Je. Castle et al., 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/, Corrosion science, 36(6), 1994, pp. 1093
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 a
nd 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 a
re applied to analyse elemental maps created by scanning Auger microsc
opy. 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 expo
sure around these intermetallics. This has been confirmed by using sca
tter diagrams to analyse the data contained within the map. The phenom
enon 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 i
ts good conductivity, the intermetallic compounds are expected to act
as cathodes. This is confirmed by the finding of magnesium hydroxide o
n the surface of the intermetallics after exposure of the specimen in
MgCl2 solution. The onset of this deposition can be used to estimate t
he cathodic current density at the cathode. Current density at the SiC
particles, estimated by this method was negligible, suggesting that t
he principal cause of corrosion is found in the interaction between th
e aluminium alloy and the intermetallics.