Abstract
Radioactive liquid waste has been stored in underground carbon steel tanks for nearly 70 years at the Hanford nuclear facility. Vapor space corrosion of the tank walls has emerged as an ongoing challenge to overcome in maintaining the structural integrity of these tanks. The interaction between corrosive and inhibitor species in condensates/supernates on the tank wall above the liquid level, and their interaction with vapor phase constituents as the liquid evaporates from the tank wall influences the formation of corrosion products and the corrosion of the carbon steel. Ammonia inhibition of vapor space corrosion has been observed on an anecdotal basis and through limited laboratory testing. Ammonia is produced predominantly in the liquid waste through thermal and radiolytically induced reactions between organic waste components and nitrate and nitrite anions. The demonstration of corrosion inhibition by ammonia may provide a technical basis for the observed lack of propensity of corrosion in the vapor space of the waste tanks. New corrosion chemistry limits for the waste have been recommended to minimize the threat of stress corrosion cracking (SCC) of the carbon steel. Previous testing demonstrated that the new SCC inhibitor requirements for the waste were not sufficient for mitigation of pitting in the vapor space for the anticipated waste chemistry envelope. Coupon tests demonstrated that ammonia concentrations typically observed in the tank vapor space (i.e., between 50 and 550 ppm) are sufficient to mitigate vapor space corrosion.
