Abstract
This paper presents a study of corrosion and stress corrosion cracking (SCC) of Alloy 600. The primary purpose of this work is to relate the corrosion susceptibility of the alloy after various heat treatments to the microstructure produced by these heat treatments. Three commercial alloys and three laboratory alloys were used. Corrosion susceptibility was evaluated with the 25% boiling nitric acid test. Stress corrosion susceptibility was determined by tests in 288 C water containing either 0.2 or 8 ppm of oxygen. It was found that grain boundary chromium depletion resulting from the growth of grain boundary carbides caused the alloy to be very susceptible to corrosion and SCC in both environments. If the chromium depletion was accompanied by phosphorus segregation, the corrosion in the nitric acid test was very severe; however, phosphorus segregation alone did not cause significant corrosion. It also appears that phosphorus segregation did not play a major role in determining the outcome of the stress corrosion tests. The results of this study show that the amount of chromium depletion that occurs at grain boundaries depends on the entire heat treatment schedule. Of particular significance is the mill-anneal treatment. If this treatment produces a structure with a high density of intragranular carbides and a low density of grain boundary carbides, aging treatments at temperatures between 400 and 650 C will not reduce the corrosion resistance of the alloy. If the mill-anneal treatment leaves many intergranular carbides or dissolves most carbides, low-temperature aging can cause the alloy to be very susceptible to corrosion.
