Inhibition of Mild Steel Corrosion in Acid Medium

Since inhibition is the simplest mechanism used for mitigating the corrosion of metals and alloys, particularly in acidic environments, the present work aims to investigate the inhibiting effect of N-benzyl-N?-phenyl thiourea (BPTU) and N-cyclohexyl-N?-phenyl thiourea (CPTU) on mild steel corrosion in 0.1M HCl medium using the Tafel extrapolation technique. Tafel experiments were conducted with ±250 mV vs. rest potential (RP) in steps of 20 mV from the cathodic side for recording the corrosion currents, and then, the Tafel plot of potential vs. current was drawn for determining the corrosion current density (i_corr). The linear polarization method was also used for validating the Tafel results. It was performed by polarizing the specimen with ±20 mV vs. RP in steps of 5 mV, and the corrosion currents were noted. The plot of potential vs. current was drawn for calculating i_corr. The study reveals that both BPTU and CPTU act as anodic inhibitors for mild steel in the HCl medium, and good inhibition efficiency (>97%) was evidenced from both the compounds even at elevated temperatures. The study also reveals that the investigated compounds get adsorbed quickly on the steel surface, following Temkin’s adsorption isotherm. The kinetic parameters obtained from the study indicated that the inhibition was governed by a chemisorption mechanism and the presence of inhibitors substantially reduced the metal dissolution in the studied temperature range. The investigation shows that there was a good correlation between the Tafel extrapolation and linear polarization results.

Acid pickling; Activation energy; Hydrochloric acid; Inhibition of mild steel corrosion; Polarization

2010). The inhibition action of N-(furfuryl)-N?-phenyl thiourea (FPTU) on the corrosion of mild steel in HCl medium was studied using the potentiodynamic polarization technique (Divakara et al., 2007). The study revealed that good inhibition efficiency (>97%) was achieved even in very low concentration of FPTU and the molecules of FPTU was chemically adsorbed on the  steel surface. The study on the corrosion inhibition of 304 SS in HCl solution by N-(furfuryl)-N?-phenyl thiourea (FPTU) was carried out using potentiodynamic polarization methods at different temperatures and inhibitor concentrations (Herle et al., 2011). The investigation showed that FPTU worked as an efficient anodic inhibitor and the adsorption of the compound obeys Temkin’s adsorption isotherm. The inhibition of the corrosion of stainless steel (304 SS) by N-(2-mercaptophynyl)-N?-phenyl thiourea (MPTU) was studied using different techniques, such as weight loss, potentiodynamic polarization and impedance spectroscopy (Ramdev et al., 2013). The study demonstrated that MPTU was a mixed inhibitor that yielded protection efficiency more than 90%. Dicyclohexyl thiourea (DCTU) was used as an inhibitor for inhibiting the corrosion of stainless steel (304L) in 2 M HCl medium and found that DCTU acts as an excellent anodic inhibitor for the corrosion of steel. Good protective efficiency of 90% was observed in HCl medium (Shen, 2011).

The choice of suitable organic compounds as corrosion inhibitors primarily depends on the type of acid and its concentration, solution temperature, and the kind of metal or alloy exposed in the acidic medium (Shetty et al., 2007). Since mild steel has wide applications in various industries owing to its easy availability, low cost, and uncomplicated fabrication method, the assessment of corrosion behavior of mild steel in various corrosive environments has received a significant amount of interest (Ostovari et al., 2009). HCl is known to be the working horse in the pre-treatment of mild steel components and is normally used for acid pickling and de-scaling, and acidizing of oil and gas wells (Emregul & Hayvali, 2006).

