Effects of grain boundary chemistry on the intergranular cracking behavior of nickel-16\%chromium-9\%iron in high -temperature, high -purity water and 50\% sodium hydroxide.

Abstract

The objective of this work is to provide a better understanding of the role of grain boundary chemistry in intergranular stress corrosion cracking (SCC) of controlled purity Inconel 600-type alloys in high purity water and 50% NaOH. High purity heats of Ni-16Cr-9Fe alloys with controlled additions of carbon and phosphorus were heat treated to produce chromium depletion, grain boundary impurity segregation, as well as inter- and intra-granular precipitation of chromium carbides. In deaerated ($<$10 ppb O$\sb2$) high purity water, extension rate (CER) tests were conducted at 360$\sp\circ$C at an initial strain rate of 3 $\times$ 10$\sp{-7}$ sec$\sp{-1}$. The results of CER tests in water and argon at 360$\sp\circ$C, and creep tests in argon at 430$\sp\circ$C show similarities in many respects. Based on these similarities, a systematic approach is made to evaluate the relation between IG cracking and creep behavior. The effect of carbon, phosphorus, cold work, grain size and strain rate on the IG cracking behavior in water is interpreted in terms of creep behavior. Special emphasis is made on the carbon-dislocation interaction to explain the effect of carbon is solution on IG cracking. Also, the effect of environment of IG cracking is discussed. In caustic solution, CER tests were conducted at $-$900 mVSCE and 140$\sp\circ$C at an initial strain rate of 1.78 $\times$ 10$\sp{-6}$ sec$\sp{-1}$. The results of CER tests in caustic solution show that chromium depletion and carbon segregation appears to have a significant effect on IGSCC susceptibility. The role of chromium depletion and carbon segregation is discussed in the light of the electrochemical behavior and the results of exposure tests.