Corrosion of Nickel Alloys and Stainless Steels in Pressurized Water Reactors

The materials used in pressurized water reactors (PWRs) are chosen for their mechanical properties, but also for their resistance to corrosion. Nevertheless, the majority of material degradation problems encountered in service have been due to corrosion, mainly stress corrosion, occurring either in a nominal environment (which is the subject of the article [BN 3 755] ), or in environments progressively or transiently contaminated by impurities, which is the subject of the presentation below.

Steam generators installed in the 1970s-1980s were by far the components most affected by corrosion phenomena due to the progressive accumulation of impurities contained in their feedwater. On the secondary side, the alloy 600 tubes of these steam generators were subjected to a combination of intergranular corrosion and stress corrosion, generally intergranular, leading first to the plugging of many tubes, and then to the gradual replacement of the majority (all in France) of first-generation steam generators.

The transient presence of oxygen, generally linked to primary circuit filling procedures at the end of unit shutdown periods, is also the cause of stress cracking, generally transgranular, of austenitic stainless steels in confined areas, notably dead arms, where traces of impurities, mainly chloride and sulfate ions, may also be present, due to poor surface cleanliness, or the presence of these ions in certain gasket or gland materials. This latter form of degradation is recurrent but represents a limited problem, nowhere near as critical as that of steam generator tubes.