doi: 10.4152/pea.201701013

Corrosion Inhibition of Carbon Steel in Well Water by L-Cysteine^-Zn²⁺ System

J. A. Thangakani^a,* , S. Rajendran^b , J. Sathiabama^c , R. J. Rathish^d and S. Santhanaprabha^d

^a C.E.O.A. Mat. Hr. Sec. School, Madurai - 625 017, Tamil Nadu, India

^b Department of Chemistry, RVS School of Engineering and Technology, Dindigul - 624 005 Tamil Nadu, India

^c Corrosion Research Centre, Department of Chemistry, GTN Arts College, Dindigul - 624 005, Tamil Nadu, India

^d PSNA College of Engineering and Technology, Dindigul, Tamil Nadu, India

The environmental friendly inhibitor system L-cysteine^-Zn²⁺ has been investigated by weight loss method. A synergistic effect exists between L-cysteine and Zn²⁺ system. The formulation consisting of 250 ppm of L-cysteine and 50 ppm of Zn²⁺ offers an excellent inhibition efficiency of 99%. Polarization study reveals that this formulation functions as anodic inhibitor. AC impedance spectra reveal that a protective film is formed on the metal surface. FTIR spectra study leads to the conclusion that the Fe²⁺L- cysteine complex formed on the anodic sites of the metal surface controlled the anodic reaction, and Zn(OH)2 formed on the cathodic sites of the metal surface controlled the cathodic reaction. A suitable mechanism of corrosion inhibition is proposed based on the results obtained from weight loss study and surface analysis technique. Synergism parameters have been calculated. They are found to be greater than 1, suggesting that a synergistic effect exists between L-cysteine and Zn²⁺.

Keywords: L-cysteine corrosion inhibitor, synergistic effect, carbon steel, well water, zinc ion, aminoacids.

Introduction

There has been a recent awareness about the health hazards of corrosion inhibition, and its best practices for health and safety. As hazardous chemicals have been restricted from contact with the environment, there has been an increasing look-out for non-toxic, eco-friendly corrosion inhibitors. Using inhibitors is one of the most pragmatic methods for protecting metals from corrosion.

Corrosion inhibitor is a chemical which, when added to the corrodible surface in optimum concentration, significantly decreases the corrosion rate of metals (or) alloys. However, many common corrosion inhibitors are highly toxic and hazardous to health, such as chromates [1], nitrite [2] and aromatic heterocyclic compounds [3]. Hence, environmentally safe inhibitors [4-6] are much recommended. Researchers have investigated and found that amino acids are both effective and environmental-friendly metal corrosion inhibitors, [6-13]. This is because aminoacids are non-toxic, biodegradable, economical and completely soluble in aqueous media, and produced with high purity at low cost. L-cysteine was selected as environmental-friendly corrosion inhibitor for the present project. The document presents some studies about the capability of amino acids to prevent corrosion in iron [14], steel [15-17], aluminium [18, 19], nickel [20] and copper [21-25]. Polarization and AC impedance spectra [26-30] and cyclic voltametry [19] have been studied by using amino acids. The adsorption of amino acids on carbon steel in an acidic environment has been researched by Adiyama et al. [31].

The aim of the present study is:

1. To assess the inhibition efficiency of L-cysteine in controlling the corrosion of carbon steel in the absence and presence of Zn²⁺.

2. To evaluate the protective film on carbon steel by FTIR spectrophotometry.

4. To propose a suitable mechanism for corrosion inhibition based on the results from the above study.

Experimental procedure

Preparation of specimens

Carbon steel specimens (0.0267% S, 0.067% P, 0.4% Mn, 0.1% C and iron), with the dimensions 1.0 cm × 4.0 cm × 0.2 cm, were polished to mirror finish, degreased with trichloroethylene, and used for weight loss method and surface examination studies.

Weight loss method

Carbon steel specimens, in triplicate, were immersed in 100 mL of well water and various concentrations of L-cysteine in the presence and absence of Zn²⁺ (as ZnSO4, 7H2O) for a period of seven days. The corrosion products were cleaned with Clarke`s solution [32]. The weight of the specimens before and after immersion was determined using a Shimadzu balance AY62. The corrosion inhibition efficiency was calculated with equation (1).

 

]]> where W1 is the corrosion rate in the absence of the inhibitor, and W2 is the corrosion rate in the presence of an inhibitor.

Potentiodynamic polarization study

Potentiostatic polarization studies were carried out using a CHI electrochemical impedance analyzer, model 660 A. A three^-electrode cell assembly was used. The working electrode was a rectangular specimen of carbon steel with one face of the electrode (1 cm² area) exposed, and the rest shielded with red lacquer. A saturated calomel electrode (SCE) was used as reference electrode, and a rectangular platinum foil was used as counter electrode. Polarization curves were recorded using IR compensation. The results, such as Tafel slopes, and Icorr, Ecorr and LPR values were calculated. During the polarization study, the scan rate (v/s) was 0.01; hold time at Ef(s) was zero and quit time(s) was 2.

