Water side corrosion of ferritic steel steam generator tubing under acid sulphate–chloride conditions

Abstract

The corrosion behaviour of 9Cr–1 Mo ferritic steel tubes containing artificial defects (holes formed by spark erosion) has been investigated under realistic steam generator conditions (355°C, heat flux = 860 k W m⁻²) with acid sulphate-chloride fault water chemistry and compared with previous tests under good water chemistry conditions (all volatile treatment (AVT) conditions). Corrosion on the tube surface was found to be unaffected by the presence of heat flux, while under AVT conditions the same heat flux. caused a 50% reduction in corrosion rate. The difference is ascribed to the magnetite layer deposited under acid sulphate-chloride being more porous than that laid down under AVT conditions. Two types of corrosion were observed in defects: accelerated general corrosion to depths of ∼ 40 μm - ascribed to acid sulphate attack; and intergranular attack (IGA)/stress corrosion cracking (SCC) to depths of ∼ 330 μm along grain boundaries perpendicular to the tube surface. Comparison with other tests has led to the provisional conclusion that IGA/SCC can occur in 9Cr–1Mo ferritic steel steam generator tubing when the following conditions are present concurrently: a sensitised structure (produced by spark erosion in this work), an acid sulphate-chloride environment, and a tensile stress.