Changes induced in anodic behaviour of stainless steel in H2SO4solutionsby preanodic treatment and potential sweep rate

Abstract

The influence of preanodic treatment and sweep rate on the anodic polarisation curves of AISI Type 304 stainless steel immersed in 1 M H2SO4 solutions has been studied by cyclic voltammetry, SEM, X-ray dispersive analysis, and Auger spectroscopy. Open circuit corrosion and polarisation at potentials in the active → passive transition region produced pitting, which started at sulphide inclusions. As a result of the attack, precipitates rich in sulphur, nickel; and copper were formed. Their oxidation during a subsequent anodic scan has been associated with the occurrence of a current maximum in the passive region. At the high sweep rate used (0·1 V S−1) hydrogen absorbed during preanodic charging, with or without prior sensitisation, did not induce any maximum. The passivation potential–sweep rate curve showed a minimum at 5 m V S−1, which is attributed to the passivation current becoming significantly smaller than that required to oxidise the precipitates. The critical current density for passivity icrit decreased with the number of cyclic polarisations, as a result of dissolution at the sites where pitting occurred. This process caused icrit to decrease to ∼ 20% of its initial value, indicating that, in the region of the passivation maximum, a larger proportion of the current is used in dissolving the most active sites than in forming a passive film.