A novel, low-temperature carburization process has been developed that allows dissolving up to 12 at% carbon in austenitic stainless steels without carbide formation, due to carbon diffusion under paraequilibrium conditions.  Applying the process to 316L austenitic stainless steel, the “colossal” supersaturation with carbon extends over a “case” with a depth of about 25 µm. This leads to a significant improvement in aqueous corrosion resistance. For example, the critical pitting temperature, pitting potential, and crevice corrosion behavior are all greatly improved in standard NaCl solutions.  The treatment is perfectly shape conformal, so that samples with rough surfaces display the same improvement in corrosion performance as samples with a mirror-smooth finish.  The enhancement in corrosion resistance is correlated with the increased carbon concentration in the passive oxide layer, not with any variations in the Fe:Cr ratio.