SLIP-STEP DISSOLUTION AND MICROMECHANICAL ANALYSIS TO MODEL STRESS-CORROSION CRACK-GROWTH OF TYPE-321 STAINLESS-STEEL IN BOILING MGCL2

It is hypothesized that for ductile austenitic stainless steels expose d to boiling MgCl2 solution, the relevant crack propagation mechanism is slip dissolution. This model relates crack advance to oxidation or anodic dissolution that occurs on the bare surface that is created whe n a thermodynamically stable, protective film at the crack tip mechani cally ruptured. Based on the model of slip-bare metal dissolution repa ssivation and crack-tip strain analysis, a theoretical equation of str ess-corrosion crack growth rate as a function of crack-tip strain rate and potential for 321 stainless steel in boiling 42 pet MgCl2 solutio n is proposed. The theoretical prediction shows that when the crack-ti p strain rate changes from 10(-4) to 10(-2) s(-1) the crack propagatio n rate changes from 0.01 to 3 mm/h at the free corrosion potential (-0 .35 V-SCE). if the crack-tip strain rate is above 10(-2)/s, the crack propagation rate should correspond to the upper bound determined by th e maximum metal dissolution rate. When the crack-tip rate is below 10( -4)/s, the crack propagation rate is below 0.01 mm/h. The slip-step di ssolution model predicted that there exists a critical potential E(c), above which the crack propagation rate is independent on potential, b ut below which the crack propagation rate decreased with decreasing po tential. The theoretical prediction has been verified by slow strain r ate tests of 321 stainless steel under potential control (above -0.35 V-SCE) in 42 pet MgCl2 solution.