Part X – October 1969 - Papers - Intergranular Corrosion of Austenitic Stainless Steels

It is proposed that the intergranular corrosion of austenitic stainless steels is associated with the presence of continuous grain houndary paths of either second phase, or solute segregate resulting from solute-vacancy interactions. Experimental observations of structural changes and crrosion behavior of different types of austenitic stainless steel provide support for this poposal. On the basis of this model, it is shown that the intergranular -corrosion susceptibility of austenitic stainless steels in nitric-dic hromate solution may be substantially reduced either by suitable heat treatments or by impurity control. AUSTENITIC stainless steels, such as Type 304, generally have excellent corrosion resistant properties when properly solution heat-treated and used at temperatures where carbide precipitation is slow. However, several corrosion environments have been found which produce intergranular corrosion of solu-tion-treated stainless steels, that is, those steels with no detectable carbide precipitation.''2 Of the various corrosion environments, the most widely used test solution has been the boiling nitric-dichromate solution. In these acid solutions, stainless steels have been found to be susceptible to intergranular attack despite the addition of carbide-forming elements such as titanium or columbium, or despite lowering of the carbon content or use of high-temperature solution treatments. Studies of the electrochemical mechanism of corrosion attack have been made by several worke1s3'4 who found that oxidizing ions such as crt6 depolarize the cathodic reactions and consequently raise the open-circuit potential of stainless steel immersed in nitric acids. As a result of this, the anodic reaction is accelerated. The reason for the localization of anodic activity at the grain boundaries, and resulting intergranular corrosion, has not been conclusively determined. Several workers, e.g., Streicher,3 and Coriou et al.,4 have suggested that the strain energy associated with grain boundaries provides the driving force for the accelerated intergranular corrosion. This argument would predict that alloys of high purity would still be susceptible to intergranular attack. However, work by chaudron5 and by ArmijO,6 has shown that high-purity alloys are immune to attack, in disagreement with this argument. An alternative suggestion is that chemical concentration differences exist between grains and grain boundaries, that is, impurity segregation at boundaries, and that these chemical differences provide the driving force for localized attack. It is this impurity segregation which can lead to accelerated dissolution of grain boundaries when the alloy is exposed to a suitable corrodant. This mechanism would predict the immunity of high-purity alloys to inter-granular attack, which is in agreement with experi-mental observations. In the present paper, some recent studies on inter-granular corrosion of austenitic stainless steels which were conducted by coworkers and myself will be re-tibility A simple model will be described in which it is proposed that the intergranular corrosion of aus-tenitic stainless steel is associated with the presence of continuous grain boundary paths of either second phase or solute-segregated regions.* On the basis of this model, it is suggested that the intergranular corrosion rate can be markedly reduced by the formation of a discontinuous second phase at the grain boundaries if the discontinuous second phase incorporates the major part of the segregating solute, drained from the grain boundary region. Results are presented of corrosion tests and electron microscopic studies of different types of austenitic stainless steel after various heat treatments which provide experimental support for this model. Finally, a solute clustering mechanism, based on a solute-vacancy interaction, is shown to be consistent with the results obtained for inter-granular corrosion of solution-treated austenitic stainless steels. EXPERIMENTAL Corrosion tests using weight loss measurements were made on sheet specimens, which were lightly electropolished, washed, and immersed in boiling (115°C) 5 N HN03 containing 4 g crt+6 per liter added as potassium dichromate. Studies in which the inter-granular penetration depth was measured both by electrical resistance and metallographic methods have shown an empirical correlation between the rate of intergranular penetration and the weight loss per unit time for identically treated specimens of stainless steel." As a result, although all the corrosion data reported here are in terms of simple weight loss measurements, these data are considered to reflect primarily the rate of intergranular dissolution. Fig. 1 shows a typical result of intergranular attack of a solution-treated Type 304 stainless steel after 4 hr in a boiling nitric-dichromate solution. The wide grain boundary grooving at the surface, and the attack at incoherent twin boundaries, are evident; very little corrosion attack is seen at the coherent twin boundaries. INTERGRANULAR CORROSION MODEL