Corrosion of Low Alloyed Steel in Flowing Pure Water under High Temperature and High Pressure Conditions

摘要

It frequently reported that corrosion damage progresses at an extraordinary high rate (>=1 mm/y) on the inside wall of heat conduction tubes in power plants and in the heat recovery boilers of chemical plants. In previous studies, the influence of dissolved oxygen (DO) and the pH of boiler feed water on the corrosion of carbon steel was investigated at elevated temperatures under high pressure. As a result, it was found that an increase in DO or pH was not useful to prevent the corrosion. In this paper, the application of low alloyed steel, a material that is currently used to resist corrosion through material improvement, was examined. The maximum corrosion depth rate of low alloyed steel reached levels as high as 1 mm/y at pH 9.0. This level decreased slightly with increasing pH and then showed a sudden and drastic decrease in the pH range from 9.25 to 9.75 depending the direction of flow of water over the specimen surface. Concerning the influence of temperature, the low alloyed steel was superior to carbon steel at temperatures over 180 deg C due to the oxide film which is uniformly deposited over the entire surface. Ditch corrosion, however, occurred on the surface of low alloyed steel at temperatures below 160 deg C, and developed to a greater extent than on a carbon steel surface. It was also found that vortexes generated in the fluid flow are intimately connected with the formation of ditch corrosion. In conclusion, low alloyed steel cannot be recommended for use in boiler tubes since it might suffer ditch corrosion badly under some conditions although it is excellent under conditions where a single and uniform oxide film is formed on the metal surface.