Abstract
The corrosion and stress corrosion cracking (SCC) behavior of iron- and nickel-base austenitic alloys in pure supercritical water is studied for the purpose of determining the suitability of these alloys for use in the core of the supercritical water reactor (SCWR) concept. SCC of 304L, 316L, 625 and 690 was investigated using constant extension rate tensile experiments in deaerated supercritical water at 500°C. One test was performed on 304L in 550°C, non-deaerated water. The oxide layer on tensile bars or exposure coupons was analyzed for weight gain, thickness, composition and bonding state. Results reveal that 304L is susceptible to intergranular SCC in 550°C non-deaerated water and, to a lesser extent, in 500°C deaerated water. 625 exhibited the greatest degree of IG cracking as measured by crack size and density. 316L and 690 failed by ductile rupture and displayed only minimal cracking on the gage section surface. 304L had a very non-uniform film thickness and alloy 625 had the thinnest oxide and exhibited a low density of micrometer-sized pits. Oxides were composed of a two-layer structure with a chromium-rich inner layer covered by an outer layer with higher iron content. Iron was the dominant element in the oxides on the iron-base alloys and nickel was the dominant element in oxides of the nickel-base alloys. The oxide layers on the 304L and 316L alloys were primarily made up of iron and chromium oxides and hydroxides. The iron oxides and hydroxides made up the majority of the composition and were present in approximately a one to one ratio to one another.