The following are the reasons for selecting and synthesizing the novel thiourea derivatives, such as BPTU and CPTU, for the corrosion of mild steel in HCl medium: (a) From the literature, it is observed that thiourea derivatives are the most effective inhibitors for ensuring excellent protection to the metals and alloys against corrosion in acid media; (b) In general, acid pickling and descaling operations are carried out at elevated temperature. The addition of inhibitor to the acidic medium helps to prevent the further dissolution of metals by forming a thin protective film on the surface of the metal and thereby preventing the metal - media interaction. It has been observed from the previous studies (Shetty et al., 2005; Shetty et al., 2008) that both BPTU and CPTU exhibit excellent performance. This motivates the authors to further investigate the effect of these compounds on the corrosion of mild steel in HCl medium at different temperatures up to 60°C. The experiments were also conducted at lower temperatures for determining the thermodynamic parameters to determine the kind and nature of adsorption; (c) The selection of these compounds as inhibitors for the corrosion of mild steel in acid medium is also attributed to the fact that the method used for synthesizing these compounds is simple, cost-effective and effective at forming an insoluble stable film on the mild steel surface owing to the presence of S and N atoms in their molecular structures. The main focus this study is to determine how these derivatives of thiourea maintain their stability and protection ability against metallic corrosion at higher temperatures in HCl medium.

2.1.    Specimen Preparation

The mild steel specimen with an exposed area 0.786 cm² and the chemical composition (wt %): C: 0.205; Si: 0.06; Mn: 0.55; S: 0.047; P: 0.039: and balance Fe was used in this study. The test sample was mirror polished with emery papers, cleaned with distilled water, and dried in air.

2.2.    Inhibitors Preparation

The compounds BPTU and CPTU were synthesized according to the procedure of synthesizing the similar compounds (Moore & Crossly, 2003). The BPTU is the condensation product of benzyl amine and phenyl-isothiocyanate whereas, CPTU is the condensation product of cyclohexyl amine and phenyl-isothiocyanate and the re-crystallization of these compounds were done by ethanol. The purity of the compounds was tested by elemental analysis and the melting point values shown in Tables 1 & 2, respectively.  The molecular structures of the compounds are shown in Figure 1.

Table 1 Elemental analysis of the compounds studied

Table 2 Physical properties of inhibitors studied

Figure 1 Structural formula of thiourea derivatives

2.3.    Tafel Extrapolation Study

Tafel extrapolation studies were carried out by using a Wenking potentiostat (LB95L), and the electrochemical cell, which contains three electrodes (platinum electrode used as auxiliary electrode, saturated calomel electrode used as reference electrode and the mild steel specimen is used as working electrode), as shown in Figure 2.

The steady state rest potential (RP) was noted after about 25-30 min. and then, Tafel experiments were carried out at 30°C with ±250 mV vs. RP in steps of 20 mV/min and the corrosion currents (I) were noted in the absence and presence of different concentrations (0.0001-0.0004 mol/L) of inhibitors. The plot of potential vs. log I was drawn for different concentrations of BPTU and CPTU for determining the corrosion current density (i_corr) and the corrosion potential (E_corr). The corrosion current (I_corr) and the corrosion potential (E_corr) were obtained by extrapolating the Tafel regions (straight line regions) of both anodic and cathodic curves shown in the schematic diagram (Figure 3). The experiments were repeated for 45 and 60°C in the absence and presence of 0.0001, 0.0002, 0.0003, and 0.0004 mol/L of inhibitors.

Figure 2 Experimental set up

The corrosion rate was calculated using Equation 1, and the linear polarization method was used for validating the Tafel results obtained for the corrosion of mild steel in HCl medium.

Corrosion rate (CR), mpy =                           (1)

where, i_corr has units in µA/cm², Eq. wt is the equivalent weight of the specimen taken as 27.925 g, and D is  the density of the specimen taken as 7.86 g/cc.

Figure 3 Schematic representation of Tafel plot

2.4.    Linear Polarization Study

The linear polarization study was performed from ±20 mV vs. RP with a scanning rate of 5 mV/min, and the corresponding steady state corrosion current (I) was noted for uninhibited (blank) and inhibited system for different temperatures. The plot of potential (E) vs. I was drawn, and the slope of the E vs. I curve was used for determining i_corr, which is calculated as follows:

i_corr = 0.026/slope                                                                (2)

Figure 4 illustrates the schematic representation of the linear polarization curve and the slope of E vs. I is taken in the straight line region of the curve for calculating the i_corr values.

