REPASSIVATION OF RAPIDLY SOLIDIFIED MAGNESIUM-ALUMINUM ALLOYS

Electrochemical techniques and electron microscopy were used to investigate the effects of rapid solidification processing (RSP) and aluminum content on the repassivation of binary magnesium-aluminum alloys in aqueous solutions of potassium chromate and sodium chloride (pH 9.1). This electrolyte resembles environments causing pitting corrosion and stress corrosion cracking in Mg-Al alloys, where chromate ions, applied as a corrosion protection measure, coexist with chloride ions indigenous to sea water and other natural electrolytes. Microcrystalline alloys containing 1 and 9 weight percent (w/o) aluminum were prepared using a melt-spinning process which yields continuous ribbons 15-25-mu-m thick. Ribbon microstructures were characterized using optical and transmission electron microscopy. Breakdown of passivity was initiated by a potential pulse or physical scratch, and repassivation was then evaluated using transient current measurements and scanning electron microscopy. The rapidly solidified alloys experienced relatively uniform attack, and repassivated faster and more completely than their as-cast counterparts. In both the as-cast and RSP conditions, increasing the aluminum content improved the repassivation rate in chromate/chloride solution of Mg-Al alloys containing 1 and 9 w/o Al.