
31.05
4.70
6.32
0.96
79
3
A
t
+
1 (22 + 3 h)
34.58
4.64
6.40
0.85
82
4
B (3 h)
3.33
3.72
0.50
0.50
96
5
A
C (3 h)
3.53
3.94
0.08
0.08
97


## 2.3. Potentiodynamic Polarization Measurement

The specimen was molded into epoxy resin in order to cover the sides of the rod, while the circular cross section area of the cylinder was exposed to the solution. The specimen was polished and washed with distilled water before the experiment. The potential scan path was 50 mV/5 min. The observed current was plotted against applied potential.


## 2.4. Galvanostatic Polarization Measurement

The galvanostatic polarization measurements were also made in 10% HC1 solution on 10 cm2 cylindrical surface of the working electrode. Saturated calomel electrode (SCE) was used as a reference electrode. An AJCO Electronics (model VT S5016) attached to a multiplex galvanometer was used for the galvanostatic studies. The polarization currents were measured by using a cycle of 10 s and changing potential by 10 mV. Both anodic and cathodic cycles were used alternately. The corrosion current density was plotted against the applied potential.


## 2.5. Voltammetric and Polarographic Measurements

An Elico Pulse Polarograph (model CL-90) was used for these studies. A cell consisting of three electrodes, namely, saturated calomel electrode as a reference, a coiled platinum wire as an auxiliary, and a dropping mercury electrode (for DCP and DPP)/glassy carbon fiber electrode (for DPASV) as a working electrode, was used. The test specimens were polished as discussed earlier, and one such specimen was suspended in 10% HCl solution at room temperature (25°C). Nitrogen gas was bubbled throughout the solution through the experiment to avoid the oxidation of dissolved Fe(II) to Fe(III). A definite aliquot of the solution was withdrawn from the test solution at different intervals (5, 10, 20 min, 1, 2, and 24 h), and polarograms and voltammograms were recorded in deaerated 0.1 M ammonium tartrate + 0.001% gelatin at pH9.0±0.1. The pH of the test solution was adjusted using ammonia solution. A similar experiment was performed using 170 mg/L NTMP in 10% HC1 solution.


## 3. Results and Discussion

### 3.1. Gravimetric Measurement

The results of weight loss determination by gravimetric are shown as corrosion rates in the presence and absence of NTMP inhibitor in Table1. The data obtained for the periods A1 and B suggest that solution corrosiveness increases in the base solution (10% HCl) while in the presence of NTMP it decreases (B<A1). The higher corrosivity of the solution can be attributed to the increase in concentration of iron ions in the solution. This suggests that NTMP molecules reduce the stimulating effect of iron ions. By comparison with the data relating to B and Ac it can be concluded that metal corrodibility decreases (Ac<B) as a function of time in the presence of the NTMP. This indicates that the formation of an inhibitor film requires some time; that is, the inhibition process has an induction period. The increased inhibition effect of NTMP may be explained on the argument that nitrogen of NTMP facilitates complex formation between metal ions coming out in the solution with NTMP and also the formation of complexed film at the metal surface.


### 3.2. Potentiodynamic Polarization Measurements

Figure2 shows the results of potentiodynamic polarization measurements in the presence and absence of NTMP inhibitor in 10% HC1 solution. The curve clearly shows the cathodic and anodic inhibition properties of inhibitor. It is quite clear from the curves that the corrosion potentials are more positive (anodic) than the rest of potentials. The inhibitor shows a remarked effect in lowering the current. However, in the cathodic region the effect is not significant. Thus it could be conduced that NTMP affects the anodic part of the corrosion process. Thus they work as anodic protector.Figure 2
Potentiodynamic polarization curves of carbon steel in 10% HC1 with, and without inhibitor.


