Effects of alloying additions on the spontaneous passivation of stainless steels containing copper at different temperatures

Spontaneous passivation of AISI 304 (UNS S 30 400)double dagger austenitic stainless steel containing 2% Cu was affected by 0.02-0.03% P and 1% Mo and 2-3% Si. The steels were investigated in deaerated 0.5 M H2SO4 at different temperatures by electrochemical and surface analysis (XRD, SEM, STM and AES) techniques. The decrease of P impurity to 0.001 wt% increased the ability of steel alloy to self-passivate at temperatures from 30 degrees C to more than 45 degrees C. P segregated at grain boundaries increased the active dissolution of the alloy, the accumulation of Cu, and hydrogen adsorption on the surface. However, some beneficial effect was observed by the presence of P during ageing. Stainless steels with P impurity and containing Mo or Si exhibited spontaneous passivation at temperatures higher than 50 degrees C. Mo addition inhibited active dissolution and Si participated in the formation of the oxide film enhancing the positive role of Cu. Copyright (C) 1996 Elsevier Science Ltd.