THE USE OF SCANNING AUGER MICROSCOPY TO LOCATE CATHODIC CENTERS IN SICP/6061 AL MMC AND TO DETERMINE THE CURRENT-DENSITY AT WHICH THEY OPERATE

The corrosion of SiC(p)/6061 Al in 3% NaCl solution, at pH values of 3.8, 7.2 and 10.2 has been studied using scanning electron microscopy and combined scanning Auger electron/X-ray microscopy. The metal matrix composites fabricated by powder metallurgy, contain approximately 15 vol% SiC as less-than-or-equal-to 10 mum particles. Scatter diagrams are applied to analyse elemental maps created by scanning Auger microscopy. The experiments reveal that some impurity elements such as Fe, Ti etc., introduced during fabrication, form intermetallic compounds and that these are the dominant factors causing microgalvanic corrosion. The Auger maps show, by inspection, that oxygen is enriched after exposure around these intermetallics. This has been confirmed by using scatter diagrams to analyse the data contained within the map. The phenomenon is probably caused by the dissolution of aluminium, resulting in the deposition of porous corrosion products such as Al(OH)3 and AlOOH. Because of the more noble electropotential of the intermetallic and its good conductivity, the intermetallic compounds are expected to act as cathodes. This is confirmed by the finding of magnesium hydroxide on the surface of the intermetallics after exposure of the specimen in MgCl2 solution. The onset of this deposition can be used to estimate the cathodic current density at the cathode. Current density at the SiC particles, estimated by this method was negligible, suggesting that the principal cause of corrosion is found in the interaction between the aluminium alloy and the intermetallics.