STUDIES ON THE ROLE OF HVOF COATINGS IN IMPROVING RESISTANCE TO HOT CORROSION AND EROSION

Abstract

The operating conditions in power station boilers are conducive to fireside corrosion and erosion, both in the furnace wall and in the super heater and reheater areas, and these effects cause tube wall thinning and premature failure. In the coal fired boiler, corrosion is occurs in several fashions due to combustion products which change their state of matter and become salts as a result of the high temperature. Combustion of coal generates very corrosive media particularly near the superheater tubes, forming highly aggressive ash deposits that contain alkali metals of sodium and potassium, and sulphur. This phenomenon can also be seen in the gas side of power station boilers when the fuel is a residual oil. Ashes formed due to the combustion of such fuels have a high concentration of compounds formed by vanadium, sodium and sulphur, mainly as Na2SO4—V205 complex and sodium—vanadates mixtures. Some mixtures of these compounds have low melting points 550°C. These compounds easily liquify at the operating temperatures of boilers and cause accelerated corrosion. Erosion is even more localized in its effect, and results from impact of particulates, such as coal ash, dolomite and unburned carbon particles on the surface of heated boiler tubes. High temperature oxidation and erosion are recognized as being one of the main causes of downtime in these installations. Maintenance costs for replacing failed tubes in these installations are also very high. The materials used in these installations are fabricated from low-alloy carbon steels with chromium and molybdenum as the primary alloy additions. Although chromium is expected to impart corrosion resistance to high-temperature alloys through the formation of a passive oxide layer on the alloy surface, its concentration in boiler tube alloys in not sufficient to form a protective external scale. Present materials being capable of resisting erosive and corrosive environments are highly alloyed, and thus expensive. In search for cost-effective solutions for Erosion —Corrosion problems, various coatings like thermal sprayed coatings have become attractive. The high-velocity oxy-fuel (HVOF) processes belong to the family of thermal spraying techniques, and are widely used in many industries to protect the components against erosion, corrosion and wear. This process has been shown to produce coatings with better density, coating cohesive strength and bond strength than many thermal spray processes. The possibilities of applying the HVOF process to a much wider range of materials are now being addressed. The role of the coating, in this case, is to provide a metal surface composition that will react with the environment to produce the most ii protective scale possible, combining corrosion-erosion resistance with long term stability and resistance to cracking or spallation under mechanical and thermal stresses induced during operation of the component. HVOF spraying has been carried out using a HIPOJET 2100 equipment (M/S Metallizing Equipment Co.Pvt.Ltd, Jodhpur, India), which utilises the supersonic jet generated by the combustion of liquid petroleum gas and oxygen mixture. The complex formulations containing Ni, Al, Cr, Si, Mo, WC, Co and boron have been used to develop combined corrosion and erosion resistant coatings. Four types of commercially available feed stock powders namely NiCrAl, NiAlCrFeMo, NiCrFeSiB and WC- Co/NiCrFeSiB were HVOF sprayed on three kinds of boiler tube steels designated as ASTM-SA210-Grade A 1 , ASTM-SA213-T11 and ASTM-SA213 -T22. Differences in chemical compositions of coatings and substrate alloys can lead to inter-diffusion across the coating substrate interface, which can modify the oxidation and hot corrosion resistance of the coatings. Under the given spray parameters, seemingly dense laminar structured coating with thickness in the desired range of 260-295 i.tm and porosity less than 2 % has been achieved. All the coatings have retained the phases observed in starting powder, containing nickel rich FCC structure as a principal phase and has not undergone significant phase transformation. Oxidation, decomposition and decarburization of the coating powder are not serious due to higher particle speed and chosen parameters during HVOF spraying. Understanding the behavior of these coated steels at elevated temperature in various environments has become an object of scientific investigation. Thermo cyclic oxidation studies are performed in static air as well as in molten salt (Na2SO4-60%V205) environment at 900°C for 50 cycles. The cumulative weight gain for all the HVOF coated GrAl , T11, and T22 steels are significantly lower than that of uncoated steels subjected to oxidation and molten salt hot corrosion. Based on the thermogravimetric data, the relative oxidation and hot corrosion resistance of the various coating under study can be arranged in the following sequence: In Static air: NiCrFeSiB > WC-Co/NiCrFeSiB > NiCrAl > NiAlCrFeMo In molten salt: NiCrFeSiB > NiCrAl > NiAlCrFeMo > WC-Co/NiCrFeSiB The superior performance of NiCrFeSiB coating can be attributed to continuous protective oxide scale of amorphous glassy Si02 and Cr203 formed on the surface. The Si02 scale found to be passive in acidic molten NaVO3 melt and hence all the NiCrFeSiB coated steel showed slow oxidation kinetics which indicated that the reaction rate is diffusion limited. The oxidation resistance of NiCrAl coatings can be ascribed to the thermodynamically stable phase of a-A1203 and Cr203 formed on the outermost surface. The preferential oxidation of