General and Intergranular Corrosion of Austenitic Stainless Steels in Acids

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 ...

Hydrophobicity and Improved Localized Corrosion Resistance of Grain Boundary Etched Stainless Steel in Chloride-Containing Environment

“…Grain boundaries are preferential etching sites under certain potential conditions. 38,39 In this system, highly selective grain boundary etching is achieved at an applied potential of 1.3 V, which accentuates the intrinsic grain structures on the SS316L surface. Potentiostatic polarization at an anodic potential of 2.4 V shows little to no selectivity toward grain boundary etching, thereby generating an electro-polished SS316L surface.…”

Austenitic and Ferritic Stainless Steels

“…Once formed, its slow dissolution in this environment then determines the corrosion rate of the metal. From observations [14,15] made on passive stainless steels exposed to boiling 50% sulfuric acid with ferric sulfate inhibitor, it is apparent that the passive state is not an inert or static state but a dynamic condition in which there is continuous dissolution and repair of the passive film at discrete points in the surface. A similar view of the passive state has also been evolved by Tomashov [16].…”

Improvement of Transpassive Intergranular Corrosion Resistance of 304 Austenitic Stainless Steel by Thermomechanical Processing for Twin-induced Grain Boundary Engineering

“…11,15,16) Meanwhile, intergranular corrosion possibly occurs for stainless steel due to the preferential dissolution of phosphate and/or phosphorus segregated at grain boundaries under corrosion environment at transpassive potentials. [21][22][23][24][25][26][27] GBE may probably be also effective to suppress the transpassive intergranular corrosion of austenitic stainless steels. The present study aimed to examine the effect of GBE on the transpassive intergranular corrosion of type 304 austenitic stainless steels containing different concentrations of phosphorus.…”