### 3.3. Galvanostatic Polarization Measurement

Figure3 depicts the galvanostatic polarization curves for the corrosion of mild steel in 10% HC1 solution with and without NTMP inhibitor. It is clear from the figure that both anodic and cathodic currents decrease in the presence of inhibitor. However, the corrosion potentials are shifted to more electropositive values with NTMP inhibitor, but the Tafel slopes of the curves remain almost the same. The shift in corrosion potential with NTMP is +50 mV. Thus, it could be concluded that the film formed by the inhibitor acts through the blocking effect, mainly on the anodic reaction.Figure 3
Galvanostatic polarization curves of carbon steel in 10% HC1 with and without inhibitor.


### 3.4. Voltammetric and Polarographic Measurements

Figures4(a), 4(b), and 4(c) are the direct current polarogram (DCP), differential pulse polarogram (DPP), and differential pulse anodic stripping voltammogram (DPASV) for corrosion sample after 5 min in 0.1 M ammonium tartrate and 0.001% gelatin at pH 9.0±0.1. The half-wave potential (E1/2)/peak potential (EP) values for Fe(II) and Fe(III) are −0.81 V/−0.83 V and −1.23/−1.25 V, respectively, in DCP/DPP mode, and EP values are −0.4 V and −0.72 V for Fe(II) and Fe(III) in DPASV mode [9]. Figure 5 shows the DPASV curves of corrosion mixtures of mild steel at different time intervals. These curves show that by using voltammetric and polarographic methods, it is possible to analyze the presence of Fe(II) and Fe(III) in the solution, simultaneously. Fe(II) and Fe(III) produce two separate well-defined polarographic peaks in each case. The wave height/peak height for Fe(II) and Fe(III) over the whole of the working concentration range in DCP, DPP, and DPASV modes is proportional to the Fe(II) and Fe(III) concentrations. The corrosion rates at different time intervals with respect to Fe(II) and Fe(III) are summarized in Table 2.Table 2
Corrosion rates with respect to concentration of Fe(II), Fe(III), and total Fe(II + III) for mild steel in 10% HCl without inhibitor using voltammetric  and  polarographic methods.
(a)
DPP mode
Time
Fe(II)
Fe(III)
Total Fe(II + III)
Conc*
CR**
Conc*
CR**
Conc*
CR**
(1) 5 min
5.91
2.37
8.23
3.31
14.14
5.69
(2) 10 min
6.13
1.23
9.89
1.99
16.02
3.22
(3) 20 min
7.93
0.79
11.21
1.12
19.14
1.92
(4) 1 h
12.16
0.40
12.55
0.42
24.17
0.83
(5) 2 h
15.98
0.26
16.81
0.28
32.78
0.55
(6) 24 h
50.57
0.07
56.17
0.07
107.28
0.15
(b)
DPASV mode
Time
Fe(II)
Fe(III)
Total  Fe(II + III)
Conc*
CR**
Conc*
CR**
Conc*
CR**
(1) 5 min
5.92
2.38
8.24
3.31
14.16
5.70
(2) 10 min
6.15
1.23
9.87
1.98
16.02
3.22
(3) 20 min
7.93
0.79
11.21
1.12
19.11
1.92
(4) 1 h
12.16
0.40
12.50
0.42
24.57
0.82
(5) 2 h
15.98
0.26
16.80
0.28
32.78
0.55
(6) 24 h
50.57
0.07
56.17
0.07
107.28
0.15
*Corrosion concentration × 102 milligram.
**Corrosion rate× 102 milligram centimeter−2 hour−1.Figure 4
(a) Direct current polarogram, (b) Differential pulse polarogram, (c) Differential pulse anodic stripping voltammogram of corrosion solution (10 mL) in 0.1 M ammonium tartrate + 0.001% gelatin at pH9.0±0.1 after 5 min. of exposure.Figure 5
