The role of passivity in the corrosion resistance of metals and alloys

Some of the physico-electrochemical properties of passive oxide films that form on reactive metal and alloy surfaces, and which protect the underlying metals from reaction with corrosive environments, are reviewed within the context of the Point Defect Model (PDM) and the Mixed Potential Model (MPM). These models yield analytical expressions (PDM) and/or rapidly convergent numerical algorithms (MPM) for the steady state current and film thickness that can be used to predict deterministically the accumulation of general corrosion damage to metal surfaces, provided that the evolutionary path to the future state is continuous and can be specified. The calculation of damage is illustrated by estimating the loss in wall thickness of Alloy-22 canisters containing high-level nuclear waste in a c(dry) (above the water table) geological repository. The models predict that the canisters will lose about 1.6 mm in wall thickness, compared with a design thickness of 2 cm, over the 10,000-year design life. Finally, a brief discussion is given of the conditions that must be met for a reactive metal to become passive, as it is the retention of passivity that makes possible the use of reactive metals in our metals-based civilization.