CORROSION TESTING IN SIMULATED TANK SOLUTIONS

Hoffman, E.

Savannah River Site (S.C.)

关键词: Corrosion;    Nitric Acid;    Carbon Steels;    Radiolysis;    Irradiation;   

DOI  :  10.2172/1004203
RP-ID  :  SRNL-STI-2010-00509
RP-ID  :  DE-AC09-08SR22470
RP-ID  :  1004203

美国|英语

来源: UNT Digital Library

【 摘 要 】

Three simulated waste solutions representing wastes from tanks SY-102 (high nitrate, modified to exceed guidance limits), AN-107, and AY-102 were supplied by PNNL. Out of the three solutions tested, both optical and electrochemical results show that carbon steel samples corroded much faster in SY-102 (high nitrate) than in the other two solutions with lower ratios of nitrate to nitrite. The effect of the surface preparation was not as strong as the effect of solution chemistry. In areas with pristine mill-scale surface, no corrosion occurred even in the SY-102 (high nitrate) solution, however, corrosion occurred in the areas where the mill-scale was damaged or flaked off due to machining. Localized corrosion in the form of pitting in the vapor space of tank walls is an ongoing challenge to overcome in maintaining the structural integrity of the liquid waste tanks at the Savannah River and Hanford Sites. It has been shown that the liquid waste condensate chemistry influences the amount of corrosion that occurs along the walls of the storage tanks. To minimize pitting corrosion, an effort is underway to gain an understanding of the pitting response in various simulated waste solutions. Electrochemical testing has been used as an accelerated tool in the investigation of pitting corrosion. While significant effort has been undertaken to evaluate the pitting susceptibility of carbon steel in various simulated waste solutions, additional effort is needed to evaluate the effect of liquid waste supernates from six Hanford Site tanks (AY-101, AY-102, AN-102, AN-107, SY-102 (high Cl{sup -}), and SY-102 (high nitrate)) on carbon steel. Solutions were formulated at PNNL to replicate tank conditions, and in the case of SY-102, exceed Cl{sup -} and NO{sub 3}{sup -} conditions, respectively, to provide a contrast between in and out of specification limits. The majority of previous testing has been performed on pristine polished samples. To evaluate the actual tank carbon steel surface, efforts are needed to compare the polished surfaces to corroded and mill-scale surfaces, which are more likely to occur in application. Additionally, due to the change in liquid waste levels within the tanks, salt deposits are highly likely to be present along the tank wall. When the level of the tank decreases, a salt deposit will form as the solution evaporates. The effects of this pre-existing salt, or supernate deposit, are unknown at this time on the corrosion effect and thus require investigation. Additionally, in the presence of radiation, moist air undergoes radiolysis, forming a corrosive nitric acid condensate. This condensate could accelerate the corrosion process in the vapor space. To investigate this process, an experimental apparatus simulating the effects of radiation was designed and constructed to provide gamma irradiation while coupons are exposed to a simulate tank solution. Additionally, ammonia vapors will also be introduced to further represent the tank environment.