Corrosion of PHWR PHT system structural materials by dilute chemical decontamination formulations containing ascorbic acid

摘要

Decontamination of primary heat transport (PHT) systems of PHWRs is normally carried out using reducing type formulations that contain reducing and complexing agents. Oxalic acid is used as a reductant in many of the commercial and non-commercial formulations. This Paper presents an attempt to evaluate ascorbic acid as a reductant in a chemical formulation. Thermal and radiation stability and corrosion behaviour of ascorbic acid have been evaluated and compared with those for oxalic acid. A formulation containing ethylenediaminetetraacetic acid (EDTA) (400 mg/dm~3), ascorbic acid (300 mg/dm~3) and citric acid (300 mg/dm~3) was evaluated for its compatibility with PHT system construction materials viz. carbon steel, monel-400 and zircaloy-2. In a dilute chemical decontamination process, which is operated in a regenerative mode, the formulation is regenerated using ion exchange resins. It has been observed that the rate at which the formulation is regenerated determines the corrosion rate. The importance of maintaining a suitable volume to surface area ratio for estimating the corrosion rate of carbon steel has also been discussed. During the dissolution of magnetite from the carbon steel, the base metal participates in the dissolution process. If the magnetite layer is thin then base metal corrosion and magnetite dissolution processes take place simultaneously. It has been observed that the suppression of base metal corrosion by an inhibitor enhances the magnetite dissolution. In addition, the effect of Fe~(3+) on carbon steel corrosion and the influence of oxygen on monel-400 corrosion have been studied.