Solution Annealing of Super Duplex Stainless Steel: Correlating Corrosion Performance with Grain Size and Phase-Specific Chemistry

A super duplex stainless steel was subjected to solution annealing at 1323 K. This led to grain coarsening, ‘limited’ (~ 7 pct by area) austenite-to-ferrite phase transformation and changes in phase-specific chemistry. No changes in the phase boundary nature or galvanic coupling between phases were, however, noted. The general corrosion behavior was determined by the electrochemical performance of the individual phases. The corrosion performance of the ferrite phase degraded monotonically with annealing time. This was related to a combined effect of grain coarsening and dilution in alloying (chromium and molybdenum content) elements. Corrosion of the austenite phase was, however, non-monotonic: as the effects of grain coarsening was counter-balanced, at least during the initial stages of annealing, by relative molybdenum enrichment. This study thus provided clear experimental data relating phase-specific corrosion performance with annealing induced changes in grain size and local chemistry in a super duplex stainless steel.