SURFACE CONTAMINATION AND ITS EFFECT ON THE CORROSION OF RAPIDLY SOLIDIFIED MG-AL ALLOY SPLATS

Contamination with copper particles of the surfaces of rapidly solidified Mg-3.5 wt% Al alloy splats during processing is discussed. Two batches of splats produced with copper substrates of different surface finish, were examined by atomic absorption spectrometry (AAS), electron probe micro-analysis (EPMA) and Rutherford back scattering (RBS). Lower copper content was detected on the well-polished splats (splats B) by AAS, while EPMA and RBS analysis with a micro-beam showed fine copper particles on the surfaces of the splats prepared with pistons of inferior surface finish (splats A). Immersion corrosion tests carried out in a 3% NaCl solution saturated with Mg(OH)2 resulted in higher pit density and earlier pitting times for splats A. Pitting is associated with copper particles (splats A) and with surface cracks and macro-porosity (splats A and B). A mechanism for pitting is suggested in which Mg(OH)Cl is envisaged to be an intermediate reaction product before decomposing to Mg(OH)2 in the pitting process.