HIGH TEMPERATURE CORROSION STUDIES OF SPUTTER DEPOSITED NANOSTRUCTURED COATINGS

Abstract

High temperature oxidation and hot corrosion are the serious problems in aircraft, marine, industrial and land-base gas turbines. It is because of the usage of wide range of fuels coupled with increased operating temperatures, which leads to the loss of mechanical strength and catastrophic failure of turbine engines. 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, NaC1, 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 (m.p. 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. 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. MCrAlY coatings play a significant role in protection of hot section components in gas turbine engine system, either as overlays or as bond coats. In order to increase the efficiency of gas turbine engines and protection from high temperature oxidation and hot corrosion over a wide range of temperature, nanostructured MCrA1Y bond coats have been developed as reported in the literature (Jiang S.M et al. 2008). However, limited studies have been reported on the Cr/Co-Al bond coat and Co-Al bond coat on Ni-base superalloy deposited by using magnetron sputtering, which is an excellent technique to produce high-quality films and coatings (Ulrich S et al. 2009). Nanostructured MCrAl Y(M=Ni or Co) coatings deposited on superalloy by magnetron sputtering showed an improved high temperature oxidation and hot corrosion resistance at 900 °C as compared to bare superalloy as reported in the literature (Rahman A et al. 2008, 2009, 2010 and 2011). Since nanosized grain possesses high density of grain boundaries, it provides the more favorable sites of diffusion for the easier formation of protective scales. The present work has been focused to study the high temperature oxidation and hot corrosion behaviour of nanostructured Cr/Co-Al coatings and Co-Al coatings deposited on Superni-718 in air and molten salts (Na2SO4-60%V205) environment at 900 °C and also bare Superni-718 substrate to assess the performance of coated superalloy. Superni-718 superalloy was developed by Mishra Dhatu Nigam Limited, Hyderabad (India) for the high temperature applications such as turbine blade of jet engine and high-speed airframe parts such as wheels, buckets, spacers, high temperature bolts and fasteners. A study on the behaviour of aforementioned coatings in air and molten salts environment will be helpful in choosing the suitable coating on 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 into the corrosion mechanism in both coated and bare superalloy. The whole thesis is presented in 7 chapters. Chapter 1 contains a brief introduction of the oxidation and hot corrosion effects and its deleterious impact on the various engineering equipments and components. The importance of nanostructured coatings for high temperature applications is also briefly discussed.. ii 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 Ni- based alloys in air and molten salt environments. The problem has been formulated based on the available literature on oxidation and hot corrosion behaviour of coated and bare superalloy used in high temperature applications. Chapter 3 presents the experimental techniques and procedures employed for deposition of the coating and their characterisation, oxidation studies in air, and molten salts environment. The specification of the equipments and other instruments used for the present investigation and the techniques used to analyze the corrosion products are discussed below. Nanostructured coatings were deposited by Magnetron Sputtering process at Institute Instrumentation Centre Roorkee, IIT Roorkee on Superni-718 superalloy. Superni-718 superalloy was procured from Mishra Dhatu Nigam Ltd., Hyderabad (India). In the present work Cr/Co-Al and Co-Al coatings were deposited on Superni-718 at different substrate temperature in the range of 250-700 °C. The as-deposited coatings were characterised by X-ray diffraction, FE-SEM/EDS, and AFM. The oxidation and hot corrosion behaviour of bare and magnetron sputtered coated superalloy has been studied in the air, and molten salts (Na2SO4-60%V205) in the laboratory furnace for 50-100 cycles under cyclic conditions. Each cycle consisted of 1 hour heating at 900 °C in a silicon carbide tube furnace followed by 30 minutes cooling at room temperature (25 °C), these studies were performed for uncoated as well as coated specimens for comparison. At the end of each cycle, the specimens were critically examined regarding the color, 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 (3-5 mg/cm2) of the mixture of Na2SO4-60%V205 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 FE-SEM/EDS. Chapter 4 deals with the detailed investigation of bare substrate and magnetron sputtered Cr/Co-Al coatings on Superni-718 superalloy, which include deposition, iii characterisation, oxidation and hot corrosion studies in