What is it about?
Buried steel tanks are used to store legacy radioactive wastes emanating from World War II and Cold War nuclear weapons production. These tanks have to maintain their integrity for the next few decades until all the wastes are extracted and processed for permanent disposal. An important factor in the long-term integrity of these tanks is their corrosion potential, which is the natural electrochemical potential (also called the open-circuit potential) of the inner tank wall exposed to various chemical species in the waste. If the electrochemical potential exceeds certain threshold values, localized corrosion and stress corrosion cracking can ensue leading to waste leakage. This paper examines the rate of change of the electrochemical potential with time and correlates it to the various chemical species in the waste, the pH of the waste, and the surface condition of the steel. These correlations are supported by fundamental examination of the protective film that forms on the steel. Such time-based correlations can be useful in determining when to inspect the tanks and which tanks to inspect first.
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Why is it important?
Until recently, it has been assumed that the corrosion potential of the tank steel attainst a steady-state value quite rapidly (within a few days at most). This misimpression is created when plotting the time evolution of potential on a linear time scale. However, our studies have shown that the potential increases slowly over time over three years and this is evident when it is plotted on a log scale. We hope to use these relationships to classify different waste types and investigate the long-term health of the tanks.
Perspectives
In the corrosion literature, attention is often focused on threshold potentials above which localized corrosion or cracking occurs. The corrosion potential (i.e. the natural potential of the metal under exposure) is not studied over long time periods under controlled conditions. Potential monitoring of the tanks is done in the field, but these monitoring results need to be compared to long-term measurements under controlled conditions to understand the mechanisms of degradation of the tanks in the field. Ultimately, even our "long-term" studies are not truly representative of the multi-decadal exposure of the real tanks. Therefore, a different paradigm is needed to evaluate the true steady-state potential (assuming that the waste chemistry history is known). Such an evaluation of true steady-state potential cannot be obtained by extrapolating shorter-term laboratory data, but only by modeling the evolution of passive films on the steel. This is the direction we are taking going into the future.
narasi sridhar
DNV GL AS
Read the Original
This page is a summary of: Long-Term Evolution of Corrosion Potential of Carbon Steel in Alkaline Radioactive Waste Environments, CORROSION, January 2019, NACE International,
DOI: 10.5006/2979.
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