What is it about?

Ni-Cr-Fe alloys used as steam generator tubing materials may suffer two different pitting corrosion processes depending on the potential range and the thiosulfate and chloride concentrations. High-potential and low-potential pitting corrosion processes are identified. They have different features: low potential pits are round and grow slowly while high potential pits have a lacy cover and grow quickly.

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Why is it important?

For the first time, two different a pitting corrosion processes are identified for alloy 690 and 800, which are the current choices of tubing materials for nuclear steam generators.


Low potential pitting corrosion may occur in conditions of steam generators wet lay-up; i.e., near-room temperature , low potential due to oxygen scavenging, presence of chlorides from thermal crevices (hide-out return) plus minute amounts of thiosulfate (from sulfate reduction). Ongoing research is focused on stating the environmental and metallurgical conditions for the occurrence of low potential pitting corrosion. It would not be the first time for the nuclear industry to first observe a phenomenon in a laboratory that is later found in service.

Dr Martín A Rodríguez
Comision Nacional de Energia Atomica

Read the Original

This page is a summary of: Effect of thiosulfate on pitting corrosion of Ni-Cr-Fe alloys in chloride solutions, CORROSION, August 2018, NACE International,
DOI: 10.5006/2923.
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