Effect of Inhibitors on the Corrosion of Copper and its Alloys by Using Electrochemical Impedance Spectroscopy and Cyclic Voltammetry

Abstract

In this work the corrosion process occurring on the surface of copper and four types of brasses i.e. CuZnAl, CuZnSn, CuZnPb and CuZnMn was investigated using elec- trochemical techniques. Electrochemical Impedance Spectroscopy (EIS), Cyclic Volt- ammetry (CV) and Potentiodynamic methods were the three techniques used. As the corrosion process is pH dependant, the investigations were carried out at three pH - slightly acidic (6.4), slightly alkaline (8.4) and strongly alkaline (10.4) in borate buffer. It was observed that the variation in pH altered the mechanism and also the rate of corrosion. In case of the four types of brasses it was observed that dopant in the brasses affected the corrosion rate i.e. CuZnMn had the highest corrosion rate (CR) of 0.019mm/yr and CuZnAl had the least CR of 0.009mm/yr in strongly alkaline medium. However the corrosion rate of all the brasses was found to be smaller than that of pure Cu (0.132 mm/yr). The same trend was observed for slightly alkaline and acidic pH. The corrosion phenomenon was also studied in the presence of azoles which inhibited the process of corrosion. The five azoles were benzotriazole (BTA), mercaptobenzothiazole (MBT), benzimidazole (BIMD), mercaptobenzimidazole (MBIMD) and thiadiazole (TDA As the inhibitory action of the five azoles was attributed to their adsorption on the sur- face of Cu and brasses, the adsorption isotherms were plotted and thermodynamic pa- rameters for the adsorption process were calculated. High negative value of G indi- cated highly spontaneous process of chemisorption. Inhibitor efficiencies for the five azoles were also calculated from CV, EIS, and PD methods and it was observed that MBT and TDA were better inhibitors due to the presence of S atom. The relative in- hibitor efficiency (I.E) was found to be in the order MBT > TDA > MBIMD >BIMD > BTA. The effect of pH and concentration of azole on its corrosion inhibitory action was also investigated. It was observed that azoles had the highest I.E values at pH = 8.4. Impedance spectra were characterized in terms of the charge transfer resistance and transport of copper ions through the oxide layer. The results indicate that the surface layer is of dielectric nature, and its protection increases with increasing inhibitor con- centration with a maximum value at pH 8.4. The corrosion process was also studied in the presence of halide ions (F-, Cl-, Br- and I-) which are known to decrease corrosion resistance at the three pH. The I- ion was found to be the most aggressive halide ion with different mechanism of corrosion. The relative order of aggressiveness of the halide ion was I-> Br- >Cl- > F-. This order was confirmed from the experimental results obtained from the three methods i.e. CV, EIS and potentiodynamic measurements. DFT calculation with 6-31 G* basis set were carried out on the five azoles and the theoretical parameters were correlated with inhibitor efficiencies. Keywords: copper, brass, adsorption isotherms, borate buffer, azole corrosion inhibi- tors, halide ions, electrochemical impedance spectroscopy, cyclic voltammetry.