STRESS-CORROSION CRACKING OF RAPIDLY SOLIDIFIED MAGNESIUM ALUMINUM-ALLOYS

Stress corrosion cracking (SCC) of rapidly solidified magnesium-alumin um alloys in aqueous solutions of potassium chromate and sodium chlori de was investigated using electrochemical techniques, constant displac ement rate tests, and optical and electron microscopy. Microcrystallin e alloys containing 1 and 9 wt% aluminum were prepared using a melt-sp inning process which yields continuous ribbons 15-25 mum thick. Potent ial-pulse and scratched electrode experiments showed that repassivatio n kinetics are improved both by rapid solidification and increased alu minum content. The melt-spun alloys experienced relatively uniform att ack, and repassivated more rapidly and more completely than their as-c ast counterparts. Both failed by transgranular stress corrosion cracki ng (TGSCC) in aqueous 0.21M K2CrO4 containing 0.6M NaCl at displacemen t 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 r easonable estimates of the diffusivity of hydrogen in magnesium, analy sis of the constant displacement rate and potential-pulse tests for Mg -9Al supports a model involving the formation of magnesium hydride ahe ad of the crack tip.