A new technique of Ni-Cr 50/50 coating processing has been developed for fossil fuel boiler tube protection against fire-side corrosion, The technology involves the electric arc process where molten wires are atomized by a supersonic jet of hot propane-air combustion products. The small size and high velocity of atomized particles lead to the formation of dense coatings, while the reducing environment in the starting jet section prevents the material from rapid oxidation. Gas permeation measurements revealed that combustion arc sprayed coatings are substantially less permeable than high velocity oxygen fuel and regular electric arc coatings. Elevated temperature corrosion tests proved the superior performance of combustion arc sprayed coatings in reducing H2S containing environment and under KCI+ Na2SO4 salts deposits in air.
Oxidation and sulfidation attack as well as corrosion under low melting point slag deposits are the most common problems associated with fire-side corrosion of superheaters and water-wall tubes in coal-fired boilers [1- 4]. Fitting of low NOX burners to coal-fired power plants aggravates these problems due to exposure of metal surfaces to reducing or alternating reducing and oxidizing conditions [5]. Thus, the development of methods aimed at reducing boiler tire-side corrosion is a significant concern.
The application of protective coatings is an effective method to combat boiler tubing high temperature corrosion. Despite different mechanisms of fire-side corrosion appearing due to different boiler designs, types of coal, and cord-treatment procedures, the beneficial effect of high chromium nickel- or iron-based alloys for thermal sprayed coatings is quite evident [5-7]. Ni-Cr 50/50 type alloys, applied by electric arc (ARC) and high velocity oxygen-tie] (HVOF) spraying, are widely used by Metalspray USA Inc. for protection of water-wall tube panels and superheater tubes for fossil fuel combustors as a single-layered coating or as a component of multi-layered coating.
Based on 10 years of application results, these coatings perform rather well under different kinds of slag deposits, withstanding salt-ash corrosion in the presence of alkali metal sulfates, as well as sulfidation and oxidation under thick iron sulfide deposits. The type of corrosion suffered by the coating is different if the surface is deposit-flee, i.e. filly open to gas attack.
The problem may be explained by the following example, Fig. 1 shows a cross-section micrographs of a Ni-44Cr-Ti ARC coating after two years of service on the waterwall tube of a pulverized coal- fired boiler, burning 2.7% sulfur coal. Retrieved from service, one side of the tube was covered with thick layer of a slag, while another side was bare. With slag cover (Fig.1 a), the coating surface wastage was estimated at 75-100 µm per year without internal degradation or interface corrosion. Without slag cover (Fig.1 b), the coating surface wastage was estimated at 25 µm per year while internal degradation and abnormal oxide/sulfide scale growth at the coating-substrate interface was revealed. As determined in a number of cases, such interface corrosion finally leads to coating spallation, while visually the coating suffers minor surface corrosion.
