Examination of oxides and deposits on tubes removed from Bruce units

At the Bruce Nuclear Power Generating Station (BNPGS), sensitized Alloy 600 steam generator (SG) tubing in different units has exhibited different secondary side corrosion behavior at and/or near the upper roll joint transition zone (RTZ) of the tubes. Wastage and pitting was observed on tubes from Unit 3, while Unit 4 SG tubing has exhibited circumferential stress corrosion cracking (SCC) and intergranular attack (IGA) in addition to wastage and pitting. Intergranular attack in one SG from Unit 5 has been documented, whereas only micropitting has been observed on tubes from Units 6, 7 and 8. In preheaters (PH), which are also tubed with sensitized alloy 600, only a small number of shallow flaws with a crystallographic appearance were observed on some of the tubes. Of these corrosion mechanisms, the SCC currently affecting Unit 4 is of the most concern as it challenges the operating life of all of the SGs in that unit. To investigate the causes of the different corrosion behavior observed in different units, detailed surface and microscopy analysis was performed on the SG tubing (within the sludge pile and in the free span regions) and PH tubing (in the free span region) removed from different units. The results indicated that the oxides formed on all tube specimens were enriched in chromium at the outer surface with its absolute concentration being close to that in the base metal, suggesting that the pH of the crevice environment is from mildly acidic to mildly basic. It was also observed that the oxide morphology and thickness for SG tubes that suffered degradation was different compared to those tubes that appeared unaffected. Oxide thickness also appeared to be significantly greater on the PH tubing compared to the SG tubing. Finally, the degraded SG tubing also exhibited a greater level of contamination from the secondary side environment, with elements such as aluminum, silicon and copper being detected. Results from Raman spectroscopic analyses further indicated that the oxide layer on SG tubing from Units 3 and 4 exhibited a slightly different composition compared to those from the Units 6 and 7 SG tubes. The results from surface and microscopic analytical measurements are presented in this paper and their significance in terms of the corrosion behavior of the SG and PH tubing are discussed. (author)