Gas-side corrosion in AGR boilers - a chemist's prescription

The central evaporator section of the AGR boiler is of 9% Cr - 1% Mo steel. Laboratory tests in CO2 at 1000C in excess of operating temperatures showed that under these conditions the period of slow oxidation can be followed by a more rapid oxidation at a constant rate. A treatment has been developed for delaying the onset of 'breakaway' condition that could be applied to installed boilers during routine shut-down and to future AGR boilers at the construction stage. The mechanism of the corrosion is discussed and the development of protective treatments is described. The most successful 'in-situ' treatment is a two stage process consisting of preferential oxidation of the chromium in the steel, using a chemical 'buffer' system, combined with a surface silicon enrichment treatment achieved by the heterogeneous pyrolysis of monosilane gas in a dry non-oxidising atmosphere. On laboratory and pilot scales, this protective treatment reduced corrosion rates by a factor of 10. (U.K.)