SYMPOSIA PAPER Published: 01 January 1996

The Use of Electrochemical Noise to Investigate the Corrosion Resistance of UNS Alloy N04400 Nuclear Heat Exchanger Tubes


Underdeposit corrosion of freshwater-cooled shutdown heat exchangers (SHX) and steam generators (SG) resulted in unscheduled unit shutdowns very early in the life of the Ontario Hydro Pickering B Nuclear Generating Station, whereas the same SHX and SG in the identical A station had not experienced any problems in over 19 years of operation. UNS alloy N04400 tubes were used for SHX and SG at both stations. Because detailed metallurgical analysis of tubes removed from the A and B stations suggested that alloy differences may be a major factor to the early failures at the B station, an investigation to characterize the corrosion resistance of the two alloys was carried out. From standard accelerated corrosion test methods it was inferred that the A and B alloys had similar corrosion resistance, suggesting that subtle but important material differences may have been masked by the aggressiveness of the test methods. Therefore, the electrochemical noise measurement (ENM) technique in conjunction with less aggressive test environments was used to characterize better the corrosion resistance of the A and B alloys. Variations in the corrosion resistance of the material removed from Pickering A and B heat exchangers (HX) were evident. During an initial exposure to deionized (DI) water at a pH of 5, the B alloy generally displayed greater passivity than the A material. However, the B alloy exhibited far less corrosion resistance than the A material when the pH of the water was reduced to 1 by the introduction of a nitric acid/acetic acid mixture. The predominant mode of degradation in the B material was intergranular attack with some localized corrosion in grain interiors. The A alloy, on the other hand, exhibited general corrosion.

The study also showed that the corrosion resistance of the A alloy was markedly reduced by heat treatment. A solution heat treatment at 900°C for 1 h followed by a cold-water quench or furnace cool from this temperature led to a corrosion performance similar to the nonheattreated B alloy. The same heat treatment did not significantly alter the corrosion performance of the B alloy. Changes in the mode of attack of the A and the B alloys were also observed when samples had been thermally treated. Both exhibited crystallographic attack, which was very localized in some areas. Transmission electron microscopy and secondary ion mass spectrometry revealed changes in the microstructure, which may account for the differences in the mode of attack. The predominant precipitate chemistry in the A alloy was Cr23C6, whereas in the B material it was titanium carbide. The B alloy also displayed boron segregation at the grain boundaries. This seemed to influence the extent of intergranular attack in this alloy.

Author Information

Brennenstuhl, AM
Ontario Hydro Technologies, Toronto, Ontario, Canada
Palumbo, G
Ontario Hydro Technologies, Toronto, Ontario, Canada
Gonzalez, FS
Ontario Hydro Technologies, Toronto, Ontario, Canada
Quirk, GP
CML Ltd., Manchester, United Kingdom
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Developed by Committee: G01
Pages: 266–287
DOI: 10.1520/STP37965S
ISBN-EB: 978-0-8031-5562-6
ISBN-13: 978-0-8031-2032-7