_STUDY OF DETONATION GUN COATED ALLOYS FOR RESISTANCE TO EROSION AND CORROSION

Abstract

Erosion-corrosion of materials at elevated temperatures is a major problem in various industrial processes which range from advanced coal-conversion processes, to the turbine blades of jet engines. Erosion-corrosion is a generalised wear phenomenon where the combined effect of each degradation mechanism generates more extensive mass loss than the sum of each mechanism acting independently. The operating conditions in power station boilers are conducive to fireside corrosion and erosion both in furnace wall and in the superheater and reheater areas. The erosion corrosion phenomenon results in tube wall thinning and premature failure. Erosion corrosion has become a topic of continuous investigations and great concern as it consumes the material at an unpredictably rapid rate. In steam thermal power plants where coal is used as primary fuel, the corrosion occurs in special fashions due to combustion products which change their state of matter and becomes salt as a result of high temperature. Combustion of coal generates very corrosive media particularly near the superheater tubes, forming highly corrosive ash deposits that contain alkali metals of sodium, potassium and sulphur. Ashes formed due to combustion of low grade fuels have a high concentration of compounds formed by vanadium, sodium and sulphur mainly as Na2SO4-V205 complex and sodium vandates mixture. These compounds have very low melting points as low as 550°C. These metals easily liquefy at the operating temperatures of boilers and cause accelerated corrosion. The erosion is even more localised in its effects, and results from impact of particulates, such as coal ash, dolomite and unburnt carbon particles on surface of heated boiler tubes. So high temperature oxidation and erosion are recognized as being one of the main cause of downtimes in these installations. The materials used in these installations are fabricated from low alloy carbon steels with chromium and molybdenum as the primary alloy additions termed as boiler steels. Unfortunately TI I and T22 steels, which are commonly used in the manufacturing of conventional boilers experience accelerated erosion corrosion in high temperature boiler conditions. Although Chromium is expected to impart corrosion resistance to high temperature alloys but is not sufficient enough to form protective external scale. Currently, the superalloys are used to increase the service life of the boilers especially in the super-heater zones of the new generation ultra-supercritical boilers. These materials being developed with high strength at elevated temperatures are highly alloyed and thus very expensive and also prone to degradation to certain environments for example Nickel based superalloys readily react with the sulphur in coal leading to high corrosion rate. The presence of combustion gases constitutes an extreme environment and the hot corrosion is inevitable when the superalloys are used at high-temperatures for longer periods of time. A number of countermeasures are presently in use or under investigation to combat the hot corrosion such as, the use of inhibitors, control of the process parameters, development of suitable industrial alloys, and deposition of protective coatings. So in search of cost effective solutions for erosion corrosion problems, various coating techniques like thermal spraying have become attractive. The boiler steels T11 and T22 were obtained from S.G.G.S. Thermal power Plant Ropar and superalloys, namely Superni 600, Superni 718 as Ni-based, and Fe-based Superfer 800, were provided by Mishra Dhatu Nigam Limited, Hyderabad (India), for devising some means to protect them against high temperature erosive corrosive environment applications. Detonation gun spraying belongs to thermal spraying family and is used for many applications_ This process has shown the development of coatings with better properties like low porosity, high strength etc. The high energy created by detonation involved in the process makes the powder closely conjoint on the surface, thereby resulting in a dense coating. This thesis investigates the erosion and corrosion performance of A1203-3 wt % Ti02 thermal spray coatings deposited by detonation gun technique. As-sprayed coatings were characterized by using the combined techniques of optical microscopy, microhardness testing, X-ray diffractometry (XRD) and scanning electron microscopy/energy-dispersive analysis (SEMfEDX). The corrosion performance was evaluated in oxidising and molten salt environment and also in actual working conditions of boiler. The erosion performance was evaluated at room temperature conditions as well as under high temperature conditions, high erodent velocity, and actual boiler conditions. The corrosion behavior and morphological development were investigated by weight change kinetics, metallographs, depths of attack, metal thickness losses, and XRD analyses. Previous research has highlighted several theoretical mechanisms under this generalized process, ranging from the erosion induced breakdown of oxide scales in corrosive environments, through to the development of oxide layers in highly erosive environments. Prior to this current work erosion corrosion experimental mechanisms has focused on bulk alloy materials with well characterised oxidation responses, under conditions of low temperature, low erodent impact velocity conditions which are readily generated within laboratory scale rigs. Few works have addressed erosion-corrosion under simulated boiler conditions of high temperature, high