Effect of grain boundary structure on sensitization and corrosion of stainless steel
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I.
INTRODUCTION
MANYexamples of uneven attack at grain boundaries of a sample are available in the literature. The energy associated with grain boundaries makes them favorable sites for solute segregation and precipitation reactions, either of which can lead to corrosion at the grain boundary. Grain boundary energy is a function of grain boundary structure and therefore varies among the boundaries of a sample. This can cause variations in the extent of these processes at the grain boundaries. Many theoretical models and calculations in the literature consider the relationship of the grain boundary structure and grain boundary energy. Notable among them are the dislocation models proposed by Burgers, ~ Bragg, 2 and Read and Shockley. 3 Cusps of lower grain boundary energy were predicted in the energy vs misorientation angle at intermediate values of grain boundary misorientation.3 The cusps have been rationalized on the basis of the coincident site lattice (CSL) model advanced by Bollman. 4 Coincident sites are those where atoms of the adjacent grains forming the grain boundary exactly register or coincide. Low energy cusps occur when coincidence occurs on the same atom at regular intervals of small (atomic) spacing. An increase in grain boundary energy is associated with both a deviation from a CSL orientation and a closer spacing of associated secondary grain boundary dislocations. Atomistic calculations can lead to the determination of the cusped nature of the grain boundary energy with misorientation, such as those performed by Vitek, et al. 5 This paper describes a study of the relationship between the extent of the sensitization process in stainless steels and the grain boundary structure. Sensitization is now generally accepted for austenitic stainless steels to be explained by the chromium-depletion theory, first proposed by Strauss, Schottky, and Hinnuber 6 in 1930 and by Bain, Aborn, and B.W. BENNETT is Member of Technical Staff, Bell Communications Research, 331 Newman Springs Road, Red Bank, NJ 07701. H.W. PICKERING is Professor, Department of Materials Science and Engineering, The Pennsylvania State University, 326 Steidle Building, University Park, PA 16802. Manuscript submitted September 30, 1985. METALLURGICALTRANSACTIONS A
Rutherford in 1933. 7 For the ferritic stainless steels several results have appeared in the literature that are consistent with the Cr-depletion theory, e . g . , Bond, 8 and an experimental proof can be found in the work of Frankenthal and Pickering. 9 For the determination of grain boundary structure, it was necessary to determine the orientations of the adjacent grains. 10,11Electron channeling patterns are widely used for such purposes, but not all scanning electron microscopes can be used in this way. On the other hand, the electron back scattering (EBS) technique can be used with virtually all scanning electron microscopes for the determination of crystal orientation. 10-~4
II.
EXPERIMENTAL
The EBS technique, originally developed by German researchers, ~2'13can be ope
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