Corrosion Susceptibility of Secondary Liner Carbon Steel of Hanford Double-Shell Tanks Exposed to Supernatant Waste and Ground Water

The primary liner failure event of double-shell Tank 241-AY-102 observed in 2012 resulted in direct contact of tank waste to the inside surface of the secondary liner. The potential also exists for ground water to contact the outside surfaces of all double-shell tank secondary liners. The objective of this work was to evaluate the corrosion susceptibility of the secondary liner in contact with tank waste and ground water under double-shell tank environmental conditions. Crevice corrosion assemblies that sandwiched an A515 Grade 60 carbon steel coupon between two blocks of refractory material were used to test refractory and steel interactions when exposed to tank supernatant waste. During the year-long test in tank waste, corrosion potential and corrosion rates were monitored periodically and were compared to those obtained from a non-creviced coupon exposed to the same conditions. Crevice corrosion assemblies that sandwiched the two ends of carbon steel coupons between concrete were submerged in ground water up to 28 months to test corrosion susceptibility in contact with ground water along with coupons set above the water surface to test steel corrosion in contact with moisture. Results showed that the contact of tank waste with double-shell tank secondary liner steel creviced with refractory material over 12 months had an insignificant effect on the corrosion susceptibility of the secondary liner. However, the secondary liner material was much more susceptible to corrosion under exposure to ground water and moisture compared to the same material exposed to tank waste. The severity of corrosion under exposure to ground water, especially localized corrosion, increased with exposure time. More corrosion mechanisms, such as pitting corrosion inside the steel-concrete crevice, were evident on the 28-month exposure coupon. The alkaline condition and the presence of nitrite as a corrosion inhibitor in the tank waste may have contributed to the high corrosion resistance of the secondary liner in contact with tank waste. The absorption of carbon dioxide from the atmosphere in the ground water over extended exposure duration may have contributed to the high localized corrosion rate in the materials that were in contact with ground water.