The Effect of Increased Nickel Content on the Intergranular Corrosion of Alloy 20 Cb

Abstract

It had been found from previous work that increasing the nickel content from 28 to 33% eliminated stress corrosion cracking in sulphuric acid of a commercial niobium-stabilised Cr-Ni-Mo-Cu austenitic steel. This relatively small increase in nickel content, however, made the alloy more susceptible to intergranular corrosion. On the bases of X-ray and electron diffraction, electron microscopy and corrosion tests, it was concluded that raising the nickel content increased the solubility of Nb(C, N) at 1065° and also promoted the formation of deleterious carbides at lower temperatures. From this study an optimum annealing temperature of 980°–1010° for the new alloy (Carpenter 20 Cb-3) was determined.