DEVELOPMENT OF COATINGS FOR INCINERATOR ENVIRONMENT USING D-GUN TECHNIQUE

Abstract

Incinerators are widely used to burn waste such as medical waste and municipal solid waste. Burning of such waste generates large amount of corrosive species and flue gases which may contaminates the environment. Medical waste can be infectious and may spread diseases. Hence to destroy the contagious microorganism completely very high temperature is required. Due to the high temperature combined with aggressive species, the metallic components of the incinerators encounter catastrophic corrosion. A possible approach to solving high-temperature materials corrosion problems that can arise in such systems is either to choose some alternative material or depositing the metallic coating on the substarte alloy before installation of the component. Thermal spray processes have emerged as one of the vital surface engineering tool and has proved to be the best method to develop the coatings that can be used at high temperature corrosion and wear application (Yamada et al. 2002). D-gun is one of the thermal spray technique having minimum porosity and higher bond strength. Guilemany et al. (2008) reported that compact structure of coatings deter the impacts of the molten salt that could deplete the coating if it has interconnected porosity. A wide range of materials that can be used, and variety of properties of coatings that can be formed, makes it usable in many areas of industry (Gruzdys 2011). The chromium carbide nickel chromium coating is widely used in erosion, high temperature corrosion and wear resistance. This coating can be effectively used till the working temperature of 900°C under the corrosive conditions (Picas et al. 2006). In view of that, in the present study, oxidation and hot corrosion behaviour of some superalloys designated as Superni 718, Superni 600 and Superco 605 as per the manufacturer’s specifications, have been investigated with and without the application of D-gun sprayed coatings as received Cr3C2-(NiCr) and further incorporating it with 0.2wt.%Zirconium and 0.4wt.%Ceria respectively. All these uncoated and coated alloys were tested in air, 40%Na2SO4-10%NaCl-40%K2SO4-10%KCl, Na2SO4-25%NaCl at 900°C for 100 cycles in laboratory and also in actual medical waste incinerator environment under cyclic conditions. A thorough investigation on the behaviour of the aforementioned coatings in different environment is very essential to choose the suitable coating and substrate to preclude the oxidation and hot corrosion problems manifested in the incinerators and bio-fuel fired boilers environment. The results of the present research work are critically analyzed and discussed in light of the ii existing literature to propose an insight in to the corrosion mechanism in both coated and bare superalloys. The whole thesis is presented in 10 chapters. Chapter 1 deals with brief introduction regarding the application of incinerator and bio-fuel fired boilers for incineration of medical waste, municipal waste and hazardous waste. An application of these for the generation of power was briefly discussed. Burning of waste especially hazardous waste involves high temperature operation and generation of corrosive gases thereby creating corrosion problems for the components of incinerator and bio-fuel fired boilers. These corrosive problem and remedial measures has been discussed in this chapter. Chapter 2 deals with a comprehensive review of the literature with references to Incineration, types of incinerators, types of fuels used in incinerators, types of medical, municipal and hazardous waste and composition of environment present in incinerator, usage of Incineration around the world , High temperature corrosion problems in incinerators and bio-mass fuel fired boilers. General mechanism of Oxidation and corrosion was also discussed. Studies reported on oxidation and hot corrosion of Ni and Co based superalloys. Detailed description of D-gun process and studies done on Cr3C2-NiCr coatings were also covered. Studies reported on 40Na2SO4-10NaCl-40K2SO4-10KCl and Na2SO4-25NaCl environment was also described in details. The effect of adding rare earth in various alloys and coatings has also been discussed. The problem has been formulated after critical analysis of the literature regarding the corrosion studies done under incinerators environment. Chapter 3 presents the experimental techniques and procedures employed for applying the coatings and their characterization. Procedure for oxidation studies in air, molten salt corrosion in simulated incinerator environment and in actual incinerators environment for both bare and coated alloys has been discussed. The specification of the equipments and other instruments used for the present investigation and the techniques employed to analyze the corrosion products are discussed. The D-gun sprayed coatings were deposited at SVX Powder M Surface Engineering Pvt. Ltd, New Delhi (India) on Ni- and Co- 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, Cr3C2-(NiCr)+0.4wt.