Corrosion of Low Carbon Steel in High Nitrate Solutions

High-level waste is stored at the Savannah River Site in large underground tanks constructed of carbon steel.  As part of storage management, the waste is evaporated in order to reclaim tank space.  The hot liquid from the evaporator is cooled in a waste tank and crystallizes forming a material referred to as salt cake.  The salt crystals tend to be rich in nitrate ion, while the interstitial liquid in equilibrium with the crystals tends to contain hydroxide and nitrite species.  In order to prepare feed for waste immobilization processes, the salt cake must be dissolved.  Water is utilized to dissolve the salt cake so that it can be transferred 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.  This occurrence has been demonstrated in laboratory experiments, material balance models, and from in-tank waste samples.  Although the addition of inhibitors to the water utilized to dissolve the salt is necessary to meet the requirements, this action can 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 nitrate stress corrosion cracking and pitting.  The basis for the inhibitor requirements for salt dissolution wastes is being revisited with the desire to investigate the risks associated with operating at these lower inhibitor concentrations for the short period of time that salt dissolution occurs.  Cyclic polarization, linear polarization resistance and long-term open circuit potential monitoring were performed to investigate the corrosion behavior of welded and un-welded samples.  Polarized U-bend tests on welded carbon steel samples and slow strain rate tests on heat treated carbon steel samples were performed to determine the inhibitor requirements for prevention of stress corrosion cracking. The behavior as a function of temperature, aggressive species (nitrate) concentration and inhibitor concentration was determined.