Materials fit for Heat exchanger applications

Vigorous pitting occurred in moderator heat exchanger tubed with Incoloy 800 in pickering nuclear production unit. Pitting occurred on the cooling water side of the tubes and perforation occurred within 24 months. It was known from corrosion evaluation at CRNL that Incoloy 800 was not sensitive to pitting in Lake Ontario water in isothermal conditions. Corrosion evaluation with heat transfer around the tube wall was conducted and it was found that extreme pitting may occur in deposits developed on the tubes in Ontario lake water. After testing conducted to find the pitting resistance of other eligible alloys : Incoloy 825, stainless steel 904l, Inconel 625, CuNi alloy 70/30 and titanium. Among these materials titanium did not received pitting corrosion in silt containing lake Ontario water. In absence of slit, so deposits, no pitting occurred on the tested alloys.

All alloys tested at room temperature in natural fresh waters did not show any sign of corrosion after exposure to 24 hours. These alloys that suffered some extent of corrosion at  45oC and 70oC are Cu-Ni alloy 70/30 and 90/10. It was invariably crevice corrosion. In availability of silt and chloride contamination, nickel based and copper based alloys experienced some corrosion at room temperature. The extent of corrosion found to increase with temperature, however this study is based on visual testing only.

Incoloy 800 was sensitive to crevice corrosion in room temperature chloride-contaminated river water, the corrosion rate was 0.1mm/a. No direct comparison of pitting rate can b made with service data for Incoloy 800, 0.5mm/a or the heat flow test results, as the isothermal tests at 70oC for contaminated Lake Ontario water were not conducted. Although, it is evident that chloride contamination seems to be important in these isothermal tests to induce pitting corrosion of Incoloy 800 and the nickel based alloys.

The comparisons of the admiralty brass results with in-service data are sufficient. In-service admiralty brass condenser tube pitting rates of 0.13 mm/a over a 10 years duration seems to be standard and this value is in good agreement with isothermal lab test results varying from 0.03 – 0.2 mm/a depending on temperature and water composition. Similarly Cu-Ni alloy 70/30 in operation in PNGS has show nominal corrosion, often an insignificant surface etching and dealloying that is consistent with results received. Cu-Ni alloy 70/30, nickel and brass increase the corrosion rate in heat transfer conditions.

Heat transfer tests

The results showed limited alloys suitable formoderator heat exchanger service Inconel 625 wire. Where no slit crevices were used, no pitting was found on the tested materials. In cases, the results were very reproducible within the duplicates, however insufficient sampling was conducted to offer statistically considerable predictability. The outcomes in these cases offer a clear guide to type the alloy that would be fit for corrosion free service as tubing in moderator heat exchangers.

Incoloy 800 is quite sensitive to pitting corrosion, Incoloy 825 less however still sensitive and stainless steel 904L subjected to occasional deep pitting. The pit shapes are feature of under deposit chloride corrosion.

PNGS has operated adequately with CuNi alloy 70/30 tubes for couple of years. The only sign of corrosion on the material was under very adherent deposits where redeposition of leached copper was found. The corrosion in this region was limited to nominal etching that had penetrated below 1 micro in six months.

In various cases, the noticed corrosion on alloys experiencing corrosion was related with dark green or black deposits that had developed at formed at the tube-silt deposit interface. Evaluations of the dark areas showed to enriched of calcium, silicon and chlorine and containing considerable magnitudes of chromium.