Effect of Applied Stress on SCC Susceptibility and Polarization Behavior of SUS 316 Type Stainless Steel

Abstract

The effect of applied stress on the SCC susceptibility and the polarization behavior was measured on SUS 316 type stainless steel in 42% MgCl₂ solution. In order to study the relation between the SCC susceptibility and the polarization behavior, the potentiostatic SCC test was also performed. Moreover, the effect of applied stress on the crack morphology was observed by a scanning electron microscope.
With the applied stress over 100MPa, the rest potential of the specimen and the pitting potential in the polarization curve shifted remarkably toward less noble potentials, and they changed more remarkably with increasing applied stress. The width of the unsteady passive region decreased owing to the shift of the pitting potential mentioned above.
From the investigation of the dependence of the SCC susceptibility on the potential, it was clarified that three different SCC susceptible regions existed and these regions were almost coincident with the steady passive region, the unsteady one and the pitting region in the polarization curve, respectively. From these results, it was concluded that the increase of the SCC susceptibility with increasing applied stress was attributable to both the decrease of the width of the unsteady passive region and the shift of the rest potential toward less noble potential due to the applied stress. As to the crack morphology, transgranular cracking occured mainly in the specimen fractured under low applied stress, whereas intergranular cracking appeared mainly under high applied stress.