Evolution of current transients and morphology of metastable and stable pitting on stainless steel near the critical pitting temperature

Type 904L austenitic stainless steel with a critical pitting temperature (CPT) of 56 °C was polarized at 750 mV (Ag/AgCl) in 1 M NaCl at 45, 49 and 54 °C. Current transients due to metastable pitting were recorded at these three temperatures, and a smaller number of stable pit current transients were recorded at 62 °C. The metastable pit current, I, followed a power law relationship I ∼ tn during initial growth prior to repassivation. The exponent n increased with temperature, from 0.5 to 1.5. To grow at the lower temperatures, the pits need to be more occluded (lower value of n; lower pit current density). The repassivated metastable pits showed two morphologies: small occluded cavities associated with the small transients, and open ones with polished surfaces for large transients recorded close to the CPT. A new type of deterministic repassivation was identified for the latter type of pit. The stable pit currents rose in proportion to t3/2, just like the faster-growing metastable pits, but only for a few seconds; then there was a drop in current and a further noisy increase occurred with a lower exponent. These pits undercut the metal surface, producing a lacy metal cover. The results are broadly consistent with a model that attributes the CPT to the onset of passivation in the saturated pit solution. A new feature requiring explanation is that, very close to the CPT, pits can precipitate a salt film but fail to undercut the surface. It is hypothesized that since a higher anodic current density is required at the undercutting site than at the pit base, there will be a range of temperatures where the former but not the latter would exceed the critical current density for passivation and is thus disallowed.