Effect of Grain Boundary Structure on Sensitization and Corrosion of Stainless Steel.

This paper reports on a study of intergranular corrosion, particularly the influence of the crystallography of a grain boundary g.b. on the sensitization and grain boundary corrosion of stainless steels. Since sensitization of stainless steels depends on g.b. energy, this study included a determination of the misorientation angle of grain boundaries in a sample. A scanning electron microscope was modified for application of the electron backscattering EBS technique in conjunction with normal SEM imaging of the sample surface. In austenitic stainless steel Fe-18Cr-10Ni samples annealed at 1200 C for 1 hour., water quenched and aged for various times at 650 C, the degree of sensitization increased with increasing aging time. This was indicated by an increasing width of the g.b. groove . A plot of grove width vs. g.b. misorientation angle obtained by the EBS technique showed a range of groove crystallography and the coincident site lattice concept provided a rationalization of this observation. Similiar results were found for ferritic stainless steel Fe-19 Cr samples. The Cr concentration profiles as a function of time were calculated, using a finite difference method, both along the g.b. and normal to the g.b.