AC impedance measurements

A CHI electrochemical impedance analyzer (model 660A) was used for AC impedance measurements. A time interval of 5 to 10 minutes was given for the system to attain its open circuit potential. The real part Z' and imaginary part Z'' of the cell impedance were measured in ohms at various frequencies. Charge transfer resistance R1, double layer capacitance Cdl and impedance values were calculated.

 

 

where Rs = solution resistance

 

 

]]> where fmax = maximum frequency

AC impedance spectra were recorded with initial E(v) = 0; high frequency (Hz) = 1; amplitude (v) = 0.05; and quiet time(s) = 2.

FTIR spectra

The structure of L-cysteine is shown in Fig. 1.

 

The carbon steel specimens immersed in various test solutions for one day were taken out and dried. The film formed on the metal surface was carefully removed and thoroughly mixed with KBr, so as to make it throughout uniform. The FTIR spectra were recorded in a Perkin-Elmer 1600 spectrophotometer.

 

Results and discussion

Analysis of results of the weight loss method

]]> Inhibition efficiencies (IE%) of L-cysteine^-Zn²⁺ systems in controlling corrosion of carbon steel immersed in well water, in the presence and absence of an inhibitor system (immersion period = 7 days), are given in

 

 

 

 

 

 

It is observed that L-cysteine alone has poor inhibition efficiency. In the presence of various concentrations of Zn²⁺ (10 and 50 ppm) the IE of L-cysteine increases. A synergistic effect exists between L-cysteine and Zn²⁺. For example, 50 ppm of L-cysteine has only 85% IE; 50 ppm of Zn²⁺ has 15% IE. However, their combination has 92% IE. This suggests a synergistic effect between L-cysteine and Zn²⁺ .

Synergism parameter (SI)

]]> Synergism parameter (SI) has been used to know the synergistic effect between two inhibitors [33 -38]. Synergism parameter (SI) can be calculated as:

 

 

where θ = surface coverage, θ₁₊₂ = (θ1+θ2)-(θ1 θ2), θ1 =surface coverage by L- cysteine, θ2 = surface coverage by Zn²⁺ , θ'₁₊₂ = surface coverage by both L- cysteine and Zn²⁺, and where

 

 

The synergism parameters of L-cysteine -Zn²⁺ system are given in Tables 4 and 5.

 

 

]]> For different concentrations of inhibitors, SI approaches 1 when no interaction between the inhibitor and compounds exists. When SI > 1, it points to synergistic effects. If SI > 1, it is an indication that the synergistic effect is not significant. From

Analysis of potentiodynamic polarization study (pH = 3)

Polarization study has been used to confirm the formation of a protective film formed on the metal surface during corrosion inhibition process [39-44]. If a protective film is formed on the metal surface, the linear polarization resistance value (LPR) increases, and corrosion current value (Icorr) decreases.

The potentiodynamic polarization curves of carbon steel immersed in well water, in the presence and absence of inhibitors, are shown in Fig. 2.

 

The corrosion parameters are given in Table 6.

 

When carbon steel was immersed in well water, the corrosion potential was -502 mV vs. SCE. When L-cysteine (250 ppm) and Zn²⁺ (50 ppm) were added to the above system, the corrosion potential shifted to the cathodic side (-523 mV vs. SCE). This indicates that the inhibitor system predominantly controls the cathodic reaction (formation of OH^-). In the presence of the inhibitor system, the corrosion potential shifted from -502 to -523 mV vs. SCE. This shift is with 50 mV/decade. Hence, it is concluded that the inhibitor system functions as a mixed type inhibitor system.

]]> The anodic reaction (Fe → Fe²⁺ + 2e^-) is controlled by the formation of Fe²⁺ inhibitor complex on the anodic sites of the metal surface.

The cathodic reaction (O2 + 2H2O + 4e^- → 4OH^-) is controlled by the formation of zinc hydroxide (Zn²⁺ + 2OH^- → Zn(OH)2↓) on the cathodic sites of the metal surface. Thus, both the anodic reaction for the generation of Fe²⁺ , and the cathodic reaction for the formation of OH^- are effectively controlled by the inhibitor system. This accounts for the mixed type of the inhibitor system and the synergistic effect between L-cysteine and Zn²⁺ system. Further, the LPR value increased from 12948.8 ohm cm² to 36560.5 ohm cm²; and the corrosion current decreased from 3.521 × 10-6 A/cm² to 1.168 × 10-6 A/cm². Thus, the polarization study confirms the formation of a protective film on the metal surface.

