Many utilities are choosing to use controlled combustion methods to reduce NOx emissions. This technology effectively reduces NOx to elemental nitrogen through the use of reducing combustion zones controlled temperatures. However, this process also produces reduced sulphur compounds in the fuel, which in turn increases the corrosion rates on traditional Cr-Mo boiler tube materials. In order to prevent rapid sulfidation of the Cr-Mo boiler tubes, a higher chromium-containing alloy is applied as a weld overlay. Several different materials have been used during the last several years with mixed success. Not only must a material resist corrosion by sulfidation, but also should match the thermal expansion coefficient of the Cr-Mo tubes and have good thermal stability to resist embrittlement at operating temperatures. Several new candidate materials for this application have been evaluated for their sulfidation resistance in laboratory tests. The data from these corrosion tests are reported here. Pertinent physical and mechanical property data are also reported to allow a complete evaluation of the weld overlay materials for this low NOx application.
Keywords Sulfidation, Oxidation, Alloy 50, Alloy 33, Alloy 59, Alloy 625, boiler tube, weld, weld overlay, low NOx, fatigue cracking, circumferential cracking