Corrosion Testing of Carbon Steel in Acid Cleaning Solutions

Abstract

Executive SummaryHigh level waste is stored in carbon steel tanks at the Savannah River Site (SRS). The site is currently in the process of waste removal from, and ultimately closure of, these tanks. One of the most time consuming steps in the waste removal process is cleaning the sludge heel from the bottom of the tanks to an acceptable residual quantity. The sludge consists primarily of metal oxides that formed after waste from the canyons was neutralized with sodium hydroxide. Since the canyon waste was originally a nitric acid solution, this acid is a prime candidate for sludge heel dissolution.A series of exploratory tests were performed to investigate the hypothesis that the corrosion rate of carbon steel in nitric acid could be inhibited with oxalic acid. These tests were performed at two nitric acid concentrations (0.3 and 3 M) and three oxalic acid concentrations (4 wt. %, 8 wt. %, and 12 wt. %) and were limited to the expected contact time for sludge dissolution (approximately 3 days). Carbon steels (ASTM A285 and A537) utilized in the construction of Type I, II and IIIA tanks were tested.The general corrosion rate, as well pit depths, were measured and compared. The results of the tests suggest that oxalic acid may inhibit steel corrosion in nitric acid solutions that have concentrations on the order of 0.3 M. For short contact times, these solutions may be viable as sludge dissolution media. In contrast, essentially no passivation was observed during the first 3 days in the 3 M nitric acid/oxalic acid solutions. Therefore, utilization of solutions with nitric acid concentrations on the order of 3 M for sludge dissolution are not recommended. More testing at better defined sludge removal conditions (i.e., perhaps higher temperatures, longer contact times and other species present) is needed to evaluate the recommendation for utilization of the more dilute nitric acid solutions for sludge dissolution. Additional studies to investigate sludge and fissile material dissolution in these dilute nitric acid/oxalic acid solutions are also necessary. BackgroundHigh level waste is stored in carbon steel tanks at the Savannah River Site (SRS). The site is currently in the process of waste removal from, and ultimately closure of, these tanks. One of the most time consuming steps in the waste removal process is cleaning the sludge heel from the bottom of the tanks to an acceptable residual quantity. In the past mechanical processes have been attempted with limited success. Therefore, an alternate chemical means of dissolving the sludge is being considered.The sludge consists primarily of metal oxides that formed after waste from the canyons was neutralized with sodium hydroxide. Since the canyon waste was originally a nitric acid solution, this acid is a prime candidate for sludge heel dissolution. However, nitric acid is very corrosive to the carbon steel waste tank. An inhibitor could be added to nitric acid to reduce its corrosivity towards carbon steel yet maintain its metal oxide dissolution efficiency. The c...

Sorption coefficients and molecular mechanisms of Pu, U, Np, Am and Tc to Fe (hydr)oxides: A review

“…For each scenario, it is commonly assumed that the steel in the engineered barriers (including rebar) for disposed waste or the carbon-steel tank liners holding the liquid waste do not sorb the radionuclides. Tank liners have been shown to form corrosion products, such as FHOs [21]. It is important to understand how extensively radionuclides sorb to corrosion products and if the current risk calculation assumptions are conservative.…”

Alternative and Enhanced Chemical Cleaning: Basic Studies Results Fy2010

“…This amount of corrosion does not significantly impact the structural integrity of the tank (McKeel, 2008) or the performance assessment analysis (Garcia-Diaz, 2010). Previous testing in oxalic/nitric acid mixtures suggested that the corrosion rate for carbon steel in dilute acid mixtures (i.e., 0.3 M nitric acid) would result in a "manageable" amount of degradation over a short period of time (Wiersma, 2002). These expectations were confirmed during recent corrosion evaluations performed in support of the EM-31 chemical cleaning program (Wiersma, 2010).…”

Waste Tank Heel Chemical Cleaning Summary

“…76 Experimental data for the corrosion of carbon steel in nitric acid at room temperature is shown in Table 27. 77,78,79…”

“…Coupon testing of carbon steel was performed in solutions of nitric acid and oxalic acid. 79 Most of the testing was conducted for 3 days at ambient temperature. The general corrosion rates on carbon steel similar to that utilized for the waste tanks are shown in Table 28.…”