STUDIES ON THE ROLE OF COATINGS IN IMPROVING RESISTANCE TO HOT CORROSION AND DEGRADATION

Abstract

The behaviour of materials at elevated temperature is gaining increasing technological importance. It is a problem that man has had to face and solve from the very beginning of his existence. Understanding the behaviour of metals at elevated temperatures and especially their corrosion behaviour became an object of scientific investigation since long. Techniques for studying reactions at high temperatures had to be developed. It is obviously difficult to observe the actual reaction between gases and metals at high temperatures and watch the reaction products build up. During the combustion stage in heat engines, particularly in gas turbines, sodium and sulphur impurities present either in fuel or in combustion air, react to form sodium sulphate (Na2SO4). If the concentration of the sulphate exceeds the saturation vapour pressure at the operating metal temperature for turbine blades and vanes (700-1100°C), then deposition of the Na2SO4 will occur on the surface of these components. At higher temperatures the deposits of Na2SO4 are molten (m. p. = 884°C) and can cause accelerated attack on the Ni-and Co-base superalloys. The accelerated corrosion can also be caused by the other salts, viz. vanadates or sulphate-vanadate mixtures as in oil ash corrosion and in the presence of solid or gaseous salts such as chlorides. When considering coal-gasification processes, hot corrosion is expected to be a problem because the gas environment generally has large sulphur and small oxygen activities and also contains substantial amounts of salts. However, the plausible mechanisms of hot corrosion are fairly well established and any one or more of these mechanisms may be operative in the degradation of a given alloy. At present, methods to minimise the extent of hot corrosion have been identified, however considerable research effort is needed for a quantitative evaluation of these methods under conditions of interest in the coal-gasification processes. One possible way to attack these problems is the use of thin anti-wear and anti-oxidation coatings. Due to the continuously rising cost of materials as well as increased material requirements, the coating techniques have been given more importance in recent ii times. Protective coatings are being used on the structural alloys in energy conversion and utilisation systems to prevent surface degradation by corrosion, erosion and abrasion or by combination of these mechanisms. The composition and structure of the coating are determined by the role that they play in the various material systems and performance environments. Their methods of application include both diffusion and deposition processes. Plasma spraying is now a versatile technology that has been successful as a reliable and cost-effective solution for many industrial problems. It allows the spraying of a wide range of high performance materials from superalloys and refractory intermetallic compounds to ceramics with continuously increasing commercial applications. In the present investigation the plasma spray coatings obtained on the boiler tube steels have been characterised. The behaviour of uncoated and coated boiler tube steel samples has been studied in air, molten salt environment (Na2SO4-60%V205) and in actual boiler environment. Some of the coated samples were laser remelted to generate the denser structure of the coating and the behaviour of these laser remelted samples has also been studied. The coatings were formulated by argon shrouded plasma spray process on the boiler tube steels namely SA-210-GrA1 (GrA1), SA-213-T11 (T11) and SA-213-122 (122). Four types of coatings formulated were Ni-22Cr-10AI-1Y (NiCrAIY), Ni-20Cr, Ni3A1 and Stellite-6 (St-6). The NiCrAIY has also been used as a bond coat of approximately 150 pm thickness before the final coating in all cases. The Ni3AI coating was also carried out by this process by mixing nickel and aluminium powders in the stochiometric ratios. Laser remelting tracks on the plasma sprayed samples have been obtained for the characterisation of coated and laser remelted coatings. Complete coatings were also laser remelted for some of the coated samples to carry out the oxidation studies. As sprayed and laser remelted coatings were also characterised by metallography, SEM, EDAX and EPMA etc. Microhardness has also been measured. Air and molten salt studies were performed in the laboratory furnace for 50 cycles, each cycle consisting of 1 hour heating at 900°C followed by 20 minutes cooling. At the end of each cycle the samples were critically examined and the change in weight was recorded. III In case of studies under the molten salt, a uniform layer (3-5 mg/cm2) of the mixture of Na2SO4-60%V205 was coated on the samples with the help of camel hair brush by preheating the sample at 200°C. Samples were also exposed to actual industrial environment in the middle zone of platen superheater of a coal fired boiler. The samples were inserted in the boiler with the help of stainless steel wire (passed through the 1 mm hole drilled in the samples) through the soot blower dummy points at 31.5 m height where temperature was around 755°C with variation of ±10°C. The samples were exposed to actual combustion gases for 10 cycles each cycle of duration 100 hours followed by 1 hour cooling at ambient temperature. At the end of each cycle the samples were critically examined regarding the colour, lustre, tendency to spell and adherence of scale and then subjected to weight change measurements. XRD and SEM/EDAX techniques were used to identify the phases formed and the elemental analysis of the surface scale. These corroded samples were then cut across the cross-section and mounted to study the cross-sectional details by EPMA. Before oxidation and hot corrosion studies the coatings were characterised. The plasma sprayed coatings were found to have some porosity and after laser remelting considerable decrease in the porosity values have been observed resulting in relatively dense coatings. The laser beam tends to melt the coating entirely as well as melts a portion of the underlying substrate. The atomic mixing of the coating with the substrate during laser-s,,, remelting leads to the surface alloying of the base metal with the coating. The microhardness plots have indicated reduction in hardness values after laser remelting. Non-orderly arrangement of atoms and internal stresses in as sprayed coatings might be responsible for the higher values of hardness. Laser remelting arranges atoms in an order and eliminates the internal stresses which lead to reduction in hardness values. Oxidation and hot corrosion products analysed in the scale are mostly oxides of the