| The corrosion of SiCp / 6061 A1 in 3% NaCl solution, at pH values of 3.8, 7.2 and 10.2, has been studied using SEM and combined scanning Auger electron/ X-ray microscopy. The MMC, fabricated by powder metallurgy, contains approximately 15 percent (volume) SiC as < 10 mum particles. Scatter diagrams are applied to analyze 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 micro-galvanic 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 analyze 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 a low overpotential for oxygen reduction, 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. It is shown that 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 is negligible, suggesting that the principal cause of corrosion is found in the interaction between the aluminium alloy and the intermetallics. The same method is used for SiC, / 6061 A1 MMC. Intermetallic particles are found much more likely to be located at a fibre / matrix interface, and they are still a sensitive factor for localized corrosion. There is some Cr in the Fe-containing intermetallics in this matrix, this type of intermetallic is not very susceptible to micro-galvanic corrosion. Another different aspect is that the reinforcing material in SiCf / 6061 A1 MMC is directly associated with the micro-corrosion cell. The reinforcing SiC fibre belongs to the SCS series which consists of three different layers. The SiC is grown on a carbon core by chemical vapour deposition and a protective coating is applied on the outside of the fibre. This coating is mainly composed of carbon which is slightly enriched in silicon at the surface. In this investigation we found that carbon core and carbon coating act as cathodes and form a galvanic cell with surrounding aluminium alloy. This is confirmed by the magnesium decoration method. Electrochemical techniques are also used. Several kinds of SiC fibres and graphite fibres are cathodically polarized in 3% NaCl solution. By comparison with the anodic polarization curve of A1 alloy, the galvanic cells between A1 alloy and carbon core or protective coating are confirmed. There is no galvanic effect between A1 and pure SiC. Al3Fe is extracted from intermetallic enriched A1 alloy, and the electrochemical performance of this intermetallic is investigated. |
