The initial stages of pitting of aluminum, under constant applied current in 1N HC1 at 65~ have been studied using millisecond current pulses and pit size distributions measured with scanning electron microscopy. Comparison of the faradaic current with the total pit area determined from SEM indicated that metal dissolution from pits is at a constant apparent current density of about 6 A/cm ~. However, the pit sizes at early times show that the equivalent current density of initial pit growth is over 100 A/cm 2. Comparison of pit volumes to charge passed confirmed that initial growth was unaccompanied by faradaic charge. However, for foils which had not been pretreated by immersion in 1N HC1 at 25~ an equivalent charge was passed during early pit growth. Implications of these observations for pit initiation mechanisms are discussed.

Transient events accompanying passivation of active surfaces in aluminum etch tunnels are investigated. Passivation is induced by pulsed reductions of the anodic etching current, of several milliseconds duration. Scanning electron microscopy is used to measure the area passivated. Potential transients are analyzed to identify a possible potential driving force for passivation. For the experimental conditions of this work, at 0.13 ms after the current step, there is a temporary reduction in the metal dissolution current in tunnels. The change in dissolution current is proportional to the cathodic surface overpotential, relative to the critical repassivation potential. The transient reduction in dissolution current is associated with passivation, since no variation in dissolution current with potential is observed during steady tunnel growth. Surface changes resulting in permanent passivation occur later, at times between 3 and 12 ms after the current step.

No abstract

No 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...