Comparative studies on the anodic behaviour of the 18Cr/10Ni stainless steel in 0.5 M H2SO4 solution in the presence of some inorganic anions

The anodic behaviour of 18Cr/10Ni stainless steel in de-aerated solutions of 0.5 M H 2 SO 4 , in the presence of various concentrations of SO 2 - 4 , Cl - , Br - , I- and NO - 3 anions was studied by means of the potentiostatic polarisation curves. These anions influence in different modes the passivation electrochemical parameters of stainless steel, depending on their nature and concentration as well as on their role in the corrosion process. The sulphate anion (> 0.5 M) determined a significant decrease of the critical current, but also an increase of the passive current. The halogen anions had an inhibitor effect both in the active and in the passive regions, at low concentrations, determining the decrease of the critical current and of the passive current density. At high concentrations, Cl - and Br - had an unfavourable effect on the corrosion resistance of 18Cr/10Ni stainless steel, Cl - (≥ 0.1 M) and Br- (≥1 M) determined the increase both of the critical current density and of the passive current density and also the reduction of the passive domain owing to the development of the pitting corrosion. In the active region, the I- anion had an inhibitor effect at all tested concentrations, but in the passive region, the iodide oxidation reaction occurred. Nitrate ions act as depolarizers in the corrosion process and determine the self-passivation of stainless steel if their concentration is ≥ 1 M.