Stress Corrosion in Mg-Al-Mn Alloy with Residual and Concentrated Stresses

The effects of residual and concentrated stress on a corrosion mode of a thixomolded Mg-Al-Mn alloy (AM-60) in a NaCl aqueous solution were studied in order to elucidate the corrosion mechanism, the time-dependent characteristics of corrosion products and an actual concentrated stress value in a grain (a-Mg) and a grain boundary (Mg 17 Al 12 ) around an artificial notch. The following results were obtained: (1) Stresses in α-Mg and Mg 17 Al 12 phases existing in a thixomolded Mg alloy plate can be measured separately using the X-ray sin 2 ψ method in the Cr-Ka irradiation area of 1.0 mm in diameter. The recommended 2θ 0 values for stress measurement are 140.0 deg for α-Mg (202) and 133.4 deg for Mg 17 Al 12 (660). (2) The measured residual stresses in a AM-60 plate were about 30 MPa compressive in α-Mg and about 30 MPa tensile in Mg 17 Al 12 phase balancing microscopic region. (3) Even in a lapped Mg-alloy a localized micro-cell exists between α-Mg grain in compressive stress state and grain boundary made of Mg 17 Al 12 in tensile stress state. The proposed model explains well the fact that the grain provides Mg 2+ ions for solution whereas grain boundary acts as a nucleation point of the hydrogen gas bubbles. Corrosion in Mg alloy occurs essentially while it is in contact with any aqueous solution. (4) Corrosion mode of AM-60 in 0.9% NaCI aqueous solution was quite different depending on the sort (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.