Analysis of AC impedance spectra

AC impedance spectra (electro chemical impedance spectra) have been used to confirm the formation of a protective film on the metal surface [45-47]. If a protective film is formed on the metal surface, charge transfer resistance (Rt) increases, double layer capacitance value (Cdl) decreases and impedance log (z/ohm) value increases. The phase angle also increases. The AC impedance spectra of carbon steel immersed in well water, in the presence and absence of inhibitors (L-cysteine^-Zn²⁺), are shown in Figs. 3 (a, b) (Nyquist plots), Figs. 4 (a, b) and Figs. 5 (a, b) (Bode plots).

 

 

 

The AC impedance parameters, namely charge transfer resistance (Rt) and double layer capacitance (Cdl) derived from Nyquist plots are given in Table 7.

]]>  

The impedance log (z/ohm) values derived from Bode plots are also given in Table 7.

It is observed that, when the inhibitors [L-cysteine (250 ppm) + Zn²⁺ (50 ppm)] are added, the charge transfer resistance (Rt) increases from 4737 ohm cm² to 7336 ohm cm². The Cdl value decreases from 1.056×10-9 F/cm² to 0.6816×10-9 F/cm². The impedance value [log (z/ohm)] increases from 3.8862 to 4.043. The phase angle increases from 34.00o to 39.00o. These results lead to the conclusion that a protective film is formed on the metal surface.

Analysis of FTIR spectra

FTIR spectra have been used to analyze the protective film formed on the metal surface [48-56]. The FTIR spectrum (KBr) of pure L-cysteine is shown in Fig. 6(a).

 

The -C=O stretching frequency of carboxyl group appears at 1640 cm^-1. The -CN stretching frequency appears at 1131.7 cm^-1. The -NH stretching frequency of the amine group appears at 2972.6 cm^-1 [51-53]. The -CS stretching frequency appears at 634.8 cm^-1.

The FTIR spectrum of the film formed on the metal surface, after immersion in well water containing 250 ppm of L-cysteine and 50 ppm Zn²⁺, is shown in Fig. 6(b).

The -C=O stretching frequency has shifted from 1640 cm^-1 to 1636.4 cm^-1. The -CN stretching frequency has shifted from 1131.7 to 1021.0 cm^-1 . The -NH stretching frequency has shifted from 2972.6 to 2920.1 cm^-1. The -CS stretching frequency has shifted from 634.8 cm^-1 to 603.6 cm^-1. This observation suggests that L-cysteine has coordinated with Fe²⁺ , through the oxygen atom of the carboxyl group and nitrogen atom of the amine group, resulting in the formation of the Fe²⁺-L-cysteine complex on the anodic sites of the metal surface. The peak at 443.9 cm^-1 corresponds to Zn-O stretching. The peak at 3399.8 cm^-1 is due to OH stretching. This confirms that Zn(OH)2 is formed on the cathodic sites of the metal surface [45, 54-56]. Thus, the FTIR spectral study leads to the conclusion that the protective film consists of Fe²⁺-L-cysteine complex and Zn(OH)2. This accounts for the synergistic effect between the amino acid and Zn²⁺.

The results of the weight-loss study show that the formulation consisting of 250 ppm L-Cys and 50 ppm of Zn²⁺ has 99% IE in controlling corrosion of carbon steel in well water. A synergistic effect exists between Zn²⁺ and L-Cys. Polarization study reveals that this formulation functions as a mixed type of 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⁺-L-Cys complex and Zn(OH)2. In order to explain these facts, the following mechanism of corrosion inhibition is proposed [57-68].

When the solution containing well water, 50 ppm Zn²⁺ and 250 ppm of L-Cys is prepared, there is formation of Zn²⁺-L-Cys complex in the solution. When carbon steel is immersed in this solution, the Zn²⁺-L-Cys complex diffuses from the bulk of the solution towards the metal surface.

Zn²⁺-L-Cys complex diffuses from the bulk solution to the surface of the metal and is converted into a Fe²⁺ -L-Cys complex, which is more stable than Zn²⁺-L- Cys [50].

On the metal surface Zn²⁺ -L-Cys complex is converted into Fe²⁺ -L-Cys on the anodic sites. Zn²⁺ is released.

 

The released Zn²⁺ combines with OH^- to form Zn(OH)2 on the cathodic sites [50].

 

 

]]> Thus, the protective film consists of the Fe²⁺-L-Cys complex and Zn(OH)2 [69,70].

 

Conclusions

1. The polarization study reveals that this formulation functions as a mixed type inhibitor system.

2. AC impedance spectra reveal that a protective film is formed on the metal surface.

3. The FTIR spectra study leads to the conclusion that the Fe²⁺-L-Cys complex formed on anodic sites of the metal surface controlled the anodic reaction, and that Zn(OH)2 formed on the cathodic sites of the metal surface controlled the cathodic reaction.

 

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^*Corresponding author. e^-mail address: angelinthangakani@gmail.com

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