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 (alpha-Mg) and a grain boundary (Mg(17)Al(12)) around an artificial notch. The following results were obtained: (1) Stresses in alpha-Mg and Mg(17)Al(12) phases existing in a thixomolded Mg alloy plate can be measured separately using the X-ray sin(2)psi method in the Cr-Kalpha irradiation area of 1.0 mm in diameter. The recommended 2theta(0) values for stress measurement are 140.0 deg for alpha-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 alpha-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 alpha-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% NaCl 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.