CORROSION MECHANISMS OF STAINLESS STEEL AND NICKEL BASE ALLOYS IN HIGH TEMPERATURE HIGH PRESSURE WATER

The electrochemical corrosion behavior and the chemical components, structures and electronic characters of oxide films for stainless steels and Ni base alloys used in nuclear power plant were reviewed. The effects of water chemistry and microstructure of materials on oxide films were analyzed in order to relate the oxide film properties with electrochemical behaviors. The corrosion mechanism of the materials used in high temperature pressurized water is different with that used in room temperature water. The corrosion mechanism of the materials in high temperature pressurized water is controlled by electrochemistry and oxidation, whereas in room temperature water it is con- trolled only by electrochemistry. Oxide films has double-layer structure. The outer layer is porous, and the inner layer is composed by nano-crystalline and has semiconductivity which controlled the corrosion process. The microstructure, chemical composition, surface status of materials and water chemistry parameters are the key parameters which affect corrosion. The wedging stress of oxide in- side the flaw could induce localized tensile stress at the tip of the flaw, which caused stress corrosion cracking initiation and propagation even in macro-compressive stressed region. The wedging stress of oxide is the key factor to induce stress corrosion cracking in compressive stressed region.