The Electrochemical and Stress Corrosion Behaviour of an Austenitic Stainless Steel in phosphate solutions at high temperatures and pressures

Abstract

KurzfassungThe present investigation concerns the effects of stress corrosion on the behaviour o f high-alloy steels in high-pressure, high-temperature phosphate solutions, with the view on service conditions in the secondary loop of pressurized water reactors.A n apparatus is described which enables electrochemical and electroochemically con trolled stress-corrosion experiments up to 300 C with tensile specimens stressed with a defined stress by a bellows arrangement.The polarization curves o f a model system of X5CrNi189 stainless steel (AISI Type 304) and 0.4 M Na2HP04 10.4 M NaH2P04 solution exhibit a large increase in current densities at higher temperatures at all potentials investigated. Howeve:, the material proves to be spontaneously passive even at 300 C, although the passive current densities are about 10-15 times higher than those at room temperature. Stress-corrosion experiments at open circuit potential and under potentiostatic control show that high stresses and exposure times o f more than 100 h are necessary to cause cracking. In addition, some possible effects o f thick phosphate layers on the sample surface on the corrosion behaviour of the material are mentioned.