Free surface contribution to sensitization of an austenitic stainless steel

Austenitic stainless steels are widely used in corrosive environments. During plastic straining these steels exhibit environmental effect. The flow stress in air at intermediate temperatures is compared with the specimens strained in vacuum. Static annealing in air and vacuum to a different degree strengthens the material at room temperature. The data reported here was collected for AISI 316 and 316L austenitic stainless steels. The specimens have been subjected to annealing at 900°C followed by water quenching+annealing at 600°C in either air or vacuum. The results obtained show that the resistance to corrosion of the grain boundaries in austenitic stainless steel subjected to recrystallization annealing at 900°C in air is particularly low in the near-surface layer of specimens. The additional annealing at 600°C either in vacuum or air per se does not lead to formation of a measurable Cr-depletion along grain boundaries in the near-surface zone. The observation of surface sensitization proves a detrimental effect, a high temperature annealing in air on corrosion resistance of an austenitic stainless steel. The results show that the harmful consequence of a high temperature annealing in air can be reversed by a subsequent annealing in vacuum at intermediate temperatures. The results also rationalize mechanical cleaning of elements subjected to a heat-treatment in air.