Chromium depletion in the vicinity of carbides in sensitized austenitic stainless steels
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INTRODUCTION
THE termsensitization refers to the breakdown in corrosion resistance which may occur if unstabilized austenitic stainless steels are slowly cooled from the solution anneal temperature (-1100 ~ or are reheated in the temperature range 600 to 750 ~ This is in contrast to the excellent corrosion resistance exhibited by these steels if they are rapidly cooled from the solution anneal temperature. In the sensitized state, the steels are susceptible to intergranular corrosion, and the original model to explain this behavior was proposed by Bain and his co-workers. ~ In this model, sensitization is associated with the precipitation of chromium-rich M23C6 carbides along grain boundaries during the detrimental heat treatments. The interstitial carbon atoms diffuse to the boundaries very rapidly and the subsequent precipitation depletes the matrix and grain boundary regions surrounding the carbide of chromium, which diffuses much more slowly. Since chromium is the essential element in the corrosion resistance of stainless steels, these chromium depleted regions are susceptible to attack and thus the corrosion proceeds intergranularly. At the present time, the chromium depletion theory is generally accepted as providing the principal explanation for sensitization, Several experimental studies 2-6 have shown the correlation of the onset of sensitization with the presence of carbides at grain boundaries. However, several other factors have been shown to affect the sensitization response ERNEST L. HALL and CLYDE L. BRIANT are Members of the Technical Staff at the General Electric Corporate Research and Development Center, P.O. Box 8, Schenectady, NY 12301. Manuscript submitted August 1, 1983.
METALLURGICALTRANSACTIONS A
of stainless steel, 7-~~and other elements besides chromium have been found to play an important role in this response, "-16 including molybdenum, nitrogen, manganese, and phosphorus. The actual chromium distribution in the vicinity of a carbide in stainless steel has been studied quite extensively using thermodynamic models by several authors. 12'~7"1s Stawstrom and Hillert 17proposed that stainless steel is susceptible to corrosion only when a zone of less than about 13 at. pct Cr and of width greater than 20 nm existed at the grain boundaries. They also studied the process of healing, which is the term used to describe the return of corrosion resistance of the stainless steel after prolonged heating in the temperature range which initially causes sensitization. They found that healing could be accounted for by the increase in matrix chromium content at the carbide-matrix interface caused by changes in the carbon activity in the steel. A major deficiency of their model was the assumption of a uniform chromium content throughout the grain boundary, which is physically unrealistic in light of experimental evidence of nonuniform carbide distributions. Tedmon et al. 18 used a more general approach which calculated the chromium distribution both along the grain boundary between carbide particles
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