INITIATION EVENTS FOR PITTING CORROSION OF STAINLESS-STEEL

Events which could conceivably trigger off pitting corrosion of 18Cr-8Ni stainless steel in chloride-containing solution have been observed upon potentiostatic polarization of microelectrodes (10-mu-m diameter) of the steel. These events were undetectable on larger electrodes because they could not be distinguished above the background current. The events had amplitude 30-80 pA, frequency up to about 0.05 s-1, and lifetime around 50 s. The average charge carried by an event was approximately 100 pC, rather less than the charge required to dissolve one monolayer of the steel over the entire micro-electrode surface (approximately 600 pC). Most of the events were characterised by a sharp rise and slow fall of the current, in contrast to the current transients (slow rise, sharp fall) associated with the nucleation and temporary propagation of micropits on larger electrodes. It is inferred that the transients were associated with a microstructural feature in the microwire used to fabricate the electrodes. The results are rationalized with a model of very slow dissolution of one component of a binary alloy, and rapid dissolution of the other, with a different rate for different types of surface site.