The use of aerated, treated geothermal or other high-chloride water in cooling systems such as the Raft River 5 MW(e) geothermal power plant or other plants presents special problems in material selection. The corrosion tests discussed here were initially conducted to determine a replacement tube material for the Raft River 5 MW(e) Pilot Power Plant isobutane carbon steel condenser. In addition,
the investigation included a preliminary evaluation of alternative materials for other cooling water systems using high-saline, aerated water.
In general, the report favours the use of highly-alloyed ferritic stainless steels, such as 29-4 (29%Cr) and a proprietary alloy Sea-Cure.
However, throughout the various report sections, Ferralium 255 is mentioned as providing equivalent performance to either of these alloys.
Example: The ferritic stainless steels examined at 100,000 ppm chloride were SEA-CURE, 29-4, 29-4-2, and 29-4-C. SEACURE and 29-4 were the only alloys exhibiting any weight loss. The corrosion rate of SEA-CURE was 0.05 mdd (0.01 mpy) after 4 weeks of exposure. This rate is approximately 1/20th of the rate encountered at 50,000 ppm. The 29-4 alloy showed greater general corrosion at 100,000 ppm than at 50,000 ppm. This alloy also exhibited crevice corrosion. None of the ferritic stainless steels exhibited pitting at 100,000 ppm chloride. Corrosion of Ferralium 255 was similar to that of 29-4.
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