Mechanism of Corrosion of Iron in Hydrochloric Acid and Its Inhibition by Iodide and Quinoline

The kinetics and mechanism of iron dissolution in acids have been the subject of numerous investigations and these are quoted by Hilbert et al ⁽¹⁾. The great number of experimental results led mainly to two theories about the anodic dissolution mechanism of pure iron worked out by Heusler ⁽²⁾, Bockris ⁽³⁾ and Kelly ⁽⁴⁾. Both theories indicate participation by hydroxyl ions in the dissolution process in H₂SO₄ solution. The generation of OH⁻ ions in H₂SO₄ solution is presumed to occur by dissolution of water molecules at the electrode surface. The effect of anions on the kinetics of metallic dissolution has been reviewed by Kolotyrkin ⁽⁵⁾. While SO₄⁻⁻and ClO₄⁻ are not surface active and therefore, do not participate in the dissolution process, chloride ions appear to influence the dissolution process and hence its participation has been taken into account in regard to dissolution in HCl^(6-10). Various reaction sequences involving the participation of chloride ions have been considered in these references. The mechanism of reduction of H⁺ ions from acid solutions on iron surface has been investigated ⁽⁴⁾. Evidence is in favour of rate determining step involving proton discharge followed by electrodic desorption. The dependence of E_corr and i_corr on H⁺ concentration has also been studied ^(6,8,9,11). These studies have enabled the satisfactory explanation of the corrosion of iron in acid solution, particularly H₂SO₄ solution.