CHARACTERIZATION AND HOT CORROSION STUDIES OF DETONATION GUN SPRAYED COATINGS

Abstract

High temperature oxidation and hot corrosion represent the most deleterious forms of surface degradation, which can lead to the loss of mechanical strength and catastrophic failure of structural and engineering components. They are routinely encountered in superalloys used in land based turbine and aero engines, which operate in high temperature and corrosive environments. Due to extensive application of the superalloys in land-based and aero gas turbine engines, the high temperature oxidation and hot corrosion behaviour of superalloys has been the subject of intense investigation for the past several years. Most of the studies have focused on the mechanism of oxidation and hot corrosion such as oxide scale growth behaviour of materials based on short-term tests. An understanding of long term high temperature oxidation and hot corrosion behaviour of coatings on superalloys is extremely important for the industrial applications. Hot corrosion may be defined as an accelerated corrosion, resulting from the presence of salt contaminants such as Na2SO4, K2SO4, NaCl, and V205 that combine to form molten deposits, which damage the protective surface oxides. Hot corrosion occurs when metals are heated in the temperature range 700-900°C in the presence of sulphate deposits formed as a result of the reaction between sodium chloride and sulphur compounds in the gas phase surrounding the metals. At higher temperatures, deposits of Na2SO4 are molten (imp. 884°C) and can cause accelerated attack on Ni-, Fe- and Co-based superalloys. This type of attack is commonly called hot corrosion. For example, alloy components in gas turbines in aircraft, thermal power plants, land-based power generators, boilers, internal combustion engines, gas turbines, fluidized bed combustion and industrial waste incinerators undergo hot corrosion. It is a life-limiting form of accelerated environmental attack that can occur on vanes and blades in the hot sections of gas turbine engines. Gurrappa reported that directionally solidified superalloys are highly susceptible to hot corrosion and the life is hardly 4 hours in a 90% Na2SO4 +10% NaCI environment and less than 2 hours in a vanadium containing environment (90% Na2SO4 5% NaCE + 5% V205) at 900 °C. During hot corrosion, a porous non-protective oxide scale is formed at the surface and sulphides in the substrate. This form of corrosion, unlike oxidation, can consume the material at an unpredictably rapid rate. Consequently, the load-carrying ability of the components reduces quickly, leading eventually to catastrophic failure. It is due to the following reasons; for example, superalloys used for high temperature applications could not meet the requirements of both the high-temperature strength and the high-temperature erosion—corrosion resistance simultaneously. Therefore, thermal spray coatings deposited on superalloys made a significant contribution for combating high temperature oxidation and hot corrosion. Use of inhibiters like MgO, Ce02, Ca0 and Mn02 applied superficially have already been investigated (Gitanjaly, 2003; Gitanjaly et al., 2002) on the superalloys exposed to most aggressive environment of Na2SO4-60% V205 at 900 °C and found the substantial reduction of damage due to hot corrosion. However, the major limitation is due to the application of these inhibiters to the hot metal surface. Hence, the viable countermeasure against the oxidation and hot corrosion constitute the use of protective coatings. The high temperature oxidation and hot corrosion of HVOF and plasma sprayed coatings on superalloys and on boiler tube steels have been investigated and the improved oxidation resistance of superalloys has been observed as reported in the literature (Prakash et al. 2005; Singh et al 2007, 2005C, 2005D; Sidhu and Prakash 2003,2005; Sidhu et al., 2006A, 2006B, 2006G, 2006H). The oxidation and hot corrosion studies of Cr3C2-NiCr, NiCrA1Y+0.4wt%Ce02 and NiCoCrAlYTa coatings developed by D-gun spray is scarce in the literature. In view of that, in the present study, oxidation and hot corrosion behaviour of some superalloys designated as superni 75, superni 718 and superfer 800H as per the manufacturer's specifications, has been investigated with and without the application of D-gun sprayed Cr3C2-NiCr, NiCrAIY+0.4 wt% Ce02 and NiCoCrAIYTa coatings in air, Na2SO4-60%V205, Na2SO4-25%K2SO4, at 900 °C, and in actual