Abstract
The effect of aluminium additions on the structure and morphology of the corrosion products formed on the surfaces of rapidly solidified Mg-Al alloy splats immersed at room temperature in a solution of 3%NaCl saturated with Mg(OH)2; has been studied under different analytical techniques. The adverse effect of contamination from copper particles during processing on the corrosion behaviour of the alloys is also highlighted. Aluminium additions were beneficial to the corrosive behaviour of the alloys with a marked improvement in their anti-corrosion resistance occurring in alloys containing more than 10 wt.% Al. This is attributed to the presence of aluminium ions in the prior oxide/hydroxide in the surface of the alloy. The thickness of the latter decreased with enrichment of aluminium ions and was 10-50nm for the Mg-16Al alloy splats as compared with 200nm for the Mg-3.5Al alloy splats. Hydromagnesite (3MgCO3. Mg(OH)2. 3H2O) formed as an overlayer on the surface of the alloy splats depending on the handling conditions. For the Mg-10Al and Mg-16Al alloy splats an admixture of a high temperature spinel (MgA12O4) in perlclase (MgO) and/or brucite (Mg(OH)2) was detected by X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD). It Is proposed that in the corrosive environment the Al3+ ions on the surface compete successfully with the chlorine ions for the anodic sites on the surface and anchor the growth of the layered brucite structure by the formation of a compound belonging to the pyroaurite-sjogrenite group of compounds. Hydroxyl ions, water, chlorine ions and carbonate ions are incorporated in The interlayers of the layered brucite structure. The formation of a double hydroxide with an acicular morphology and a structure close to that of hydrotalcite-manasseite (Mg6A12(OH)16. CO3. 4H2O) has been supported by scanning transmission electron microscopy (STEM), XPS, XRD, multi-element mapping by electron probe microanalysis (EPMA) and Rutherford backscattering spectrometry (RBS) analyses on the corroded splats. A growth mechanism is proposed on the basis of the structural chemistry, surface morphology and crystal structure of the corrosion products. The implications of this work for the design of Mg base alloys with improved corrosion properties are also discussed. The selected alloying elements are in excellent agreement with those selected from other studies on the development of corrosion resistant Mg alloys.