On the critical pit stability product of stainless steel as a function of pit depth

A stepwise galvanostatic method was developed to identify the critical pit stability product, (x‧i)crit, of 316 L stainless steel (SS) as a function of pit depth in 0.6 M NaCl at 25°C. (x‧i)crit for a flat-bottom pit was 80-90% of the (x‧i) value for salt film formation, (x‧i)sf, independent of pit depth. However, the (x‧i) threshold at which the pit propagated with a heterogeneous dissolution interface, (x‧i)crit-het, decreased with increased pit depth. The pit adapted its morphology to maintain the active propagation when (x‧i) was between (x‧i)crit-het and (x‧i)crit. The heterogeneous dissolution mechanism was also evaluated using numerical modelling.