Differential pulse anodic stripping voltammogram of the corrosion solution of carbon steel in 10% HC1 at different time intervals in ammonium tartrate + 0.01% gelatin at pH9.0±0.1.Figure6 shows the DPASV curves for the corrosion solution to which NTMP (170 mg/L) has been added. A shift in the EP value for each of the Fe(II) and Fe(III) indicates the M: NTMP complex formation in the solution.Figure 6
Differential pulse anodic stripping voltammogram of the corrosion solution of carbon steel, 0.1 M ammonium tartrate + 0.001% gelatin at pH9.0±0.1 at different time intervals in presence of Inhibitor.Since the shift inE1/2 and EP values of Fe(III) is more than that of Fe(II) in complexed system, it could be concluded that Fe(III) is more reactive in complex formation with NTMP than Fe(II) [10]. The corrosion rates in the presence of NTMP inhibitor have been tabulated in Table 3. A perusal of the data in Tables 1, 2, and 3 clearly shows that the rate of corrosion is very fast in the beginning of the experiment, which goes on decreasing with due course of time. The corrosion inhibition efficiency of NTMP is 55%, 69%, 75%, 81%, and 84% after 5, 10, 20 min, 1, and 2 h, and it becomes 93% after 24 h, respectively.Table 3
Corrosion rates with respect to Fe(II) + Fe(III) for mild steel in 10% HCl solution with NTMP inhibitor (170 mg/L).
Time
DPASV mode
DPP mode
Conc*
CR**
% inhibition
Conc*
CR**
% inibition
(1) 5 min
6.28
2.530
55.6
6.26
2.52
55.7
(2) 10 min
6.41
1.290
59.9
6.41
1.290
59.9
(3) 20 min
6.99
0.693
63.9
6.99
0.693
63.9
(4) 1 h
7.18
0.235
71.6
7.02
0.235
71.6
(5) 2 h
7.21
0.119
80.0
7.12
0.119
80.6
(6) 24 h
7.91
0.010
93.3
7.81
0.010
93.3
*Corrosion concentration × 102 milligram.
**Corrosion rate× 102 milligram centimeter−2 hour−1.Thus, the inhibitive efficiency of NTMP (organophosphonic acid) will depend on the structure of the protective inhibitor film formed by the dissolved Fe(II) and Fe(III) species on the metal surface. The NTMP used as an inhibitor for the present study has the following structure:The nitrogen atom acts as the reaction centre in the complexation reaction with the metal ions.A comparison of the corrosion rate data as calculated by gravimetric, galvanostatic, potentiodynamic polarization, and voltammetric methods clearly speaks that NTMP is a good inhibitor for corrosion inhibition of mild steel in acidic medium, that is, 10% HC1.From the above data and ongoing discussion, it could be concluded that polarographic and voltammetric methods give dependable results for corrosion rates with respect to each species appearing in the solution, that is, Fe(II) and Fe(III). It is also clear that the proposed polarographic/voltammetric methods are highly sensitive for corrosion rate determination at short-time intervals and that too with respect to each species present in the solution, which is otherwise not possible using methods prevalent in the field of corrosion rate determination [11, 12] and also in the field of trace analysis, namely, atomic absorption spectrometry (AAS).


