Corrosion cracking behaviors of materials for tubing of heavy-water collection system in CANDU nuclear power plants

摘要

Corrosion cracking behaviors of stainless steels 304L and 316L,nickel based Alloy 690 and carbon steel A106B in various high temperature liquid/vapor environments for tubing of heavy-water collection system in CANDU were studied for cracking-leakage failures in plants.The failure events were corrosion especially pitting firstly happened on the inner surface of the tubes,then stress corrosion cracking(SCC)taking place at the pit bottoms and developed to the outer surface,in the high temperature water/vapor containing LiOH and chloride concentrated as impurity.Tests performed in this work can be divided into three types.1.Corrosion behaviors in simulated service environments at both 250 and 150?C,by immersion tests.Results showed that the corrosion susceptibility rank were A106B＞316L＞304L＞690.The corrosion susceptibility of the same material was relatively lower in liquid than in the vapor above,and lower when temperature was lower.2.SCC behaviors in both the solution containing 0.7ppm Li++100ppm Cl-and its vapor above with initial gas being air or N2 at 250?C,by slow strain rate testing and U bend specimen testing.Results showed that in both the solution and the vapor plus air,316L and 304L were susceptible to SCC.The fracture morphology was all transgranular quasi-cleavage,similar to that of failed tubes in service.When the initial gas as air above the solution was replaced by pure nitrogen,SCC extent was significantly decreased.Alloy 690 exhibited excellent resistance to SCC.3.SCC behaviors in both the boiling magnesium chloride solution and the vapor above,by U bend specimen testing.316L and 304L were susceptible to SCC in both environments,mainly intergranular SCC.Alloy 690 exhibited excellent resistance to SCC.The significance of the lab results for industry are discussed.