erodent impact velocity. The trials have been run under such testing conditions that have involved only one temperature i.e. only substrate temperature and importance of surrounding temperature has been overlooked. Most of the published work on thermally sprayed A1203-3 wt % TiO2 coatings has been confined to wear and room temperature erosion. Little has been presented on the mechanism of erosion- corrosion of A1203-3 wt % Ti02 coatings. Most works have been conducted under milder conditions than used in the current work. In addressing the short comings in the current state of knowledge, the aim of this work was to characterise the mechanism of erosion and corrosion of D-gun sprayed A1203-3 wt % TiO2 thermal spray coatings under actual boiler conditions, erosion conditions and oxidising and molten salt corrosion conditions. Al203-3 wt % Ti02 coatings were deposited by D-gun spraying equipment available with SVX Powder M Surface Engineering Private Limited, Greater Noida (India) under optimised conditions. The coatings, characterised in terms of microhardness, porosity content were in good agreement with the coating quality presented in the literature. The coating was deposited on two types of boiler steels designated as ASTM-SA-213-T11 and ASTM-SA-213-T22 and three superalloys namely Superni 600, Superni 718 and superfer 800. Under the given spray parameters coatings with thickness in the range of 200-220μm and porosity less than 1% were formed. The EDX analysis has confirmed that coatings obtained in the present investigation meet the required compositions. Solid particle erosion studies have been carried out as per 076 ASTM standard at 30° and 90° impact angle with Alumina as erodent. Erosion testing was carried out using a solid particle erosion test rig TR-471-M10 Air Jet Erosion Tester (Ducom Instruments. Private Limited, Bangalore, India) capable of conducting tests at room temperature as well as high temperature. In general, Erosion resistance is measured using weight loss technique by measuring the weights before and after the test. But at high temperature, weight change measurements leads to flawed results due to oxidation of samples. In order to overcome the limitations of the weight change technique, a different technique was used for the present investigation and erosion resistance was measured in terms of volume loss after the erosion testing. The total volume loss rate, due to erosion for the test materials are compared. The effect of temperature on the coating erosion was investigated. The high temperature erosion behavior of this coating was compared with that uncoated alloy i.e. T-11, T-22 steels and Superni 600, Superni 718 and Superfer 800 superalloys. On these coated substrates, thermal cyclic oxidation studies were performed in static air as well as in molten salt (Na2SO4-60%V205) environment at 900°C for 50 cycles. The weight gain for all the coated alloys are significantly lower than the uncoated alloys subjected to oxidation and molten salt hot corrosion. The coatings showed good adherence to their respective substrates during exposure to oxidation as well as the molten salt environment. The very low porosity and the flat splat structure of the D-gun sprayed coatings have also iv contributed to hot corrosion resistance as this is the desired structure, when the coatings have to perform in corrosive environment at higher temperature. In the flat splat structure the distance from the coating surface to coating/substrate interface along splat boundaries, through which the corrosive species mostly permeate, is very long. In order to establish an understanding of the behaviour of the coatings and bare alloys in the actual working conditions, where these coatings are intended to be used, the specimens were exposed for 1500 hours to platen super-heater zone of the coal fired boiler of Shri Guru Gobind Singh Super Thermal Plant, Ropar, Punjab (India) for this study. This zone was selected for the present study as many breakdowns occurred in this power plant due to the erosion corrosion degradation of the platen superheater tubes of the coal fired boilers. The specimens were exposed to the combustion environment for 15 cycles of 100 hours heating followed by 1 hour cooling at ambient conditions. The temperature of the hanging zone was measured at regular intervals during the study and the temperature was about 700 ±10 °C with full load of 210 MW. The erosion corrosion behavior of coated alloys is promising in comparison to substrate boiler steels. 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 measurements. XRD and SEM/EDX techniques were used for detecting the phases present and for elemental analysis of the surface scale after exposure to boiler environments. The coatings have successfully imparted the erosion corrosion resistance to the substrate alloys in the boiler environment. Based on these results the coatings were considered good for industrial applications. The responses of the samples in this work to oxidation, hot corrosion and erosion were considered indicative of the response of industrially applied coatings of this composition in service. This analysis makes it clear that with an appropriate choice of processing conditions a sound and adherent ceramic coating is achievable by detonation gun process using A1203-3 wt % TiO2 powder. Based on the findings of the present study, A1203-3 wt % Ti02 coatings under investigations are suggested to be tried for applications to super-heater and re-heater tubes of the boilers for protecting them against high temperature erosive corrosive environment applications.