coal fired boiler environment under cyclic conditions. These superalloys are developed by Mishra Dhatu Nigam Limited, Hyderabad (India) for the high temperature applications such as boilers and gas turbine parts, heat exchangers and piping in chemical industries, jet engines, pump bodies, high temperature furnace parts and heat treatment jigs. A study on the behaviour of aforementioned coatings in different environment will be helpful in choosing the suitable coating and substrate to withstand against oxidation and hot corrosion problems manifested in the gas turbine and boiler applications. The outcome of the present research work are critically analyzed and discussed in the light of existing literature to propose an insight in to the corrosion mechanism in both coated and bare superalloys. The whole thesis is presented in 9 chapters. Chapter 1 contains a brief introduction as to oxidation and hot corrosion effects and its deleterious impact on the various engineering equipments and components. The remedial measures to obviate this problem are also briefly discussed. Chapter 2 contains oxidation and hot corrosion studies reported by various researchers relevant to the current study. It has been critically reviewed particularly those conducted on similar Ni- an Fe-based alloys in air and molten salt environments. The various preventive measures have been summarised along with the description of D-gun spray process. The problem has been formulated based on the available literature on oxidation and hot corrosion behaviour of coated and bare superalloys used in high temperature applications. Chapter 3 presents the experimental techniques and procedures employed for applying the coating and their characterisation, oxidation studies in air, molten salt environments and in actual coal fired boiler environment. The specification of the equipments and other instruments used for the present investigation and the techniques used to analyse the corrosion products are discussed below. The D-gun sprayed coatings were deposited at SVX Powder M Surface Engineering Pvt. Ltd, New Delhi (India) on Ni- and Fe- based superalloys. These superalloys were procured from Mishra Dhatu Nigam Ltd., Hyderabad (India). In the present work three types of coatings were formulated as given below. Cr3C2-NiCr, NiCrA1Y+0.4wt%Ce02 and NiCoCrAlYTa The as-sprayed coatings were characterised by metallography, FE-SEM/EDS, X-ray mapping analysis, mechanical properties such as microhardness of the coatings have been evaluated. Surface roughness and porosity of the as-sprayed coatings were also measured. The oxidation and hot corrosion behaviour of bare and D-gun coated superalloys have been studied in the air, two molten salts (Na2SO4-60%V205 and Na2SO4-25%K2SO4) in the laboratory furnace and actual coal fired boiler environments for 100 cycles under cyclic conditions. Each cycle consisted of I hour heating at 900 °C in a silicon carbide tube furnace followed by 20 minutes cooling at room temperature (25 °C), these studies were performed for uncoated as well as coated specimens for comparison. Whereas, in case of actual coal fired boiler environment, the specimens were exposed to the combustion environment for 10 cycles. Each cycle iii consisted of 100 hours heating followed by 1 hour cooling at ambient conditions. At the end of each cycle, the specimens were critically examined regarding the colour, luster, physical changes on the samples, scale adherence/spallation and then subjected to weight change measurements. The molten salt studies were performed by applying a uniform layer (3- 5ing/cm2) of the mixture of Na2SO4-60%V205 and Na2SO4-25%K2SO4 on the preheated specimens (250 °C) with the help of camel hair brush. XRD and FE-SEM/EDS analytical techniques were used to identify the phases and the elemental analysis of the surface scale, respectively. The coated samples were then cut across the cross-section for analyzing its elemental composition by X-ray mapping analysis. Chapter 4 deals with the detailed investigation of bare and D-sprayed Cr3C2-NiCr coating on Ni- and Fe-based superalloys, which include characterisation, oxidation and hot corrosion studies in air and in two molten salt (Na2SO4-60%V205 and Na2SO4-25%K2SO4) environments in the laboratory under cyclic condition at 900 °C for 100 cycles. The techniques such as XRD, FE-SEM/EDS and X-ray mapping were used to analyse the as-sprayed coating and the corroded specimens. A good adhesion of the coatings