ABSTRACT
Localized corrosion of solubilized and thermally aged alloys UNS N06690 and N06600, used in nuclear power plant steam generator tubing, was studied in simulated wet lay-up conditions. The results showed that, despite having half of the chromium content of alloy N06690, alloy N06600 was more resistant to crevice corrosion in dilute chloride solutions, at low temperatures. In concentrated chloride solutions, the crevice corrosion resistance of both alloys was similar. The heat treatment used for steam generator tubing, which produces carbide precipitation at grain boundaries, did not reduce the crevice corrosion resistance of tested alloys with respect to solubilized material. Pitting corrosion initiated at higher potentials than crevice corrosion for both alloys. Initiation of pitting corrosion in 0.01 mol/L NaCl, at 30 °C, occurred at higher potentials for alloy N06690 than for alloy N06600. Initiation potentials of localized corrosion for each alloy were not affected by heat treatment. Open circuit potentials of thermally aged alloys N06600 and N06690 in deaerated 10-4 mol/L LiOH, which simulates lay-up conditions, showed a significant safety margin over crevice corrosion repassivation potentials in 0.01 mol/L NaCl, at 30 °C. Dissolved oxygen might pose a risk of crevice corrosion in lay-up conditions if chloride concentration build-up in crevices becomes significant.
INTRODUCTION
Ni-Cr-Fe alloys UNS N06600 and N06690, and Fe-Ni-Cr alloy UNS N08800 have been used for nuclear power plant steam generator tubing.1-3 The tubes constitute the main primary system pressure boundary of a pressurized water reactor and they limit the lifetime of a steam generator.4 Alloy N06600 was one of the earlier choices for this application, though it has been replaced due to its high susceptibility to stress corrosion cracking (SCC) in primary water, especially in the mill annealed or solubilized conditions.5 Alloys N06690 and N08800 are the current choices for steam generator tubing. However, many nuclear power plants in operation still have alloy N06600 tubes. Alloy N06690 is the selected material for the helical steam generator tubing of CAREM (Central ARgentina de Elementos Modulares) prototype, which is a small modular reactor currently under construction in Argentina.6
Failure of steam generator tubes may occur in operation primarily due to any of several recognized SCC submodes.1-3 Use of alloy tubing with an optimized metallurgy along with improvements in steam generator design and strict control of water chemistry has led to a dramatic reduction of failures in operation.4 Alloys N06600 and N06690 are used in the thermally-aged condition since a microstructure with chromium carbides precipitated at grain boundaries improve their resistance to alkaline and primary water SCC.1 Such a microstructure may produce a sensitized material with chromium depleted zones at grain boundaries. In the case of alloy N06690, a minimum of 22 wt. % Cr is detected at grain boundaries for the most sensitized material (bulk content: ~29 wt. % Cr).7 For alloy N06600, the chromium concentration at the depleted region may be as low as 3.3 wt. % (bulk content ~16 wt. % Cr).8
