HOT CORROSION BEHAVIOUR OF CONVENTIONAL AND NANOSTRUCTURED COATINGS ON SUPERALLOYS

Abstract

Material degradation at high temperatures is a serious problem in several high technology based industries. Gas turbines in aircraft, fossil fueled power plants, refineries and petrochemical industries, and heating elements for high temperature furnaces are some examples, where corrosion either limits their use or reduces their life, considerably affecting the efficiency. Understanding the behaviour of metals and alloys at elevated temperatures, especially their corrosion behaviour and providing protective surface layers has become an object of scientific investigation since long time. Hot corrosion is an accelerated oxidation of material at elevated temperatures, induced by a thin film of fused salt deposits. In hot corrosion, metals and alloys are subjected to degradation at much higher rates than in gaseous oxidation, with porous non-protective oxide scale formed at the surface and sulphides permeating into the substrate. Hot corrosion was first recognized as a serious problem in the year 1940 in connection with the degradation of fireside boiler tubes in coal-fired steam generating plants. Later on, it became a topic of importance and popular interest in the late 60s as gas turbine engines of military aircraft suffered severe corrosion during the Vietnam conflict during operation over seawater. The hot corrosion of an alloy usually occurs in the environments where molten salts such as sulphates (Na2SO4), chlorides (NaCI) or oxides (V205) are deposited onto the surfaces. Sodium vanadyl vanadate (NazO.V204.5V205), which melts at a relatively low temperature 550°C is found to be the most common salt deposit on boiler superheaters. Superalloys are well known candidates for such high temperature applications. Although the superalloys have adequate mechanical strength at elevated temperatures, they often lack resistance to oxidizing/corroding environments. Use of inhibitors like MgO, CeO2, CaO, Mn02, etc. have already been investigated in the laboratory at Metallurgical and Materials Engg, IIT Roorkee, and decrease in the extent of corrosion in the most aggressive environment of Na2SO4-60%V205 at 900°C has been achieved. However, the major problem being envisaged is the application of these inhibitors to the hot metal surface. Another countermeasure against the hot corrosion and oxidation constitutes the use of protective coatings. Furthermore, use of plasma sprayed coatings on boiler steels and HVOF sprayed coatings on superalloys to investigate the hot corrosion resistance have been carried out in this department and the better resistance to hot corrosion in a very aggressive environment of Na2SO4-60%V205 at 900°C has been achieved. The literature on hot corrosion behaviour of Ni-SAI, NiCrAL and ceria added NiCrA1Y coatings deposited on superalloys by HVOF and sputtering techniques is very limited. Owing to the aforementioned facts, the present work has been focused to evaluate the hot corrosion behaviour of Ni-5A1, NiCrAI and NiCrA1Y-0.4wt%CeO2 coatings deposited by HVOF spray process and Ni-Al and NiCrA1 films deposited by RF magnetron sputtering process on Ni and Fe based superalloys in air, molten salt (Na2SO4-60%V205) at 900°C in the laboratory under cyclic conditions, and in actual industrial environment in a coal fired boiler of a thermal power plant. The behaviour of these coatings in different degrading environments will be helpful in choosing the suitable coating for the hot section components of the gas turbine and boiler applications. The results have been critically analysed and discussed in the light of existing literature to propose an insight into the corrosion mechanisms manifested in both coated and bare superalloys. The whole work has been presented in 9 chapters. Chapter-1 contains a brief introduction as to hot corrosion phenomenon and its deleterious impact on the various engineering equipments and components. The remedial measures to obviate this problem are also briefly discussed. Chapter-2 presents the literature review on various aspects and mechanism of high temperature oxidation and hot corrosion. The high temperature oxidation/corrosion studies reported by various researchers relevant to the current work have been critically reviewed; particularly those conducted on similar Ni- and Fe-based alloys in air and molten salt environments. The various preventive measures have been summarised along with the description of HVOF spray process and RF magnetron sputtering process. The studies related to hot corrosion behaviour of coatings have also been reviewed. The problem has been formulated based on the available literature on hot corrosion behavior of coated and bare superalloys for the high temperature applications. Chapter 3 presents the experimental techniques and procedures employed for applying the coatings and their characterisation, oxidation studies in air, molten salt environment, and in the actual coal fired boiler