Relationship of Heat Treatment and Microstructure to Corrosion Resistance in Wrought Ni-Cr-Mo Alloys

Abstract

In an investigation of the effect of heat treatment on microstructure and corrosion resistance, seven heats of wrought Ni-Cr-Mo alloy were heated in the range of 800 F to 2375 F. Corrosion was studied in reducing and in oxidizing acids. Oxalic acid etch structures produced by various precipitates were classified and correlated with corrosion rates. Two maxima in corrosion rates were found: one on specimens heated near 1400 F and the other on specimens heated near 1900 F. In hydrochloric acid, only the maximum at 1400 F is revealed, while in chromic acid, only the maximum at 1900 F is detected. Both maxima are revealed by exposure to nitric acid, pure sulfuric acid and ferric sulfate-sulfuric acid solution. Studies of the microstructures and analysis of residues from dissolution in bromine-methanol solution show that sigma phase, in microscopic or submicroscopic form, is mainly responsible for changes in corrosion resistance produced by heat treatments. carbides of the M6C type containing molybdenum and tungsten as major elements are also present but have a minor effect on corrosion. No chromium carbide, Cr23C6, was found. Incompletely dissolved precipitates in the form of submicroscopic phases have a deleterious influence on corrosion. Multiple solution-annealing treatments at 2250 F, rather than length of time at temperature, were found most effective in completely dissolving this phase. Corrosion in reducing acids is under cathodic control. Intergranular attack and the corrosion potentials are a function of the anions of the acid. In oxidizing solutions, corrosion is governed by anodic polarization. The cathodic process consists of reduction of metallic cations. Both grain face and grain boundary zones are passive. Anodic polarization is governed by the presence of such cations as ferric ions and hexavalent chromium ions. On the basis of data presented, a simple, rapid method for evaluating Ni-Cr-Mo alloys for susceptibility to intergranular attack is, proposed. It consists of electrolytic etching in oxalic acid to detect the presence of incipient fusion resulting from overheating during annealing, and a 24-hour test in boiling solution of ferric sulfate-50 percent sulfuric acid. Material free of all kinds of damaging precipitates has a low rate of corrosion in this test.