INTRODUCTION AND BACKGROUND
The Hanford tank reservation contains approximately 50 million gallons of liquid legacy radioactive waste from cold war weapons production, which is stored in 177 underground storage tanks. The tanks will be in use until waste processing operations are completed. The wastes tend to be high pH (over 10) and nitrate based. Under these alkaline conditions carbon steels tend to be passive and undergo relatively slow uniform corrosion. However, the presence of nitrate and other aggressive species, can lead to pitting and stress corrosion cracking. This work is a continuation of previous work that investigated the propensity of steels to suffer pitting and stress corrosion cracking in various waste simulants.
The focus of this work is an investigation of the sensitivity of the steels' pitting and stress corrosion cracking susceptibility to simulant pH. Previous work demonstrated that wastes that are high in aggressive nitrate and low in inhibitory nitrite are susceptible to localized corrosion. However, the previous work involved wastes with pH 12 or higher. The current work involves wastes with lower pH of 10 or 11. It is expected that at these lower pHs that a higher nitrite-to-nitrate ratio will be necessary to ensure tank integrity. This experimental work involved both electrochemical testing, and slow strain rate testing at either the free corrosion potential or under anodic polarization. The results of the current work will be discussed, and compared to work previously presented.
The Hanford tank reservation in Washington State contains approximately 50 million gallons of liquid legacy radioactive waste from cold war weapons production. The waste is stored in 177 underground storage tanks. The current plan is to construct a vitrification plant that will be used to process the waste. The underground storage tanks are expected to continue service until these processing operations are finalized. However, the tanks are approaching their original design lives, and analysis and testing are necessary to ensure that the tanks are fit for continued service and the waste processing operations are complete.
Most of the wastes being stored are highly alkaline (pH > 10). Under these conditions carbon steels tend to remain passive and undergo relatively slow uniform corrosion. However, carbon steels become susceptible to pitting and stress corrosion cracking (SCC) in the presence of certain aggressive species, such as chlorides and nitrates [1]. The original Single Shell Tanks (SSTs) at Hanford experienced some SCC failures due to the high nitrate contents and high residual stresses. Research at both Hanford and Savannah River National Laboratories (SRNL) demonstrated that cracking could be reduced by stress relieving the welds and maintaining a high alkalinity (pH > 13). These practices were incorporated into the construction and operations of the Double Shell Tanks (DSTs) to ensure their structural integrity.
Most of the wastes stored in the DSTs are within their required pH specification, and are expected to remain within these specifications until the waste processing facilities are complete and the wastes can be removed.
