Corrosion of Low Carbon Steel in High Concentration Nitrate Solutions

High-level waste is stored at the Savannah River Site in large underground tanks constructed of carbon steel.  The waste is composed of three phases: salt cake, sludge and supernate.  In order to prepare feed for waste immobilization processes, the salt cake is dissolved in water for transfer to the feed preparation tank.  During the salt dissolution process, a stage is achieved in which the inhibitor concentrations may not meet the requirements of the corrosion control program for the waste tanks.  The addition of inhibitors may be counterproductive to the efficiency of the process and to waste minimization.  Given the high nitrate concentrations in the salt cake, the primary corrosion mechanisms of concern is pitting and nitrate stress corrosion cracking (SCC).  An experimental program was undertaken to assess both the corrosion susceptibility of low carbon steel to high nitrate (4.5-8.5 M) solutions with low inhibitor concentrations (<0.6 M hydroxide and <0.2 M nitrite) and the risks associated with operating at these lower inhibitor concentrations for the short period of time that salt dissolution occurs.  Polarized U-bend tests on welded carbon steel samples were performed to determine the inhibitor requirements for prevention of stress corrosion cracking.  The behavior as a function of heat treatment, aggressive species (nitrate) concentration and inhibitor concentration was determined.