elements present in the coatings and substrate materials for the samples corroded in all the three environments of this study. In coal fired boiler environment the embedment of ash particles has been observed in the scale. The presence of continuous Cr203 layer at the substrate-scale interface is predominantly revealed in most of the cases where weight gain and corrosion rates are found to be comparatively less. Spinets of composition NiCr204 and iv CoCr204 are the most commonly observed phases in the scale for nickel and cobalt based coatings respectively those have formed due to reaction of Ni0 and Co0 with Cr203. In case of few corroded samples phases of composition NiFe204 and NiA1204 have also been observed in the scale. The results obtained for coated steel during oxidation in air and molten salt at 900°C had followed the overall trend to provide protection to the substrate steels in following order for any of the given coating: GrAl > T11 > T22 The presence of Mo in T22 base steel may be responsible for the above as it forms Mo03, a low melting phase (795°C) and this liquid oxide disrupts and dissolves the protective oxide scale resulting in catastrophic oxidation. Very inferior resistance of this T22 bare steel in molten salt is also attributed to the reaction of this Mo03 with molten salt resulting in formation of low melting Na2Mo04 which might have led to the acidic fluxing of the protective oxide scale. However, the effect of the base steels was less significant in case of boiler environment. Stellite-6 coating has shown superior resistance to oxidation and hot corrosion in all the three given environments of study. XRD analysis has indicated the presence of CoCr204 in all the environments for this coating. The presence of which might have blocked the diffusion activities through the cobalt oxide (Co0) by suppressing the further formation of CoO. The internal oxidation for the complete coatings and diffusion of iron from the base steel to the upper scale has been observed for the studies performed in all the three environments. The cracks in the scale and some times across the coatings have also been observed in case of studies in air and molten salt environment. The cracking might be due to the difference in composition of coatings, bond coat, substrate and oxides formed, Their thermal expansion coefficients may not be fully matching, which inevitably resulted in generation of thermal-stresses. These cracks were found to be absent in case of studies conducted in the boiler environment and this might be partially attributed to the lower temperature prevailing in that region of the boiler. The oxide protrusions from the beneath have been observed mainly through the cracks during studies conducted in air and molten salt environments. These protrusions were identified to be of iron oxide in case of all the coatings except Stellite-6, where the protrusions contained the oxides of cobalt and iron. The cracking of scale and oxide protrusions from beneath has been observed to be maximum when T22 steel was the substrate and the minimum in case of GrA1 steel as a substrate. Among the uncoated steels comparatively maximum resistance to oxidation was indicated by T11 steel and minimum by GrA1 steel during oxidation in air at 900°C. Whereas maximum protection was provided by Stellite-6 coating to GrA1 steel where the weight gain was only 5% of that for uncoated steel. The overall protection trend observed in case of different coatings to protect the base steels is: Stellite-6 > Ni3A1> Ni-20Cr > NiCrAIY The oxidation resistance of Ni-20Cr coated T22 steel was found to improve after laser remelting. In this case the weight gain value of laser remelted steel is around 2/7 of that for similar type of as coated steel. In case of studies under molten salt, the XRD diffractograms and EDAX analysis have indicated the phases formed identical to those formed during oxidation in air in almost all the corroded coatings. The minimum weight gain value was indicated by Stellite-6 coated GrA1 steel which was only 5% of that of the uncoated steel. The overall corrosion resistance of coatings in the molten salt environment followed the sequence as below: Stellite-6 > Ni-20Cr > NiCrAlY > Ni3AI However, in this environment laser remelted coatings have shown comparatively lower protection than that of unmelted coatings, in general, whereas laser remelted Ni3Al coated T22 steel has shown superior corrosion resistance. Relatively lower corrosion resistance in case of laser remelted samples inspite of dense structure may perhaps be due to attack through the small oxide inclusions introduced at the time of formation of coatings and the stresses developed during the cooling of samples after laser remelting." The uncoated T22 steel showed better resistance to hot corrosion in boiler environment as compared to T11 and GrA1 bare steels. This behaviour is different from the one in air and molten salt environment at 900°C where molybdenum present in the steel has been found to vi be deleterious. In case of exposure in the actual coal fired boiler lower temperature (755°C) is encountered where MoO3 is proposed to be solid. Inferior behaviour of GrA1 uncoated steel in boiler environment may perhaps be due to observed heavy spalling. The sequence wise overall behaviour of coatings for the protection of base steels in the actual coal fired boiler environment is as given below: Stellite-6 > NiCrAIY > Ni-20Cr > Ni3Al Among the coated steels the maximum resistance to corrosion was indicated by Stellite-6 coated T11 type of steel that has shown a loss of thickness which is only around 22% of that for uncoated T11 steel. Laser remelted coatings have also shown relatively good resistance when exposed to boiler environment. The maximum erosion-corrosion resistance might be attributed to the compact and less porous structure of laser remelted coatings. Among the coated and laser remelted steels the maximum resistance was indicated by NiCrAIY coated and laser remelted T22 steel which has indicated only 6% and 42% loss in the metal thickness of that for uncoated and as coated steels respectively. The coatings have been proved to be effective in all the three environments inspite of development of cracks and protruding of oxides from the beneath in case of air and molten salt environments. In most of the cases the substantial reduction in the weight gain values and corrosion rates have been observed for the coated steels. The bond coat has also played significant role for the protection of base steels. The continuous chromium rich layer has also been observed in most of the cases at the bond coat substrate interface which has blocked the transport of reacting species to the substrate steels and thereby enhancing corrosion resistance.