Changes induced in anodic behaviour of stainless steel in H 2 SO 4 solutionsby 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 i_crit decreased with the number of cyclic polarisations, as a result of dissolution at the sites where pitting occurred. This process caused i_crit 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.