Abstract
Several million gallons of high-level radioactive waste is stored in underground carbon-steel storage tanks at the Hanford site. The waste chemistries are complex mixture of organic and inorganic species and are predominantly alkaline. Carbon steel is expected undergo passive dissolution under the alkaline conditions, but certain corrosive species in the waste chemistries such as chloride and nitrate could cause pitting corrosion and stress corrosion cracking (SCC). The risk of pitting corrosion is determined using cyclic potentiodynamic polarization (CPP) tests and by estimating pitting factor which is a function of inorganic ion concentrations and temperature. However, for certain waste chemistries, the risk cannot be clearly determined as CPP test responses are mixed, and corresponding pitting factors fall close to delineation boundary of pitting versus no pitting. In such cases, difference between corrosion and repassivation potentials is used as key parameter to determine the risk level. However, corrosion potential can change due to long-term evolution of open circuit potential, thus complicating evaluation of the risk which increases when open-circuit potentials (OCPs) drift in anodic direction compared to the initial values. Electrochemical experiments were conducted to determine extent of drift in the OCPs as a function of surface conditions and waste simulant chemistry consisting of only inorganics. The electrochemical experiments were conducted for approximately 4 months, and OCP evolutions were recorded. Electrochemical test results are presented to provide quantification of the OCP evolutions as a function of surface characteristics and waste chemistries.
