PROBABILITY BASED CORROSION CONTROL FOR LIQUID WASTE TANKS - PART III

Hoffman, E. ; Edwards, T.

Savannah River Site (S.C.)

关键词: Corrosion;    Probability;    Carbon Steels;    Liquid Wastes;    36 Materials Science;   

DOI  :  10.2172/1001183
RP-ID  :  SRNL-STI-2010-00246
RP-ID  :  DE-AC09-08SR22470
RP-ID  :  1001183

美国|英语

来源: UNT Digital Library

【 摘 要 】

The liquid waste chemistry control program is designed to reduce the pitting corrosion occurrence on tank walls. The chemistry control program has been implemented, in part, by applying engineering judgment safety factors to experimental data. However, the simple application of a general safety factor can result in use of excessive corrosion inhibiting agents. The required use of excess corrosion inhibitors can be costly for tank maintenance, waste processing, and in future tank closure. 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 overall costs associated with overly conservative use of inhibitor. Furthermore, when using nitrite as an inhibitor, the current chemistry control program is based on a linear model of increased aggressive species requiring increased protective species. This linear model was primarily supported by experimental data obtained from dilute solutions with nitrate concentrations less than 0.6 M, but is used to produce the current chemistry control program up to 1.0 M nitrate. Therefore, in the nitrate space between 0.6 and 1.0 M, the current control limit is based on assumptions that the linear model developed from data in the <0.6 M region is applicable in the 0.6-1.0 M region. Due to this assumption, further investigation of the nitrate region of 0.6 M to 1.0 M has potential for significant inhibitor reduction, while maintaining the same level of corrosion risk associated with the current chemistry control program. Ongoing studies have been conducted in FY07, FY08, FY09 and FY10 to evaluate the corrosion controls at the SRS tank farm and to assess the minimum nitrite concentrations to inhibit pitting in ASTM A537 carbon steel below 1.0 molar nitrate. The experimentation from FY08 suggested a non-linear model known as the mixture/amount model could be used to predict the probability of corrosion in ASTM A537 in varying solutions as shown in Figure 1. The mixture/amount model takes into account not only the ratio (or mixture) of inhibitors and aggressive species, but also the total concentration (or amount) of species in a solution. Historically, the ratio was the only factor taken into consideration in the development of the current chemistry control program. During FY09, an experimental program was undertaken to refine the mixture/amount model by further investigating the risk associated with reducing the minimum molar nitrite concentration required to confidently inhibit pitting in dilute solutions. The results of FY09, as shown in Figure 2, quantified the probability for a corrosion free outcome for combinations of nitrate and nitrite. The FY09 data predict probabilities up to 70%. Additional experimental data are needed to increase the probability to an acceptable percentage.