## 3.1. Gravimetric Measurement

The results of weight loss determination by gravimetric are shown as corrosion rates in the presence and absence of NTMP inhibitor in Table1. The data obtained for the periods A1 and B suggest that solution corrosiveness increases in the base solution (10% HCl) while in the presence of NTMP it decreases (B<A1). The higher corrosivity of the solution can be attributed to the increase in concentration of iron ions in the solution. This suggests that NTMP molecules reduce the stimulating effect of iron ions. By comparison with the data relating to B and Ac it can be concluded that metal corrodibility decreases (Ac<B) as a function of time in the presence of the NTMP. This indicates that the formation of an inhibitor film requires some time; that is, the inhibition process has an induction period. The increased inhibition effect of NTMP may be explained on the argument that nitrogen of NTMP facilitates complex formation between metal ions coming out in the solution with NTMP and also the formation of complexed film at the metal surface.


## 3.2. Potentiodynamic Polarization Measurements

Figure2 shows the results of potentiodynamic polarization measurements in the presence and absence of NTMP inhibitor in 10% HC1 solution. The curve clearly shows the cathodic and anodic inhibition properties of inhibitor. It is quite clear from the curves that the corrosion potentials are more positive (anodic) than the rest of potentials. The inhibitor shows a remarked effect in lowering the current. However, in the cathodic region the effect is not significant. Thus it could be conduced that NTMP affects the anodic part of the corrosion process. Thus they work as anodic protector.Figure 2
Potentiodynamic polarization curves of carbon steel in 10% HC1 with, and without inhibitor.


## 3.3. Galvanostatic Polarization Measurement

Figure3 depicts the galvanostatic polarization curves for the corrosion of mild steel in 10% HC1 solution with and without NTMP inhibitor. It is clear from the figure that both anodic and cathodic currents decrease in the presence of inhibitor. However, the corrosion potentials are shifted to more electropositive values with NTMP inhibitor, but the Tafel slopes of the curves remain almost the same. The shift in corrosion potential with NTMP is +50 mV. Thus, it could be concluded that the film formed by the inhibitor acts through the blocking effect, mainly on the anodic reaction.Figure 3
Galvanostatic polarization curves of carbon steel in 10% HC1 with and without inhibitor.


