Slip-Step Dissolution and Micromechanical Analysis to Model Stress-Corrosion Crack Growth of Type 321 Stainless Steel in Boiling MgCl_2

摘要

It is hypothesized that for ductile austenitic stainless steels exposed to boiling MgCl_2 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 when a thermo-dynamically stable, protective film at the crack tip mechanically ruptured. Based on the model of slip-bare metal dissolution repassivation and crack-tip strain analysis, a theoretical equation of stress-corrosion crack growth rate as a function of crack-tip strain rate and potential for 321 stainless steel in boiling 42 pet MgCl_2 solution is proposed. The theoretical prediction shows that when the crack-tip strain rate changes from 10~(-4) to 10~(-2) s~(-1) the crack propagation rate changes from 0.01 to 3 mm/h at the free corrosion potential (-0.35 VSCE). If the crack-tip strain rate is above 10~(-2)/s, the crack propagation rate should correspond to the upper bound determined by the 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 dissolution model predicted that there exists a critical potential E_c, above which the crack propagation rate is independent on potential, but below which the crack propagation rate decreased with decreasing potential. The theoretical prediction has been verified by slow strain rate tests of 321 stainless steel under potential control (above -0.35 V_(SCE)) in 42 pet MgCl_2 solution.