Grain boundary segregation in austenitic stainless steels and its effect on intergranular corrosion and stress corrosion
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I.
INTRODUCTION
DURINGthe last decade
a large amount of research has been devoted to determining the causes of intergranular stress corrosion cracking in austenitic stainless steels. One of the primary reasons for carrying out this research has been the occurrence of intergranular stress corrosion cracking in these materials while they are in service in nuclear reactors.1 Cracks often occur in environments typical of boiling water reactors when the grain boundaries of the material are depleted of chromium. This depletion occurs because chromium-rich carbides (M23C6) precipitate along the grain boundaries. As they grow, carbon rapidly diffuses to the carbide-matrix interface and enters the growing carbide. Chromium diffuses much more slowly and is usually drawn from a volume of material around the grain boundary. This process leads to depletion of chromium along the grain boundaries, 2-5 and a steel in this condition is often referred to as being sensitized. A number of recent experimental results have shown that intergra~aular stress corrosion cracking can occur in simulated BWR environments in materials that have not been sensitized. 6'7'8 These results raise the question of whether or not some other change in grain boundary composition has made the grain boundaries particularly susceptible to corrosive attack. The most likely possibility would be the segregation of impurity elements such as phosphorus and sulfur. However, there have been only a few studies reported in the literature that have measured grain boundary segregation of impurity elements and correlated this segregation with intergranular corrosion and stress corrosion cracking in stainless steels. 6'9-~4 In this paper we address the issue of what causes intergranular corrosion and stress corrosion cracking in nonsensitized stainless steels. First, we will present results C.L. BRIANT and P.L. ANDRESEN are Staff Scientists at the Corporate Research and Development Center, General Electric Company, E O. Box 8, Schenectady, NY 12301. Manuscript submitted April 17, 1987. METALLURGICALTRANSACTIONS A
concerning grain boundary segregation of nitrogen, phosphorus, and sulfur in types 304L and 316NG stainless steels. We will briefly review the data presented in three previous papers ~2'14a5 as well as report new results. In this way we will be able to present a rather complete picture of the segregation behavior of these three elements. We will then consider the effect of segregation on intergranular corrosion in both the modified Strauss test and the Huey test. Finally, we will consider the effect of segregation on stress corrosion cracking in a high temperature aqueous environment. The results will show that nitrogen, phosphorus, and sulfur segregate to the grain boundaries but that the extent of segregation of each element is affected by the segregation of the others. None of these elements appears to accelerate intergranular corrosion in the modified Strauss test, but phosphorus clearly accelerates corrosion in the Huey test. Sulfur appears to be a major
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