Investigations on the effect of chloride on the general corrosion behavior of low-alloy steels in oxygenated high-temperature water

Abstract

This project focused on investigations on the effect of chloride contaminations on the general corrosion and crack initiation behavior of low-alloy steel (German reactor pressure vessel steel 22NiMoCr3 7) in oxygenated high-temperature water (HTW). Therefore, tests were performed in oxygenated HTW without chloride and at different chloride contamination levels up to 50 ppb. Chloride was added either permanently or temporarily to simulate a chloride transient during plant operation. During these tests, electrochemical noise (EN) and electrochemical impedance spectroscopy (EIS) measurements were performed to monitor the electrochemical behavior depending on the adjusted environment conditions, especially the effect of chloride on the degradation of low-alloy steel. After the tests, the specimens were examined macroscopically and microscopically. In addition, the oxide layer thickness was determined using the focused ion beam (FIB) technique and different surface analysis techniques as, e.g., TOF-SIMS were performed to analyze the composition of the oxide layer. A change of the corrosion behavior of the tested specimens was revealed by the applied electrochemical methods EN and EIS during high-temperature testing. In addition, the applied post-test investigations showed a decrease in the oxide layer thickness due to permanently increased chloride concentrations in the HTW. Temporary transients, however, did not cause a long-term memory effect as shown by both, the electrochemical and the metallographic post-test investigations.