environment. The specifications of the equipments and other instruments used for the present investigation and the techniques used to analyse the corrosion products are described bellow. The HVOF spray coatings were deposited at M/s Metallizing Equipments Company Pvt Ltd, Jodhpur, India, on Ni- and Fe- based superalloys. These superalloys were procured from Mishra Dhatu Nigham Ltd., Hyderabad (India). Three types of coatings formulated were Ni-5A1, NiCrAl and NiCrA1Y-0.4wt%CeO2. As-sprayed coatings were characterised by iii metallography, FESEM, EDAX and X-ray mapping analysis. Mechanical properties such as bond strength and microhardness of the coatings have been evaluated. Surface roughness of the as sprayed coatings was also measured. The RF magnetron sputtering process was used to deposit nanostructured coatings (Ni-Al and NiCrAI) on the superalloys. The thin films were formulated in the Centre of Nanoscience Laboratory, Indian Institute of Technology Roorkee. The as deposited films were characterised by XRD, AFM, FESEM/EDAX analyses. The corrosion behaviour of uncoated and coated superalloys has been studied in the air as well as in the aggressive environment of molten salt (Na2SO4-60%V205) in the laboratory furnace for 100 cycles. Each cycle consisted of 1 hour heating at 900°C followed by 20 minutes cooling to ambient temperature. At the end of each cycle, the specimens were critically examined regarding the colour, luster, tendency to spall and adherence of scale 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 samples (250°C) with the help of camel hair brush. XRD and FESEM/EDAX analytical techniques were used to identify the phases obtained and the elemental analysis of the surface scale, respectively. The corroded samples were then cut across the cross-sections for analyzing its composition by X-ray mapping analysis. Chapter-4 describes the detailed investigation of HVOF sprayed Ni-S Al. coatings and RF magnetron sputtered Ni-Al film on Ni- and Fe-based superalloys, which includes, the characterisation of HVOF sprayed Ni-5A1 coatings and RF magnetron sputtered film, oxidation studies in air at 900°C under cyclic conditions and hot corrosion studies in a very aggressive environment of (Na2SO4-60%V203) at 900°C in laboratory under cyclic conditions. Techniques such as XRD, FESEM, EDAX and X-ray mapping were used to analyse the as sprayed coatings and the corroded specimens after oxidation studies in air and molten salt environment. The porosity of the coating was found to be around 2.0%. The bond strength of the coating was measured as per ASTM standard C633-01 and found to be 43 MPa. The splats formed are rich in nickel and aluminum moved to the periphery of the nickel rich splats. The as sprayed Ni-5A1 coating indicates minor diffusion of alloying elements from the substrate into the coating. Oxidation studies were carried out on both bare and coated superalloy substrates in air at 900°C for 100 cycles. Among the three-coated superalloys, Superfer 800 substrate has shown the better resistance to oxidation. The protective nature of the Ni-5A1 coated superalloys was due to the formation of protective oxide scales such as NiO, A1203 and Cr203. Hot corrosion studies were conducted on bare as well as HVOF-coated superalloy iv specimens in a molten salt (Na2SO4-60%V205) environment at 900°C under cyclic conditions. The coatings and the oxide scale formed on the exposed surface were found to be intact with the superalloys. Superfer 800 with Ni-5A1 coating has provided a good protection to the superalloys in the given molten salt environment. In case of RF magnetron sputtered Ni-Al coating, the weight gain was comparatively lower than the HVOF sprayed coatings in oxidation studies. The lower weight gain may be attributed to the formation of oxide layer on the surface at a rapid rate due to the nanograins formed on the as deposited film. In molten salt tests, the RF sputtered Ni-Al film has shown comparatively higher weight gain. The possible reason could be sputtering of the thin scale during the initial period of exposure. Chapter-5 contains the investigation of HVOF sprayed NiCrAl coatings and RF magnetron sputtered NiCrAI film on Ni- and Fe-based superalloys. The characterisation of HVOF sprayed NiCrAI coating and RF sputtered film has been discussed in detail. Oxidation studies in air at 900°C under cyclic conditions and hot corrosion studies in a very aggressive environment of (Na2SO4-60%V205) at 900°C under cyclic conditions are reported in detail. XRD, FESEM, EDAX and X-ray mapping techniques were used to analyse the as sprayed and corroded products after oxidation studies in different environments. The porosity of the coating was found to be around 1.7%. The microhardness of the coating was in the range of 278 - 351 Hv. The bond strength of the coating was found to be 59 MPa. Nickel and chromium were coexisting in the splats and aluminum along the splat boundaries. In the as sprayed coating, minor diffusion of alloying elements