DEVELOPMENT OF CORROSION-RESISTANT MAGNESIUM ALLOYS .2. STRUCTURE OF CORROSION PRODUCTS ON RAPIDLY SOLIDIFIED MG-16AL ALLOYS

The structure and morphology of the corrosion products formed on the surfaces of splat quenched (SQ) Mg-16Al (wt-%) alloy specimens immersed in a 3% NaCl solution saturated with Mg(OH)2 have been studied. X-ray photoelectron spectroscopy, Rutherford backscattering spectrometry, and analytical electron microscopy of the corrosion products and X-ray diffraction analysis of the corroded alloys have been used to determine the composition and growth characteristics of the corrosion products. The corrosion products formed on the surfaces of the corroded SQ specimens had an acicular morphology and the structural chemistry of double layered hydroxycarbonate/chloride brucite belonging to the pyroaurite-sjogrenite group of minerals. The crystal structure of the corrosion products is hexagonal/rhombohedral with lattice parameters corresponding to the hexagonal unit cell with c0 = 1.58 and 2.34 nm and a0 = 0.306 nm and with an interlayer spacing c varying from 0.72 to 0.79 nm. The accommodation of chloride ions and carbonate ions in this interlayer is determined by the Al ions which replace the Mg ions in the brucite structure, which in turn governs the interlayer spacing. With increasing corrosion time, the depletion of Mg at the surface is accompanied by an increase in Al concentration, which reduces the interlayer spacing. A growth mechanism is proposed for the formation of the double layer brucite structure which is based on the crystal structure, surface morphology, and chemistry of the corrosion products. (C) 1993 The Institute of Materials.