Grain boundary phenomena strongly depend on grain boundary structure and character, i.e., coincidence site lattice (CSL) boundaries, as contrasted with random boundaries, are highly resistant to grain boundary deterioration. Grain boundary engineering (GBE) primarily intends to prevent the initiation and propagation of intergranular degradation along random boundaries by frequent introduction of CSL boundaries into the grain boundary networks in materials. A high frequency of CSL boundaries by GBE leads to high resistance to grain boundary deterioration. By twin-induced GBE utilizing optimized one-step thermomechanical processing with small pre-strain and subsequent annealing, a very high frequency of CSL boundaries was introduced into type 304 and 316 austenitic stainless steels. The resulting steels indicated remarkably high resistance to intergranular corrosion during corrosion tests. The high CSL frequency resulted in a very low percolation probability of random boundary networks in the per-threshold and a remarkable suppression of intergranular deterioration during twin-induced GBE.