Roles of different components of complex inclusion in pitting of 321 stainless steel: Induction effect of CaS and inhibition effect of TiN

Pitting behavior of 321 stainless steel (SS) in sodium chloride solution was studied by potentiostatic pulse technique triggering pits. Pure TiN inclusion and complex inclusion with core-shell structure are found in 321 SS. The former has no effect on pitting performance, while CaS in the latter is the exact inducer of pitting. Mech-anism of pitting initiation and propagation induced by complex inclusions was proposed. When CaS content differs little, inhibition effect of TiN shell on pitting propagation is the main reason why a complex inclusion with higher TiN content is more resistant to pitting.