The use of corrosion resistant weld overlay materials has proven to be a very effective method to extend the life of boiler tubes in coal fired boilers. In order to properly select the best material to use for a weld overlay, the corrosive conditions need to be understood in the various regions of the boiler. The cost of the protection method and ease of application also need to be considered when selecting an overlay material.In the furnace region where combustion occurs, the water-wall tubes are exposed to high heat inputs along with corrosive combustion gases and deposits. These conditions can cause rapid corrosion by a mixed sulfidation/oxidation mechanism. The corrosion rates increase if low NOx combustion practices are used, since this causes a reducing atmosphere that forms more corrosive sulfur species such as H_2S gases and FeS deposits. The corrosion rates increase with tube metal temperatures, which are controlled by pressure of the water used to cool the boiler tubes. In the highest pressure units that operate above the water triple point (supercritical plants), cracking is also an issue. This cracking is caused by a corrosion fatigue mechanism and is generally referred to as "circumferential cracking" in the industry.In the case of waterwall corrosion, it has been found that higher chromium materials best resist the corrosion conditions. The main materials currently used to overlay waterwall tubes are Type 309 stainless steel (23Cr-13Ni-58Fe), Alloy 22 (22Cr-57Ni-13Mo-3W) and Alloy 33 (33Cr-32Fe-31Ni). This last material has the highest chromium content and is therefore the most corrosion resistant. Overlay material selection for supercritical units is further complicated by the possibility of cracking. While not yet fully understood, it is generally agreed that certain material properties improve resistance to cracking; these include the materials strength, ductility, thermal conductivity, thermal expansion coefficient, and of course corrosion resistance. No material has yet been identified that is completely immune to cracking in supercritical units.
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