%CeO2 and Cr3C2-(NiCr)+0.2wt.%Zr iii The as-sprayed coatings were characterized by the techniques such as visual examination, XRD, FE-SEM, EDS, and X-ray mapping analysis. The oxidation and hot corrosion behavior of bare and D-gun coated superalloys have been studied in the air, two molten salts (40%Na2SO4-40%K2SO4-10%NaCl-10%KCl and Na2SO4+25wt.%NaCl) in a laboratory furnace at 900°C for 100 cycles under cyclic conditions. Each cycle consisted of 1h heating followed by 20min cooling. At the end of each cycle, the specimens were critically examined regarding the colour, lustre, physical changes on the samples, scale adherence/spallation and then subjected to weight change measurements. XRD, FE-SEM/EDS and X-ray mapping analytical techniques were used to identify the phases and the elemental composition in the corroded coatings. The samples were tested in actual medical waste incinerator environment for 1000h with 10 cycles of 100h each. Chapter 4 deals with the characterization of D-gun sprayed coatings. Porosity measurements, Microhardness and Surface roughness measurements have been conducted in the laboratory for the as sprayed coatings. SEM/EDS and XRD techniques were used for the characterization. A good adhesion of the coatings to the substrates were evident from the absence of cracks and gaps at the interfaces as well as due to the uniform distribution of carbide particles along NiCr matrix with the porosity value in the range of 1.0-1.5%. The microhardness of the Cr3C2–25%NiCr coatings was found to be in the range of 720 –975Hv due to high volume fraction of carbides, dispersed uniformly in the matrix. Coating thickness ranges from 200-250μm, where as surface roughness varies from 4.92-6.05μm as observed in the present work. Ceria and Zirconium incorporated in the coating were found to be distributed around the splat boundaries. Chapter 5 The D-gun sprayed Cr3C2-NiCr coatings on three different superalloys subjected to cyclic oxidation in air at 900oC for 100 cycles. The coating remain adherent to the substrate and did not show any spalling of the oxide scale. All the coatings showed lower weight gain as compared to substrate alloys. However, both bare and coated alloys showed good oxidation resistance in air under cyclic condition. SEM/EDS and XRD analysis showed formation of dense and compact oxide mainly consisting of chromium oxide in case of all the bare and coated substrates. Chapter 6 Bare and coated Superni 718, Superni 600 and Superco 605 were exposed under high temperature in presence of Na2SO4-K2SO4-NaCl-KCl at 900°C for 100 cycles. Bare Superni 718 and Superni 600 showed minor weight loss and suffered from internal oxidation iv in the above salt environment. Bare Superco 605 suffers from catastrophic corrosion resulting in severe spallation of the oxide scale. Cr3C2-(NiCr) coated Superni 718 and Superni 600 developed good resistance to the corrosion but coated Superco 605 showed the failure and disintegration of the coating during corrosion run in the above environment. Incorporation of ceria and zirconium in the coating powder showed lower weight gain of the resulted coated alloys as compared to original Cr3C2-(NiCr) coated. In case of Co-based alloy, rare earth doped coating got partially disintegrated during the corrosion run. Chapter 7 deals with behaviour of the bare and coated superalloys in Na2SO4-25%NaCl environment at 900°C for 100 cycles. All the three alloys suffered internal oxidation. The two nickel based alloys had minor weight change whereas Co-based alloy underwent catastrophic degradation. Cr3C2-(NiCr) coating deposited on both the Nickel based superalloys provided sufficient corrosion resistance. Addition of rare earth elements namely zirconium and ceria further reduce the overall weight gain as compared to original Cr3C2-(NiCr) coatings. But all the three viz original and rare earth doped Cr3C2-(NiCr) coatings were not very successful for Superco 605 under the above corrosive environment and suffered with coating delamination in this environment also. Chapter 8 Bare and coated specimens were exposed under the actual medical waste incinerator environment for 1000h under cyclic condition with cycles of 100h each. Samples were kept in secondary chamber of dual chamber incinerator where the temperature is about 1050°C available in 92 Base Hospital, BB Cantt., Srinagar, Kashmir, India. The specimens showed almost analogous results to the laboratory test. All the three bare alloys undergo internal oxidation. Whereas Cr3C2-(NiCr) coating provide sufficient protection to the substarte. Addition of 0.4wt.%CeO2 in Cr3C2-(NiCr) increases the corrosion resistance. Similar results were observed for 0.2wt.%Zr doped coating. Chapter 9 In this chapter all the results of oxidation and hot corrosion of bare and coated superalloys has been compared and their performance has been discussed. Chapter 10 Final conclusions from the present study have been summarized.