Pitting Corrosion of Inconel 600 in High-Temperature Water Containing CuCl2

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Abstract
Pitting corrosion of Inconel 600 was studied in aqueous sodium and cupric chloride solutions at 60 and 280 C. The pit nucleation potential, Enp, was evaluated in two different concentrations of sodium chloride. Enp decreased with increasing concentrations of the chloride ion and with temperature. On specimen surfaces exposed to cupric chloride solutions, pitting occurred at open circuit potentials nearly equal to or higher than the Enp determined by anodic polarization in 0.01 M NaCl solution. The number and size of the pits increased with increasing concentrations of cupric chloride and dissolved oxygen. On specimens partly covered with polytetrafluorethylene (PTFE) tape (i.e., in the presence of artificial crevices), pitting occurred more easily at low concentrations of CuCl2 (≤ 20 ppm CuCl2 in deaerated solutions at 280 C). Tubes covered with oxide films that formed during the operation of model boilers exhibited greater pitting resistance than tubes with clean surfaces at 280 C, but less resistance at 60 C. Corrosion products contained in the pits were enriched in chromium with small amounts of copper, sulfur, and chlorine. The composition of corrosion products covering the pits was similar to that in the pits, but with the additional enrichment of iron. Presumably, sulfur present in Inconel 600 as an impurity was significant in the pitting process. The probable mechanism of the processes leading to pitting of Inconel 600 tubing in high-temperature water is discussed.

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