Role of Stress in the Stress Corrosion Cracking of a Mg-Al Alloy

It is shown that the correlation of the threshold stress for stress corrosion cracking (SCC) of plain specimens and of K_ISCC for precracked specimens in a Mg 7% Al alloy immersed in K₂CrO₄/NaCl solutions is explicable in terms of the formation of cracks that do not propagate to total failure in plain specimens stressed below the threshold stress. It is suggested that whether or not a crack continues to propagate relates to the competition between the lateral film growth rate, tending to inhibit corrosion, and the strain rate, tending to create reactive bare metal. Study of the effects of various strain rates has shown that SCC only occurs within a restricted range of strain rates, with ductile failures at higher or lower rates, lending support to the suggestion that if the effective strain rate is too low film growth occurs as fast as bare metal is created and stress corrosion failure does not occur. It is concluded that in SCC in this and possibly some other systems, stress or stress intensity, per se, may be less important than the strain rates they produce.