Fossil fuel fired boilers and circulating fluidized bed boiler tubing in the power industry are normally protected against corrosion using nickel based clad coatings. Weld overlay is the most common form of cladding that utilizes the pulsed gas metal arc welding process, however thermal spray processes using cored wires, powder, and solid wire nickel based consumables have been used as well. Good bond integrity, heat transfer, corrosion resistance, and cladding density are the essential cladding characteristics that provide optimum life of water wall tubes in the transition areas, air injection ports, super-heater and re-heater areas of a boiler. Increasingly, nickel base alloy cladding is not only used for repair and maintenance purposes, but is also used as a cost-effective and efficient alternative for new boiler installations.
This paper describes field experience of nickel based materials of construction and repair, and reports laboratory results of a new, low NOx test method developed to screen various alloys and coatings for the power industry. Case history examples presented include the effective application of nickel alloy claddings for new boiler systems, as well as evaluations of materials that were previously exposed to high temperature and corrosive operating conditions. The inherent corrosion resistance, mechanical and physical properties of NiCr, NiCrMo, NiCrFe and NiCrFeAl qualify them as materials that meet or exceed high temperature corrosion and corrosion-erosion requirements.
Corrosion resistant alloy (CRA) weld overlay of boilers began in the late 1970?s and early 1980?s when waste-to-energy (WTE) boilers showed a need for protection. At that point in time, ERNiCrMo-3 welding filler metal was the most popular choice in the U.S. ERNiCrMo-3 is a solid solution superalloy designed for low and intermediate temperature corrosive environments. This high nickel alloy performed very well on WTE waterwall boiler tubes. With passage of the 1990 Clean Air Act, utility power boiler operators began to install devices designed to reduce oxides of nitrogen (NO, NO2, etc.) commonly referred to as NOx. Due to its outstanding performance in WTE boilers, ERNiCrMo-3 was the obvious choice for low NOx waterwall overlay. However, when used as an overlay for low NOx waterwall tubes, the ERNiCrMo-3 overlays were subject to corrosion fatigue failures.
One method used to reduce the emissions of NOx from coal-fired boilers has been the use of low NOx burners coupled with staged combustion. The low NOx burners introduce less than stoichiometric air, and thereby create a relatively low temperature, fuel- rich flame, which retards potential NOx producing reactions. In addition staged combustion inhibits potential NOx reactions by operating the burner zone under fuel-rich conditions.
While the new low NOx burning / staged combustion method is successful in reducing NOx emissions, it produces a more aggressive environment inside the boiler. Reducing NOx emissions is good for our environment but one of the costs was a dramatic increase in corrosion and erosion of boiler waterwalls, and superheater and reheater tubes.
This paper presents the current status of waterwall protection and presents laboratory test data of selected candidate alloys. It also presents details of successful superheater and reheater overlays in a number of low NOx boilers. In this regard some of the most prominent corrosion issues are coal ash corrosion, sulfidation, reducing atmosphere, oxidizing atmosphere, carburization, and increasing operating temperatures. The problem is made worse because there are multiple forms of corrosion / erosion occurring in the same individual Lo