General and Intergranular Corrosion of Austenitic Stainless Steels in Acids: Effect of Cations in the Acids and the lnfluence of Heat Treatment and Grain Size of the Steel

Abstract

Corrosion of stainless steels by nitric acid is determined largely by the crystallography of grain boundaries. There is preferential attack even on annealed steels. Increasing the rate of dissolution, either by an anodic current or by oxidizing cations, intensifies intergranular penetration. The same crystallographic factors which determine preferential corrosion also determine the precipitation of chromium carbides. Their presence leads to a very great intensification of intergranular attack, as does sigma phase, even as an invisible, pre‐precipitation constituent.Oxidizing cations, such as Cr+6, Ce+4, and Fe+3, increase the corrosion of stainless steels in nitric acid by cathodic depolarization, by shifting the open‐circuit potential of cathodic areas toward more noble values and, probably, by depolarization of anodic areas.The effect of surface finish on corrosion rate is largely a function of the tru (absolute) area produced by various finishes.Ferric ions in ferric sulfate‐sulfuric acid solution greatly inhibit general, or grain‐face, corrosion by anodic polarization. In place of the reduction of hydrogen ions and the evolution of hydrogen gas at cathodic areas, ferric ions are reduced. The consumption of ferric ions is electrochemically equivalent to the weight of steel dissolved. On steels containing intergranularly precipitated chromium carbides, intergranular attack leads to dislodgment of grains and a readily detectable weight loss.Data obtained in sulfuric acid solutions containing ferric nitrate in place of ferric sulfate suggest that it may be possible to develop a 24‐hr evaluation test with this solution.The action of cupric sulfate in copper sulfate‐sulfuric acid solution is similar to that of ferric ions. However, intergranular attack on susceptible steels does not dislodge grains readily, and, therefore, weight loss cannot be used for routine evaluation of the results.Intergranular attack in this solution is greatly accelerated by metallic copper immersed simultaneously with the stainless steel specimen or in contact with it. This acceleration is a result of the formation of cuprous ions and of galvanic action by copper, which is the anode of this couple. The resulting galvanic current and the cuprous ions reduce anodic polarization most readily at grain boundaries containing chromium carbide precipitate and thereby greatly increase the rate of intergranular penetration. Sigma phase does not lead to accelerated intergranular attack in this solution.The influence of grain size on intergranular corrosion depends on the method used for measurement, change in electrical resistance or weight‐loss, and composition of the corroding acid solution.Dissolution of stainless steels in all three acid solutions is predominantly under anodic control. The difference in grain surface corrosion and in intergranular penetration at susceptible boundaries is attributed primarily to a lower anodic polarizability of the metal in such grain boundary zones rather than to any difference which may exist in the open‐circuit potentials of grain surfaces and grain boundaries.