Grain boundary segregation of phosphorus and
- PDF / 976,159 Bytes
- 9 Pages / 594 x 774 pts Page_size
- 10 Downloads / 290 Views
I.
INTRODUCTION
AUSTENITIC stainless steels are often used as structural materials in applications that require corrosion resistance. It has been known for many years that this corrosion resistance depends on both the composition of the alloy and on the heat treatment it has received. 1-5 Early research 1'3 clearly demonstrated that the presence of chromium in these alloys makes a major contribution to their general corrosion resistance. Therefore, it was hypothesized that the heat treatments that cause these alloys to lose their resistance to intergranular corrosion must allow a solid state reaction to occur that lowers the chromium concentration at the grain boundaries. 3'4'5 The precipitation of chromium-rich carbides was suggested as a likely candidate. In 1941 Bain, Aborn, and Rutherford6 proposed a detailed version of this model and provided extensive experimental data to substantiate it. They suggested that during these detrimental heat treatments chromium-rich carbides precipitate along the grain boundaries. As these carbides grow, carbon can rapidly diffuse from the matrix to the carbidematrix interface and be available to enter the growing carbide. Chromium diffuses more slowly. In order for it to keep up with the supply of carbon, it must initially be drawn from a volume of material near the growing carbide. Consequently, the matrix level of chromium around the carbide will be lowered. Since these carbides usually precipitate along the grain boundaries, it is the grain boundary region that will become depleted of chromium. This depletion leads to intergranular corrosion. Since the publication of this model, many additional studies have helped to substantiate its general validity. 7-11 Studies that employed analytical electron microscopy have now proven that chromium depletion does occur and that its occurrence can fully explain the results of many corrosion tests. 9'12-16 Yet, through the years a number of observations of intergranular corrosion have been made which cannot be explained by the chromium depletion model. 17-23These results were usually produced as a result of corroding samples in very oxidizing solutions such as boiling nitric acid or
C.L. BRIANT is Staff Metallurgist with General Electric Company, Research and Development Center, P.O. Box 8, MB 269-K1, Schenectady, NY 12301. Manuscript submitted June 18, 1986. METALLURGICALTRANSACTIONS A
nitric acid containing hexavalent chromium ions. The possibility of chromium depletion was ruled out because the samples were either solution annealed at a high temperature so that all carbides were dissolved or the carbon content was too low to allow precipitation of carbides. In these instances, impurity segregation to the grain boundaries was proposed to be the primary microstructural cause of the corrosion. Phosphorus, in particular, was thought to be damaging. 18-21 There has been little published work 24-28 that examines impurity segregation to grain boundaries in austenitic stainless steels, and only a few of these papers 26'28 have tried to rel
Data Loading...
