A Study of Crack Initiation in Corrosion Fatigue of AISI Type 316 Stainless Steel by Dynamic Measurement of Corrosion Current Transients

Corrosion current transients synchronized with the cyclic strain were observed when AISI Type 316 stainless steel plate was strained by reverse bending at 24 Hz in selected aqueous media. This was accomplished by extracting for oscilloscope display the regular fluctuating component of the corrosion current flowing when the sample was polarized to prescribed potentials. The amplitude of the cyclic current transient could be correlated with the expected stability of the passive condition of the steel when it was polarized to potentials within its passive range. Chloride ions increased and chromate ions decreased the amplitude of the signals. These results are interpreted as providing evidence that (a) the dominant stress-environment interaction for this material is strain-induced rupture of the passive film at persistent slip bands, accelerating crack initiation and (b) the chemical depassivating or passivating influence of the environment is additive to the mechanical depassivating effect of the applied cyclic stress.