Influence of anion adsorption on the action of inhibitors on the acid corrosion of iron and cobalt

Abstract

It has been shown by the method of current/potential ( i/ ϕ) curves and by differential capacitance measurements that the chemisorption of halogen anions from H 2SO 4 solutions on cobalt leads, as in the case of iron, to an increase in the overvoltage of the cathodic reaction of hydrogen-ion discharge and of the anodic reaction of metal ionization. Due to a positive charge of the cobalt surface in H 2SO 4 solutions, organic cations are poorly adsorbed and ineffective as inhibitors, but in the presence of halogen anions their adsorption increases and they exert an additional inhibitive effect upon the above reactions, although less pronounced than in the case of iron. Hydrogen sulphide accelerates the electrochemical reactions occurring on iron and cobalt electrodes. In small concentration in acid solutions, it increases the adsorption of organic bases, which become effective inhibitors. This leads to the disappearance of the accelerating effect of hydrogen sulphide. The distortion of the anodic i/ ϕ curves (decrease in the slope in the case of inhibitors and increase in the case of stimulators) is connected with the finite rate of the establishment of adsorption equilibrium and with the removal of the adsorbed organic substance together with the ionized metal atoms. A hypothesis is advanced that the acceleration of the anodic reaction of iron and cobalt ionization in the presence of hydrogen sulphide is due to the formation of a surface catalyst from the chemisorbed HS − ions and the metal atoms.