UNS R20033 is produced as a welding consumable and is covered under AWS A.9 under the designation of ER33-31. This material is now being widely used as weld overlay material in the fossil boiler industry. The high chromium content of ER33-31 has been shown to provide excellent corrosion protection in both waterwall and superheater/reheater tube applications. However, the main interest has been for use in convection pass tubes (i.e. superheater and reheater tubes). ER33-31 is applied to pressure tubing using a spiral clad weld overlay and is able to provide excellent resistance coal ash corrosion. Laboratory and field tests have shown ER33-31 to have among the best corrosion resistance of all materials studied. The application of ER33-31 is also easier than other more highly alloyed materials (such as FM-72) and is less expensive. A review of the favorable experiences of ER33-31 and the conditions under which ER33-31 should be considered are reviewed in this paper.
Superheaters in coal fired power plants are used to heat saturated steam from the boiler to higher temperatures in order to provide a dry high pressure steam source for the turbine. For many years the steam temperatures delivered to the turbine have been around 1000°F (538°C); however in recent years newer designs are increasing this temperature with a vision of going as high as 1400°F (760°C). The superheater tubes used to heat the steam to these temperatures therefore have metal temperatures even higher. While the actual tube temperature depends on boiler design, they are typically 50-100°F (28-56°C) higher than the target steam temperature. The operating temperatures of the superheaters result in the requirement to consider corrosion in order to establish desired lifetimes for the tubes. There are a number of corrosion mechanisms that can affect the tube life of the superheaters.