to the substrate was evident from the absence of cracks and gaps at the interfaces as well as due to the uniform distribution of carbide particles along metallic binders with the porosity value around 0.69%. The microhardness of the Cr3C2-25%NiCr coatings was found to be in the range of 775 —1200Hv due to high volume fraction of carbides, dispersed uniformly in the matrix. Coating thickness ranges from 221-246pm, where as surface roughness varies from 4.92-6.05prn as observed in the present work. The D-gun sprayed Cr3C2-NiCr coatings on three different superalloys when subjected to cyclic oxidation in air at 900 °C for 100 cycles were found to be successful in maintaining its adherencey with the substrate superalloys. The oxide scales were also found to be intact and there is no indication of any spalling in all the cases. A saving in overall cumulative weight gain for Cr3C2— NiCr coated superni 75, superni 718 and superfer 800H with respect to the bare alloys tend to be of the order of 37.3%, 26.3% and 19.6% respectively. The Cr3C2—NiCr coating after exposure to air oxidation showed the presence of mainly oxides of Cr in the upper region of the scale. In the subscale region, the phases revealed were oxides of Cr and Ni, and their spinels, below the subscale region, Ni-rich splats remained un-oxidised and provided protection to the superalloys against high temperature oxidation. iv Hot corrosion behaviour of Cr3C2—NiCr coating in two different molten salt environments shows that the Na2SO4-60%V205 is more aggressive than the Na2SO4-25%K2SO4. Among three superalloys, bare and coated superalloy superfer 800H have shown lower and higher resistance to hot corrosion in Na2SO4-60%V205 molten salt environment, respectively. The better protection of Cr3C2—NiCr coated superfer 800H might be attributed to the formation of dense, massive and continuous oxide scale mainly consisting of Cr in the upper most part of the scale with NiO layer in the subscale region where Cr is depleted. Where as in case of Na2SO4_25%K2SO4 molten salt environment, bare and Cr3C2— NiCr coated superni 718 exhibit least and highest protections, respectively. The superior corrosion resistance of coated alloy might be ascribed to the formation of oxides of Cr, Ni and their spinel, further the scale was dense, compact and without any spallation/sputtering or peeling tendency. Chapter 5 contains the investigation as to characterisation, oxidation, and hot corrosion of bare and D-gun sprayed NiCrAlY+0.4wt%Ce02 coating on Ni- and Fe-based superalloys in the laboratory under cyclic condition at 900 °C for 100 cycles. The techniques such as XRD, FE-SEM/EDS and X-ray mapping were used to analyse the as-sprayed coating and the corroded specimens. The morphology, porosity, microhardness, roughness, and thickness of as-sprayed coating were characterized. The surface morphology depicts the formation of un-melted particles in the form of globular dendritic structure. The microhardness of the coating is found to?be in the range of 697-920 Hv and the average porosity is less than 0.58%. The thickness of coating ranges from 200-250 ttm and it showed a good adhesion with the substrate. The surface roughness of the coating is found to be in the range of 6.17-6.94 litn, respectively. Oxidation kinetic was established at 900 °C for 100 cycles under cyclic conditions. The D-gun sprayed NiCrAlY + 0.4 wt% CeO2 coating on superfer 800H has provided a maximum oxidation resistance. The presence of Si, NiCr2O4, NiO, u-A1203 and respective spinels might have contributed for better oxidation resistance. Further, a thick band of chromium along the coating -substrate interface, where all other elements were depleted, might have acted as a diffusion barrier to the oxygen to enter in to the substrate. Hot corrosion kinetics was also studied on two different molted salt environments namely of Na2SO4-60%V205 and Na2SO4-25%K2SO4. The bare and D-gun sprayed NiCrAIY + 0.4 wt% CeO2 coating on superfer 800H depicts least and maximum hot corrosion resistance in both the molten salt environments, respectively. The better hot corrosion resistance of coating might be attributed to the formation of dense oxide scale with phases like NiQ, Cr203, A1203, NiCr204 and NiA1204 Further, oxides along the splat boundaries and within open pores of the coating might have acted as diffusion barrier to the inward diffusion of molten salt, also a thin continuous streaks of iron and silicon oxides on the top surface