Precipitation formation on Σ5 and Σ7 grain boundaries in 316L stainless steel and their roles on intergranular corrosion

In sensitized (700 degrees C for 10 h) AISI 316L stainless steel samples attacked by an HF solution, coincidence site lattice (CSL) Sigma 7 grain boundaries (GBs) always show corrosion while Sigma 5 GBs never do. This distinct corrosion behaviour can be related to the distinct precipitation behaviour of the different GBs, which itself depends on the GB characters. It is found that Sigma 5 GBs, although being curved both macro- and microscopically, neither show facets nor precipitates. The amount of segregation is enhanced in sensitized Sigma 5 GBs compared to solution-annealed ones. In contrast, Sigma 7 GBs show clear faceting in both the solution-annealed and sensitized state, with one facet orientation showing strong segregation while the other not. GB energy anisotropy is applied to explain the faceting phenomenon. The formation of two kinds of precipitates, i.e., C14 Laves phase and M23C6 carbide, holding responsible for two distinct morphologies of corrosion, are believed to be both linked to intensive faceting and the anisotropic segregation of Sigma 7 GBs. For the carbide, a preferred {111} orientation out of all Sigma 7 facets measured by trace analyses is presumed to facilitate its nucleation. (C) 2021 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.