Carbon steel is a potential canister material for high-level waste disposal in underground geologic repository systems where reducing conditions dominate after the thermal period. For example, the super container concept in the proposed Boom Clay repository concept in Belgium comprises a carbon steel overpack containing the waste and an outer stainless steel liner forming the boundary of an annular region containing a cementitious material. The outer surface of the carbon steel overpack would be in contact with concrete pore water with pH near 12.5, expected to persist for thousands of years. This study investigated carbon steel passivity, corrosion rate, repassivation, hydrogen generation, and hydrogen induced cracking tendency in simulated concrete pore water under anoxic conditions, and the effects of possible anions in the pore water such as Cl−, , and S2−. Carbon steel maintained its passivity at 50°C and 80°C up to a test period of 6 months and passive corrosion rates were mostly in the range of 0.1...