51312-01541-Effect of Chloride and Sulfate on Probability Based Corrosion Control for Waste Tanks

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Product Number: 51312-01541-SG

ISBN: 01541 2012 CP

Author: Elizabeth N. Hoffman

Publication Date: 2012

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A chemistry control program with the aim to reduce the pitting corrosion occurrence on tank walls has thus far been implemented in part by applying engineering judgment safety factors to experimental data. It is proposed that a probability-based approach can be used to quantify the risk associated with the chemistry control program. This approach can lead to the application of tank-specific chemistry control programs reducing the overall costs associated with overly conservative use of inhibitor. Furthermore when using nitrite as an inhibitor the chemistry control program is based on a linear model of increased aggressive species requiring increased protective species. Primarily supported by experimental data obtained from dilute solutions with nitrate concentrations less than 0.6 M this linear model was used to produce the current chemistry control program at 1.0 M nitrate or less. Data collected from recent years have been plotted and suggest a reduction of nitrite is possible particularly at higher concentrations of nitrate. The total sum of data suggested a non-linear model known as the mixture/amount model could be used to predict the probability of corrosion of ASTM A537 carbon steel similar to the steel utilized for Type III/IIIA waste tanks in varying solutions. The mixture amount model takes into account not only the total concentration of species in a solution but also the ratio of inhibitors and aggressive species. The latter was historically the only factor taken into consideration in the development of the current chemistry control program. Recent testing also suggests a significant effect of concentration of chlorine and sulfate ions in the risk for corrosion to occur. This work evaluates the effect of chloride and sulfate ions and considers that effect in relationship to the mixture/amount model developed based on varying the nitrate and nitrite concentration alone.