was noticed from the substrate into the coating. Oxidation kinetics was established at 900°C for 100 cycles by thermogravimetric analysis. The NiCrAI coated Superni 750 alloy (SN 750) provided a better protection among the coated superalloys investigated. The NiCrAI deposited coatings on Ni-and Fe-based superalloys indicate dense and adherent with the substrates. The scale formed on the coated superalloys provided a better oxidation resistance at 900°C under cyclic conditions. The NiCrAI coated Superni 750 has provided better hot corrosion resistance amongst the coated superalloys in the molten salt environment due to the formation of spinels on the surface and a thick band of Cr203 observed along the coating-substrate interface contributed to the added protection to the substrate alloy, resulting in the least weight gain among the three coated superalloys. In case of RF magnetron sputtered NiCrAI coating, the weight gain was comparatively lower than the HVOF sprayed coatings after exposure to air at 900°C for 100 cycles. However, in molten salt environment, the scale spalled during the period of exposure indicating a higher weight gain. I Chapter-6 describes the detailed investigation of HVOF sprayed NiCrAiY-0.4wt%CeO2 coatings on Ni- and Fe-based superalloys. The characterisation of HVOF sprayed NiCrAI coatings has been discussed in detail. Oxidation studies in air at 900°C under cyclic conditions and hot corrosion studies in a very aggressive environment of (Na2SO4-60%V205) at 900°C under cyclic conditions are discussed. Techniques such as XRD, FESEM, EDAX and X-ray mapping are used to analyse the as sprayed coatings and the corroded specimens after oxidation studies in air and molten salt environment. The porosity of the coating was found to be around 1.4%. The microhardness of the coating was in the range of 649 - 753 Hv. The bond strength of the coating was found to be 45 MPa. Nickel and chromium were coexisting in the splats and aluminum along the splat boundaries. In the as sprayed coating, minor diffusion of alloying elements was noticed from the substrate into the coating. Presence of Ce and Y was observed in the coating. Oxidation kinetics was established at 900°C for 100 cycles by thermogravimetric analysis. All the coated superalloys followed the parabolic rate law in the given environment. The better protection of the NiCrAIY-0.4wt%CeO2 coated superalloys may be attributed to the formation of oxides of aluminum, chromium, nickel and spinet of nickel and chromium. The coated superalloys after exposure to molten salt environment at 900°C for 100 cycles showed comparatively very little weight gain and the scale formed on the surface were dense and adherent to superalloy substrates. The NiCrAIY-0.4wt%CeO2 coating has led to the reduction in weight per unit area of about 1/20 in Superni 76, whereas 1/50 and 1/100 for Superni 750 and Superfer 800, respectively. The formations of oxides of nickel, chromium, aluminum, and spinel of nickel and chromium as indicated by XRD analysis provide a better protection to the superalloys in the given environment. Furthermore, the formation of ceria in the scale and the possible formation of cerium vanadates (CeVO4) might have contributed for better corrosion resistance of the superalloys in the given environment. ` Chapter 7 includes the detailed study of uncoated Superfer 800, HVOF sprayed Ni-5A1, NiCrA1 and NiCrAIY-0.4wt%CeO2 coatings and RF magnetron sputtered films (Ni-Al and NiCrAl) on Fe- based superalloy in the actual environment of the coal fired boiler. A 2 mm hole was drilled in the specimens and the uncoated and coated specimens were hanged in the low temperature superheater zone of the coal fired boiler of Guru Gobind Singh Super Thermal Power Plant, Ropar, Punjab, India, by using stainless steel wires. The temperature of the zone was around 700°C ±10°C. Hot corrosion studies were performed for 100 cycles, each cycle consists of 100 hours exposure followed by I hour cooling at ambient temperature. The vi specimens were visually examined at the end of each cycle for any change in colour, luster, spalling tendency and other physical changes of the scale, if any. Weight change measurements were made at the end of each cycle but it could not be of much use for predicting hot corrosion behaviour of suspected spalling and ash deposition on the samples. The extent of corrosion has been evaluated by measuring thickness of the unreacted portion of the samples after the total exposure of 1000 hours. The different phases and their distribution in the hot corroded specimens were analysed by using XRD, FESEM/EDAX and X-ray mapping. Chpater-8 describes the comparative performance of the uncoated, HVOF coated and RF magnetron sputtered films on superalloys in air, molten salt (Na2SO4-60%V205) environment as well as in the actual working environment of the coal fired boiler at 700°C under cyclic conditions. Chpater-9 includes conclusions of the present investigation and a scope for the future work.