air and in molten salt (Na2SO4 -60% V205) environments in the laboratory under cyclic condition at 900 °C for 50 and 100 cycles. The techniques such as XRD, FE-SEM/EDS and AFM were used to analyze the as-deposited coatings and the corroded specimens are characterized by XRD, and FE-SEM/EDS. This chapter is divided into two sections. The first section (Section 4.1) describes the deposition of nanostructured Cr/Co-Al coating on superni-718 at 250, 350, 450, 500, and 700°C substrate temperatures, and their high temperature oxidation behaviour at 900 °C for 50 and 100 cycles in air and compare with bare substrate. With increase in substrate temperature, the surface mobility of condensed atoms increases, which could easily diffuse from island side to lower potential zone of substrate, resulting in denser coating with reduced porosity or voids in coatings. XRD and CoAl binary phase diagram confirmed that on the surface of coatings before oxidation has 13-CoAl phase. Thickness of coatings was approximately 4.5 to 5 1.1m was calculated by FE-SEM cross-sectional analysis. Coated superalloy at 250, 350, and 450°C has higher oxidation resistance in air environment at 900 °C for 50 hrs as compare to bare superalloy, due to formation of protective and adherent scale such as A1203, Cr203, CoO, and CoCr204 spinel. Among them Cr/Co-Al coated sample at 350 °C has highest oxidation resistance. Cr/Co-Al Coated superalloy at 700 °C substrate temperature has highest oxidation resistance in air environment at 900 °C for 100 hrs as compare to Cr/Co-Al coated sample at 500 °C and bare superalloy, due to formation of dense, protective and adherent scale such as A1203, Cr203, CoO, and CoA1204 spinel as a major phase and CoCr204 spinel as a minor phase. Also, Cr/Co-Al Coated superalloy at 700 °C before oxidation has dense morphology of coating, since it belongs to structure zone 3, which is confirmed by surface as well as cross-sectional FE-SEM micrograph. Section 4.2 describes the study of cyclic hot corrosion of nanostructured Cr/Co-Al coatings on Superni-718 substrate in molten salt (Na2SO4- 60%V205) environment at 900 °C for 50 and 100 cycles and is compared with bare Superni-718 superalloy. The better hot corrosion of Cr/Co-Al coated sample at 700 °C substrate temperature might be attributed to the formation of three dimensional pyramidal scale morphology on corroded dense surface and continuous oxide scale formed on cross-section. The scale mainly consisting of NaA102, CoA1204, CoCr204, CoO, A1203 and Cr203. Chapter 5 contains the investigation of deposition, characterisation, oxidation, and hot corrosion of Co-Al coatings on Superni-718 at different substrate temperature and bare iv Superni-718 substrate in the laboratory under cyclic condition at 900 °C for 100 cycles. The echniques such as XRD, FE-SEM/EDS and AFM were used to analyze the coated samples )efore and after oxidation and hot corrosion. The thickness of coating was calculated by FE-3EM cross-sectional analysis and was approximately 4 1.1111. It showed a good adhesion with he substrate. This chapter is divided into two sections. The first section (Section 5.1) describes degradation behaviour of sputtered Co-Al coatings on Superni-718. Co-Al coatings were deposited at 500-700 °C substrate temperature. At the substrate temperature 800 °C, the deposited coatings peeled off due to the induced thermal stress in the coatings, which is confirmed by FE-SEM/EDS. Oxidation kinetic was established at 900 °C for 100 cycles under cyclic conditions. The sputtered Co-Al coated sample at 700 °C has provided a maximum oxidation resistance in air at 900 °C for 100 hrs. The presence of A1203, Cr2O3 and CoO and respective spinels might have contributed for better oxidation resistance. Section 5.2 describes the microstructural characterization and cyclic hot corrosion behaviour of sputtered Co-Al nanostructured coatings on superalloy at 500-700 °C substrate temperature in molten salt (Na2SO4-60%V205) environment at 900 °C for 100 cycles. The bare and sputtered Co-Al coated sample depicts least and improved hot corrosion resistance in molten salt environments at 900 °C for 100 hrs. The Co-Al coated sample at 700 °C shows better hot corrosion resistance might be due to dense coating before hot corrosion and after hot corrosion attributed to the formation of dense oxide scale with phases like A1203, Cr2O3, CoO, NiCr2O4 ,CoAl2O4 and CoCr2O4. Spinel phases might have blocked the all possible paths/defects in scale which has formed during cyclic hot corrosion and to enhance the hot corrosion resistance. Chapter 6 describes the comparative results of the bare Superni-718 substrate and magnetron sputtered Cr/Co-Al and Co-Al coated Superni-718 at different substrate temperature in air, and molten salt environments (Na2SO4-60%V205) at 900 °C under cyclic conditions. Chapter 7 includes conclusions of the present investigation and scope for the future work.