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I. Factors controlling pitting corrosion and laboratory methods used for its study are reviewed. An electrolytically accelerated test was developed for investigation of pit initiation. A controlled direct current was passed through a cell whose anode was the stainless steel specimen and whose electrolyte was the pitting solution. The number of pits formed depended on the current density, the steel specimen (composition and surface treatment), and the solution (composition and temperature).II. The accelerated electrolytic pitting method described in Part I was used to determine the influence of alloying elements added to 18Cr-8Ni stainless steel on pit initiation in sodium chloride and bromide solutions. Reduction in carbon content, increase in nitrogen content of these steels, and alloying additions of molybdenum and silicon increased resistance to pit initiation. Grain boundaries, rather than nonmetallic inclusions, were primary sites of pit initiation during simple immersion or in the electrolytically accelerated ~est. I. Development of an Accelerated Pit Initiation MethodPITTING CORROSION OF STAINLESS STEEL

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 ...

Because of the resistance of iron-chromium stainless steels to chloride stress corrosion, this alloy system was used as a base for developing superior resistance to various forms of corrosion by means of alloying with molybdenum, nickel, and the six metals of the platinum group. The effects of these alloying elements were evaluated by accelerated laboratory tests for pitting, intergranular, general, and stress corrosion. The optimum ductility and resistance to pitting, intergranular, and stress corrosion were found for an alloy of Fe-28% Cr-4% Mo with carbon not exceeding 0.010% and nitrogen below 0.020% (C+N <0.025%). This alloy resists pitting and crevice corrosion in 10% FeCl3-6H2O at 50 C (122 F) with six crevices on the specimen surfaces, and it resists all intergranular attack on a welded specimen in the boiling ferric sulfate-50% H2SO4 test. Addition of 2% Ni to this alloy extends its general corrosion resistance in oxidizing and organic acids to boiling 10% H2SO4 and 1% HCI, in which it is also self-repassivating. The nickel addition makes the alloy subject to stress corrosion cracking (SCC) in the boiling 45% MgCl2 test, but not in the NaCl wick test, which more nearly simulates plant exposures and cracks 18Cr-8Ni stainless steel. Additions of small -amounts of any of the six platinum metals, e.g., 0.020% Ru, also make the 28Cr-4Mo alloy passive in boiling 10% H2SO4. But only ruthenium, iridium, and osmium do this without impairing pitting resistance in halide solutions. For self-repassivation, 0.50% Ru is required, and this amount makes the alloy subject to stress corrosion in the boiling 45% MgCl2 test, but not in the wick test.