Figure 4 Schematic representation of linear polarization curve

The percentage inhibition efficiency (% IE) and the degree of surface coverage (?) for the corrosion of mild steel were calculated by using the following relations;

                                                (3)

where, i_corr and i_corr(inh) are the current densities in the nonexistence and existence of inhibitors, respectively.                                         

                                                              (4)

The results of the Tafel extrapolation studies carried out in the presence and absence of BPTU and CPTU for the corrosion of mild steel in 0.1M HCl solution at different temperatures are presented in Table 3.

Table 3 Tafel extrapolation results for mild steel corrosion in 0.1M HCl

3.1.    Effect of Inhibitors on Mild Steel Corrosion

The Tafel plot for mild steel in 0.1 M HCl in the absence and presence of 0.0004 mol/L of both BPTU and CPTU at 30°C is shown as an example in Figure 5. From Figure 5, it can be observed that the Tafel curves of both BPTU and CPTU were shifted to the left compared to the Tafel curve shown for the blank (uninhibited system). This demonstrates that there is a noticeable reduction in corrosion current in the presence of 0.0004 mol/L of inhibitors at 30°C. The decrease in corrosion current in the presence of inhibitors at different temperatures shown in Table 5 is also a clear evidence for the effectiveness of both the compounds in mitigating the corrosion of mild steel in HCl solution. From Figure 5, it can also be observed that the corrosion potential (E_corr) swings in the positive direction or the anodic direction by shifting the Tafel curves of both the inhibitors in the upward direction. This swing in E_corr in the positive region indicates that the investigated compounds predominantly act as anodic inhibitors for mild steel in HCl medium (Divakara et al., 2007). Similar trends were noticed in 0.0004 mol/L of inhibitors concentration at 45 and 60°C and also in 0.0001, 0.0002, and 0.0003 mol/L of inhibitors at different temperatures. Shift in the polarization curves (shown in Figure 5) to the left or to the lower corrosion current values in the presence of 0.0004 mol/L of inhibitors, indicating that the dissolution of the metal was reduced to a greater extent in HCl medium.

Figure 5 Tafel extrapolarization plot for mild steel in 0.1 M HCl with and without the presence of 0.0004 mol/L of inhibitors at 30°C

The investigation shows that the inhibition efficiency (IE) has a positive correlation with the concentrations of inhibitors, except in 0.0004 mol/L in the range of temperatures studied. The addition of 0.0004 mol/L of inhibitors to the corrosive medium decreases the IE of the compounds at all temperatures studied. This indicates that the optimal concentration of the inhibitors required to achieve maximum inhibition efficiencies at different temperatures is 0.0003 mol/L. This increase in IE may be attributed to the creation of adsorption film on the surface of the mild steel which reduces the effective area of attack. The decrease in efficiency for 0.0004 mol/L of inhibitors may be attributed to the desorption of the inhibitor molecules from the steel surface. Owing to this, the rate of metal dissolution or the corrosion rate starts increasing and thus decreasing the inhibition efficiency and the degree of surface coverage values. The study also reveals that 0.0003 mol/L of BPTU and CPTU can be efficiently used as inhibitors for mild steel corrosion in HCl medium in industries. The linear polarization results obtained for the corrosion of mild steel in the presence and absence of both BPTU and CPTU at different temperatures in 0.1M HCl solution are presented in Table 4. From Table 3 and Table 4, it can be observed that the Tafel extrapolation and linear polarization results closely match each other. This confirms the accuracy of the Tafel extrapolation results obtained for corrosion of mild steel in HCl medium.

Table 4 Linear polarization results for mild steel corrosion in 0.1M HCl

The polarization study reveals that the compounds investigated act as excellent anodic inhibitors for mild steel in HCl solution. The study also reveals that the presence of BPTU and CPTU enhanced the corrosion resistance of mild steel. The adsorption of these compounds on the mild steel surface follows Temkin’s adsorption isotherm, and the inhibition was governed by a chemisorption mechanism. The study results demonstrated that the IE values of the compounds were nearly constant in the range of temperatures and inhibitors concentrations studied. The exceptionally good performance demonstrated by BPTU and CPTU, even at very low concentrations and at elevated temperatures, confirmed that these compounds can be efficiently used as inhibitors for mild steel corrosion in HCl solution.

The authors would like to thank Manipal University, Manipal, India, for providing facilities for conducting experiments.

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