## 3.4. Voltammetric and Polarographic Measurements

Figures4(a), 4(b), and 4(c) are the direct current polarogram (DCP), differential pulse polarogram (DPP), and differential pulse anodic stripping voltammogram (DPASV) for corrosion sample after 5 min in 0.1 M ammonium tartrate and 0.001% gelatin at pH 9.0±0.1. The half-wave potential (E1/2)/peak potential (EP) values for Fe(II) and Fe(III) are −0.81 V/−0.83 V and −1.23/−1.25 V, respectively, in DCP/DPP mode, and EP values are −0.4 V and −0.72 V for Fe(II) and Fe(III) in DPASV mode [9]. Figure 5 shows the DPASV curves of corrosion mixtures of mild steel at different time intervals. These curves show that by using voltammetric and polarographic methods, it is possible to analyze the presence of Fe(II) and Fe(III) in the solution, simultaneously. Fe(II) and Fe(III) produce two separate well-defined polarographic peaks in each case. The wave height/peak height for Fe(II) and Fe(III) over the whole of the working concentration range in DCP, DPP, and DPASV modes is proportional to the Fe(II) and Fe(III) concentrations. The corrosion rates at different time intervals with respect to Fe(II) and Fe(III) are summarized in Table 2.Table 2
Corrosion rates with respect to concentration of Fe(II), Fe(III), and total Fe(II + III) for mild steel in 10% HCl without inhibitor using voltammetric  and  polarographic methods.
(a)
DPP mode
Time
Fe(II)
Fe(III)
Total Fe(II + III)
Conc*
CR**
Conc*
CR**
Conc*
CR**
(1) 5 min
5.91
2.37
8.23
3.31
14.14
5.69
(2) 10 min
6.13
1.23
9.89
1.99
16.02
3.22
(3) 20 min
7.93
0.79
11.21
1.12
19.14
1.92
(4) 1 h
12.16
0.40
12.55
0.42
24.17
0.83
(5) 2 h
15.98
0.26
16.81
0.28
32.78
0.55
(6) 24 h
50.57
0.07
56.17
0.07
107.28
0.15
(b)
DPASV mode
Time
Fe(II)
Fe(III)
Total  Fe(II + III)
Conc*
CR**
Conc*
CR**
Conc*
CR**
(1) 5 min
5.92
2.38
8.24
3.31
14.16
5.70
(2) 10 min
6.15
1.23
9.87
1.98
16.02
3.22
(3) 20 min
7.93
0.79
11.21
1.12
19.11
1.92
(4) 1 h
12.16
0.40
12.50
0.42
24.57
0.82
(5) 2 h
15.98
0.26
16.80
0.28
32.78
0.55
(6) 24 h
50.57
0.07
56.17
0.07
107.28
0.15
*Corrosion concentration × 102 milligram.
**Corrosion rate× 102 milligram centimeter−2 hour−1.Figure 4
(a) Direct current polarogram, (b) Differential pulse polarogram, (c) Differential pulse anodic stripping voltammogram of corrosion solution (10 mL) in 0.1 M ammonium tartrate + 0.001% gelatin at pH9.0±0.1 after 5 min. of exposure.Figure 5
Differential pulse anodic stripping voltammogram of the corrosion solution of carbon steel in 10% HC1 at different time intervals in ammonium tartrate + 0.01% gelatin at pH9.0±0.1.Figure6 shows the DPASV curves for the corrosion solution to which NTMP (170 mg/L) has been added. A shift in the EP value for each of the Fe(II) and Fe(III) indicates the M: NTMP complex formation in the solution.Figure 6
Differential pulse anodic stripping voltammogram of the corrosion solution of carbon steel, 0.1 M ammonium tartrate + 0.001% gelatin at pH9.0±0.1 at different time intervals in presence of Inhibitor.Since the shift inE1/2 and EP values of Fe(III) is more than that of Fe(II) in complexed system, it could be concluded that Fe(III) is more reactive in complex formation with NTMP than Fe(II) [10]. The corrosion rates in the presence of NTMP inhibitor have been tabulated in Table 3. A perusal of the data in Tables 1, 2, and 3 clearly shows that the rate of corrosion is very fast in the beginning of the experiment, which goes on decreasing with due course of time. The corrosion inhibition efficiency of NTMP is 55%, 69%, 75%, 81%, and 84% after 5, 10, 20 min, 1, and 2 h, and it becomes 93% after 24 h, respectively.Table 3
Corrosion rates with respect to Fe(II) + Fe(III) for mild steel in 10% HCl solution with NTMP inhibitor (170 mg/L).
Time
DPASV mode
DPP mode
Conc*
CR**
% inhibition
Conc*
CR**
% inibition
(1) 5 min
6.28
2.530
55.6
6.26
2.52
55.7
(2) 10 min
6.41
1.290
59.9
6.41
1.290
59.9
(3) 20 min
6.99
0.693
63.9
6.99
0.693
63.9
(4) 1 h
7.18
0.235
71.6
7.02
0.235
71.6
(5) 2 h
7.21
0.119
80.0
7.12
0.119
80.6
(6) 24 h
7.91
0.010
93.3
7.81
0.010
93.3
*Corrosion concentration × 102 milligram.
**Corrosion rate× 102 milligram centimeter−2 hour−1.Thus, the inhibitive efficiency of NTMP (organophosphonic acid) will depend on the structure of the protective inhibitor film formed by the dissolved Fe(II) and Fe(III) species on the metal surface. The NTMP used as an inhibitor for the present study has the following structure:The nitrogen atom acts as the reaction centre in the complexation reaction with the metal ions.A comparison of the corrosion rate data as calculated by gravimetric, galvanostatic, potentiodynamic polarization, and voltammetric methods clearly speaks that NTMP is a good inhibitor for corrosion inhibition of mild steel in acidic medium, that is, 10% HC1.From the above data and ongoing discussion, it could be concluded that polarographic and voltammetric methods give dependable results for corrosion rates with respect to each species appearing in the solution, that is, Fe(II) and Fe(III). It is also clear that the proposed polarographic/voltammetric methods are highly sensitive for corrosion rate determination at short-time intervals and that too with respect to each species present in the solution, which is otherwise not possible using methods prevalent in the field of corrosion rate determination [11, 12] and also in the field of trace analysis, namely, atomic absorption spectrometry (AAS).



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