Stress corrosion in Mg-Al-Mn alloy discussed with measured stress distributions

The effect of stress on a corrosion mode of magnesium alloy in an aqueous solution was studied with elucidating the time dependent characteristics of a corrosion product and a concentrated stress value around an artificial notch. Stress values of a thixomolded Mg alloy plate, AM-60, were measured separately in a-Mg and Mg17Al12 phases using microarea X-ray sin(2)psi method and residual stresses in the lapped plate were about 30 MPa compressive and about 30 MPa tensile, respectively. The concentrated stress in a-Mg ranged from 100 MPa tensile to 80 MPa compressive ahead the V notch root when loaded by three point bending to KI = 6 MPa m(1/2) Corrosion mode of AM-60 in 0.9% NaCl aqueous solution was quite different depending on the sign of the stresses; a fine and dense amorphous hydro-oxide formed in the region of tensile stress whereas a coarse amorphous oxide formed in that of compressive stress. The former reaction product formed in a short time, but the latter grew gradually. Concentrated tensile stress in ex-Mg phase at the notch root relaxed within two minutes by the corrosion followed by gradual relaxation of the stress to zero. This phenomenon is considered to be the compensation effect of the lattice spacing in alpha -Mg phase.