The effect of chloride on general corrosion and crack initiation of low-alloy steels in oxygenated high-temperature water

The effect of chloride on the general corrosion and its potential impact on EAC crack initiation of low-alloy steel (German reactor pressure vessel steel 22 NiMoCr 3 7) in oxygenated high-temperature water were investigated. The general corrosion behavior was analyzed by exposure tests with either permanently increased chloride concentration levels or temporary chloride transients. The potential effect on EAC crack initiation was analyzed with pre-strained C-ring specimens and in SSRT (CERT) tests with slowly rising strain. Both kinds of tests were performed under simulated BWR conditions and with different chloride levels. The chloride concentrations of 5 to 50 ppb were chosen according to the action levels of the German water chemistry guideline for the reactor coolant of BWRs (VGB R401J, 2006). In all exposure tests, none of the pre-strained C-ring specimens showed crack initiation during up to 1000 hours of exposure time with up to 50 ppb chloride. Investigations of the oxide layer thickness after immersion testing revealed a decrease with increasing chloride concentration. As shown by post-test chemical analysis of the oxide layer composition by TOF-SIMS, this effect is most likely primarily due to adsorption of chloride on the oxide layer surface, since only very limited penetration of chloride into the oxide was detected. In contrast to the tests with C-ring specimens, where no crack initiation occurred, slightly accelerated crack initiation at lower elongation levels was observed at increasing chloride concentrations in SSRT tests under simulated BWR conditions using actively loaded specimens. In addition, SSRT specimens that were cyclically loaded at the oxide fracture elongation level were used to generate a continuous, exposure of bare metal to the environment by repeated fracture of the oxide. This loading pattern did not cause crack initiation at all chloride concentrations applied (up to 50 ppb). From these results, it may be concluded that at least up to 50 ppb there is no immediate, significant effect of increased chloride concentration on the EAC susceptibility of low-alloy steel components with smooth, uncracked surfaces.