STRESS-CORROSION CRACKING OF RAPIDLY SOLIDIFIED MAGNESIUM ALUMINUM-ALLOYS

Stress corrosion cracking (SCC) of rapidly solidified magnesium-aluminum alloys in aqueous solutions of potassium chromate and sodium chloride was investigated using electrochemical techniques, constant displacement rate tests, and optical and electron microscopy. Microcrystalline alloys containing 1 and 9 wt% aluminum were prepared using a melt-spinning process which yields continuous ribbons 15-25 mum thick. Potential-pulse and scratched electrode experiments showed that repassivation kinetics are improved both by rapid solidification and increased aluminum content. The melt-spun alloys experienced relatively uniform attack, and repassivated more rapidly and more completely than their as-cast counterparts. Both failed by transgranular stress corrosion cracking (TGSCC) in aqueous 0.21M K2CrO4 containing 0.6M NaCl at displacement rates between 5 x 10(-5) and 9 x 10(-3) mm s-1. In 0.6M NaCl, TGSCC occurred only near 3.6 x 10(-3) mm s-1, while no stress corrosion was observed in chromate solution without chloride. Constant displacement rate tests in air after pre-exposure to the electrolyte indicated that TGSCC probably results from a hydrogen embrittlement process. Using reasonable estimates of the diffusivity of hydrogen in magnesium, analysis of the constant displacement rate and potential-pulse tests for Mg-9Al supports a model involving the formation of magnesium hydride ahead of the crack tip.