of hot corroded coating contributed to enhance the hot corrosion resistance. A co-existence of cerium oxide and vanadium along the cross-section shows the possible formation of cerium vanadates (CeVO4), which might have further contributed to the better hot corrosion resistance of NiCrAIY + 0.4 wt% CeO2 coated superfer 800H (Fig.5.29). Chapter 6 describes the detailed investigation of D-gun sprayed NiCoCrAIYTa coatings on Ni- and Fe-based superalloys. The characterisation of D-gun sprayed NiCoCrAIYTa coatings has been discussed in detail. Oxidation studies in air at 900 °C under cyclic conditions and hot corrosion studies in two different molten salt environments at 900 °C under cycle conditions are discussed. The techniques such as XRD, FE-SEM/EDS and X-ray mapping are used to analyse the corrosion product of oxidation and hot corrosion. D-gun sprayed NiCoCrAIYTa coating exhibits dense and adherent microstructure. The thin contrast stringers which appeared in the microstructure are presumably the oxides that are believed to be formed due to the oxidation of in-flight particles. Micrograph depicts the presence of porosity; oxide stringers (A1203), un-melted and semi-melted particles and oxide inclusions. The average thickness of the coatings on three superalloys were found to be in the range of 200-250μm. The porosity of the coating is less than 0.48 %. Roughness was found to be in the range of 6.25- 7.48(Ra) Am, where as microhardness of the substrates is found to be in the range of 290-395 Hv and it is 385-748Hv along the cross section of the coatings. Oxidation kinetics was established at 900 °C for 100 cycles under cyclic conditions by weight gain data. D-gun sprayed NiCoCrA1YTa coated superni 75 showed the lowest weight gain. The better protection against oxidation shown by the NiCoCrA1YTa-coated superni 75 might be ascribed to the formation of oxide of Al, Co, Cr, Ni, thereby suggesting the formation of spinel. The coated superalloys after exposed to molten salt environments at 900 °C for 100 cycles under cyclic conditions show that the Na2SO4-60%V205 is found to be more aggressive than the Na2SO4-25%K2SO4. It may be attributed to the penetration of oxygen little deeper in to vi the coating forming thick oxide scale, further Si and Fe has diffused upwards and reached top surface of coating. There is micro-spallation of coating along the corners and edges in the coating. Where as in case of Na2SO4-25%K2SO4 coating across the cross-section got partially oxidised thereby restricting penetration of corrosive species. NiCoCrAIYTa coating provided maximum hot corrosion resistance to superfer 80011 by reducing weight gain by 37% and 41% in Na2SO4-60%V205 and Na2SO4-25%K2SO4 molten salt environments, respectively, in comparison with bare superfer 800H. Chapter 7 deals with the study of bare and D-gun sprayed Cr3C2-NiCr, NiCrAlY +0.4 wt %Ce02 and NiCoCrAIYTa coatings on Fe-based superalloy superfer 800H. In order to establish an understanding of the behaviour of these coatings and bare superalloy in the actual coal fired boiler environments, where these coatings are intended to be used, the specimens were exposed for 1000 hours to platen super-heater zone of coal fired boiler at Guru Nanak Dev Thermal Power Plant, Bathinda, Punjab. This zone was selected for the present study as.quany break downs occurred in this power plant due to the hot corrosion degradation of the platen super-heater tubes of coal fired boilers. The samples were hanged in this zone, having temperature 900 10 °C, with the help of stainless steel wires for 10 cycles, each cycle consisting of 1000 hours of heating followed by 1 hour of cooling in open air. At the end of each cycle, the specimens were visually examined with respect to colour, luster, spallation tendency and adherence of the scale. Thereafter, the specimens were subjected to weight change measurements but at could not be of much use for predicting hot corrosion, as there is suspected spallation and ash deposition on the samples. The hot corroded superalloys with respect to different phases and their distribution were analysed by using XRD, FE-SEM/EDS and X-ray mapping. Chapter 8 describes the comparative results of all the bare and D-gun coated superalloys in air, molten salt environments (Na2SO4-60%V205 and Na2SO4-25%K2SO4) and in actual coal fired boiler environment at 900 °C under cyclic conditions. Chapter 9 includes conclusions of the present investigation and scope for the future work.