Nitrate stress corrosion cracking in waste heat recovery boilers Available to Purchase

Waste Heat Recovery Boilers (WHRB's) enhance the thermal efficiency of gas turbine power generating plants by capturing the heat from the exhaust gas and utilising it to raise steam for increased electricity production or district heating. Under normal conditions the exhaust gas from the turbine is cooled from around 550°C to 80°C. Care is taken that condensation will not occur in the cooler parts of the boiler. Nevertheless, during normal operation, dry ammonium nitrate deposits can be formed in the WHRB. These deposits will become wet when the unit is started or shut down, when the temperature falls below the dew point. The deposits may also “sweat” (i.e. absorb atmospheric moisture and become liquid) during extended plant outage periods. In consequence, Intergranular Corrosion Attack (IGA) and Stress Assisted Intergranular Corrosion Attack (SA‐IGA) (in general called stress corrosion cracking) can occur in carbon steel construction materials. The sensitivity to IGA of a total of 86 steels, of known compositions, was tested. The steels were subjected to aerated ammonium nitrate solutions of 90°C with concentrations ranging between 2 and 35 per cent. After an exposure of 65 hours, cross sections of the steels were studied. If IGA occurred, the steel was considered sensitive to ammonium nitrate at that (critical) percentage. During the investigation it was observed that alloying elements in several types of steel influenced its resistance to IGA in ammonium nitrate. Elements such as molybdenum, manganese and chromium had a positive effect on the resistance to nitrate induced IGA, SA‐IGA and stress corrosion cracking, whereas carbon and copper were detrimental. The microstructure of the steel also appeared to be important. In many specimens taken from in‐service failures, strings of carbide precipitates were found to be present at grain boundaries. The precipitates were identified to be ternary carbides. It seems, therefore, that the presence of carbides at grain boundaries increases the susceptibility of a steel to intergranular corrosion.