Effect of Heat Treatment, Composition and Microstructure on Corrosion of 18Cr-8Ni-Ti Stainless Steels in Acids

Abstract

Abstract Titanium additions to 18Cr-8Ni steels can effectively eliminate susceptibility to intergranular attack associated with the precipitation of chromium carbides. However, these additions also drastically reduce the grain size of these steels and promote the formation of sigma phase. Both of these factors modify the behavior of titanium-stabilized steels in corrosive environments. A detailed investigation of the influence of heat treatment and microstructure on the corrosion resistance of four titanium-stabilized, AISI 321, heats is described. Incomplete stabilization of carbon by titanium leads to precipitation of chromium carbides on heating in the range of 800 F to 1600 F. This type of susceptibility is readily detected in the oxalic acid etch test, the nitric-hydrofluoric acid test, the copper sulfate-sulfuric acid test, the ferric sulfate-sulfuric acid test, and in the nitric acid test. A second type of susceptibility to intergranular attack is associated with the formation of sigma phase, which may precipitate in a sub-microscopic form during heating in the range of 1150 to 1550 F. It is detected only in the nitric acid test and, to a lesser extent, in the ferric sulfate-sulfuric acid test. The action of various acid solutions on titanium-stabilized stainless steels are compared by means of metallographic studies of microstructures and of progressive corrosion, and by measurements of weight loss and change in electrical resistance of corroding specimens. The data obtained are used to explain the action of acids on these steels and to recommend procedures for evaluation testing.