Stress Corrosion Cracking Behavior in Liquid/Vapors of Boiling Magnesium Chloride Solution of 304L/316L/690 for Heavy-Water Collection Tubing in CANDU Nuclear Power Plant

Stress corrosion cracking (SCC) in high temperature water/vapor environments with chloride concentrated as impurity has been regarded as one of the main failure mechanisms for leakage events occurred in 304L and 316L stainless steel tubing of heavy-water collection system in CANDU reactors according to failure analysis. In this work, the SCC behaviors in high temperature environments with highly concentrated chloride of the 304L, 316L and Ni-based alloy 690 were characterized by performing tests with U-bend specimens in the liquid of boiling magnesium chloride solution, as well as in the vapors above the liquid, to provide basic data for selecting better materials. Effect of surface state was examined by testing the specimens with inner surfaces in as-received state and in grinding-polished state for comparison. Results showed that the stainless steels 304L and 316L were all susceptible to SCC both in the liquid and in the vapor above. No apparent difference in cracking susceptibility was found among the specimens with as-received original surface state and with grinding-polish surface state. All were mainly intergranular cracking. The 304L appeared more resistant to SCC than the 316L. Ni-based alloy 690 exhibited excellent resistance to the cracking, as no any cracks were found on all the specimens tested. However, general corrosion occurred on the specimens of 690 tested both in the liquid and in the vapor above.