Al and Cr along the nickel rich splat boundary blocks the iii transport of oxygen into the coating through pores and voids, thereby making the oxidation rate to reach steady state. Because of accelerated oxidation induced by the molten salt, which act as catalyst and oxygen carrier, metastable 0-A1203 has formed in the initial cycles of studies and shows polymorphic transformation. The oxides and spinels of Si, Cr and Co formed on surface during the oxidation of WC-Co/NiCrFeSiB coatings stabilized the formation of volatile tungsten oxide. While in molten salt environment, preferential oxidation of W and Cr during the initial stage impedes the formation of continuous layer of SiO2. Tungsten oxide decreases the oxide ion activity of the molten salt environment to cause acidic fluxing of the active elements in the coating. The massive, loosely held needle microstructure of the oxides formed on the surface leads to least hot corrosion resistance of the WC-Co/NiCrFeSiB coatings in molten salt environment. Comparatively lower oxidation resistance of NiA1CrFeMo coated steels may be attributed, to some extent, to severe internal oxidation along the splat boundary due to higher porosity content. In molten salt, the formation of nickel and chromium molybdates, which are liquid at 900°C, resulted in highly friable and porous scale. The corrosion rate reduces once the available Mo at the surface has been consumed. With the progress of oxidation, protective oxides of Ni, Cr and Al are formed at the subscale level. Iron oxide protrusions are observed on the corroded surface of all the coated T22 steels except NiCrFeSiB coated steels. These protrusions lead to higher weight gains and rapid growth of inhomogeneous oxides. The probable cause of oozing out of iron oxide might be attributed to the presence of molybdenum in the T22 steel substrate. In general, coated GrA1 steels showed lower weight gain in comparison to coated T11 and T22 steels. Solid particle erosion studies have been carried out as per ASTM G76 standard at 30° and 90° impact angles to provide the maximum erosion condition for both ductile and brittle materials under silica sand erodent. Relative erosion resistance of the various coating under study can be arranged in the following sequence: WC-Co/NiCrFeSiB > NiCrFeSiB > NiCrAl > NiA1CrFeMo The better erosion resistance of NiCrFeSiB coatings may be ascribed to its higher ductility and homogeneous microstructure. The erosion resistance of NiCrFeSiB coatings found to be increased by adding refractory WC-Co particles. The morphology of the eroded surface of WC-Co/NiCrFeSiB coatings point out craters, groove formation in binder matrix, lips and platelet formation, and carbide particle pull out as the prevailing erosion mechanism which signifies composite ductile and brittle modes of material removal. The principal erosion mechanism in NiCrAI coating is by severe plastic deformation even though fracture of some Ni-rich splats is observed. Higher erosion iv losses observed for NiA1CrFeMo coatings might be attributed to relatively higher porosity of the coatings. Indentations, craters, and lips observed on the surface indicate the ductile erosion behavior of the coatings. Substrate GrAl steel exhibit lower steady state volume erosion rate in comparison to all the HVOF coatings under similar test conditions. The higher hardness ratio between silica erodent particle and substrate steel might have caused the penetration of silica particles in to the surface which bestow some shielding effect against impacting particles leading to lower wear loss. The HVOF coated and uncoated samples were exposed to actual service conditions of the boiler for 1000 hours. The samples were tested in super heater zone of the coal fired boiler at Guru Gobind Singh Super Thermal Plant, Ropar, India. The flue gas temperature in this region is about 778±20°C. The erosion-corrosion behavior of all the HVOF sprayed coatings is promising in comparison to substrate boiler steels. Analogous to the observations in laboratory conditions, NiCrFeSiB coatings showed Minimum thickness loss as well as lower weight gain in comparison to other coatings under study. The superior performance of these coating can be attributed to the thin and continuous oxide scale of Si02 and Cr203 formed on the surface. The weight gains for WC-Co/NiCrFeSiB coated steels are almost equivalent to that of NiCrFeSiB coated steels. Both these coatings got oxidized only at the uppermost surface to for thin protective oxide scale and rest of the coatings remain unoxidised. Low porosity content and rapid growth of protective oxides during the initial cycles leads to enhanced hot corrosion'resistance of these coatings. The better performance of NiCrAl coatings can be credited to thin protective oxide scale of a-A1203 and Cr203 formed on the surface as well as preferential oxidation of Al and Cr along the nickel rich splat boundary. Relatively lower erosion-corrosion resistance offered by NiA1CrFeMo coatings might be associated with higher porosity content in the coating. The entire cross section of the coating has been partially, oxidized along the splat boundary. The coatings under study have been found to be successful in protecting the given substrate boiler steels tested in laboratory as well as in actual coal fired boiler environment. NiCrFeSi coating shows superior performance among the coatings used in the present study. In addition to applications in superheater zone of coal fired boilers, these coatings can be applied to other applications like fluidized bed combustors, industrial waste incinerators, internal combustion engines, gas turbine, steam turbines etc.