Metastable Pitting and the Critical Pitting Temperature

For any particular stainless steel, a critical pitting temperature (CPT) can be measured, below which stable pits do not occur at any potential up to the onset of transpassivity. The more highly alloyed the steel, particularly with molybdenum, the higher the CPT. The CPT for 904L austenitic stainless steel with a 240 grit surface finish has been determined as 48 to 49°C, and metastable pitting events have been analyzed in detail at lower temperatures. Below the CPT, metastable pitting activity peaks at around 300 mV (SCE), but occurs at all potentials up to the transpassive range. The morphology of these pits has been examined using SEM and a model is proposed in which precipitation of an anodic salt film within pits is the critical factor; above the CPT, a salt film is essential for stable pit growth, while below the CPT, the salt is an intermediary in oxide passivation, like that of iron in sulfuric acid. This is an outcome of a complex dynamical system and does not require the properties of the salt itself to change suddenly with temperature.