Abstract
The corrosion behaviour of un-oxidized Fe-Cr binary alloys, in atmospheres typical of the advanced gas-cooled reaction (A.G.R.) can be greatly improved by a vapour-deposited silica coat. Such a treatment would be nearly always applied to steels already partly corroded. The coatings could be damaged during reactor operation. The behaviour of samples simulating these more realistic conditions has been investigated. Silica coatings have been deposited on clean Fe−8.6 Cr binary alloy, and then scratched to simulate damage. They have been deposited on binary alloy which was already extensively corroded, and on a 9%Cr steel typical of that used in A.G.R. power station boilers. The time taken for the samples to reach their linear ‘breakaway’ oxidation rate was increased by a factor of ca. 10 in all cases. In the case of the damaged sample, the improvement was due to geometrical factors. Analysis of its behaviour has given some insight into the mechanism of breakaway corrosion. If the corroded samples were already in the breakaway condition, silica coatings did not alter their oxidation rate. To be effective the coatings had to be applied before breakaway oxidation commenced.
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