Abstract
The corrosion behaviour of standard and modified 9Cr–1 Mo ferritic steel tube containing artificial defects (holes formed by spark erosion and drilling) has been investigated under realistic steam generator conditions (334°C, heat flux 860 kW m−2) with acid sulphate-chloride fault water chemistry. Corrosion of the tube was found to be independent of steel type, the presence of heat flux, or acid sulphate-chloride water chemistry. Wide variations were seen in the rate of magnetite deposition in heat flux and non-heat flux regions of the tube. These are attributed to the presence of a thin oxide film on the outer surface of the tube which retards the escape of corrosion hydrogen and reduces the rate of magnetite deposition. Two types of corrosion were observed in defects: accelerated general corrosion to depths of 54μm– ascribed to acid sulphate attack, and intergranular attack (IGA) to depths of ∼80 μm–independent of steel type or the presence of a sensitised structure. Under these test conditions, there appears to be a threshold stress of ∼150 MNm−2 above which stress assisted IGA can occur in sensitised 9Cr–1 Mo steels.
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