HOT CORROSION AND EROSION BEHAVIOR OF NANOSTRUCTURED AND CONVENTIONAL COATINGS

Abstract

Materials degradation at high temperatures is a serious problem in several high tech industries. Power plants are one of the major industries which encounter severe corrosion problems resulting in the substantial loss. The problem is becoming more prominent as the plants are getting older. Attempts to increase the efficiency of steam generating plant by raising the final steam temperature above 600°C and the use of gas turbines for the production of cheap electric power have made working conditions more severe for the materials. The boiler tubes used for super-haters and re-heaters in the steam generating systems are subjected to fireside corrosion, resulting in tube wall thinning and premature failure. Hot corrosion has been identified as a serious problem in high temperature applications such as in boilers, gas turbines, waste incinerations, diesel engines, coal gasification plants, chemical plants and other energy generation systems. Also, erosion-corrosion by solid particles in gaseous environments at elevated temperatures and hence wastage of alloys, is a serious problem in many industrial processes. Small, solid particles propelled by generally oxidizing gases in various fluid flow patterns constitute the primary operating environments in the combustion regions of energy generating boilers operating on such fossil fuels as coal and various kinds of biomass. High-temperature erosion-corrosion and oxidation of the heat transfer pipes and other structural materials in the coal fired boilers are recognized as being the main cause of downtime at power generating plants, accounting for 50% to 75% of the total arrest time. Maintenance costs for replacing the broken pipes in such installations are also very high and are estimated to be up to 54% of the total production costs. These facts emphasize the need to develop more and more corrosion resistant materials for such applications. Therefore, the boiler steel needs to be protected. In this regard, it is learnt from the published literature that one possible, practical, reliable and economically viable way to control or prevent the high temperature corrosion and erosion problems of the superheaters and reheaters of the boilers is an application of a thin layer of corrosion resistant coatings having good thermal conductivity. The physical vapour deposition (PVD) and plasma spray (a thermal spray process) have been reported to be two major coating processing technologies used In recent years, corrosion performance of nanostructured materials/coatings is a hot topic in the corrosion field. Nanostructured materials indeed behave differently than their microscopic counterparts because their characteristic sizes are smaller than the characteristic length scales of physical phenomenon occurring in bulk materials. In many tribological applications, hard coatings of metal nitrides are now commonly used. In the past years, hard protective TiAIN coatings and AICrN coatings were widely used for wear resistant properties. Recently these coatings are gaining importance for high temperature wear, corrosion and oxidation resistance applications. It is important to understand the nature of all types of environmental degradation of metals and alloys as vividly as possible so that preventive measures against metal loss and failure can be economically devised to ensure safety and reliability in the use of metallic components. The present study has been performed to evaluate the behavior of the nanostructured and conventional metal nitride coatings (obtained on the boiler tube steels and a Fe-based superalloy); when exposed to high temperature oxidation in air, molten salt (Na2SO4-60%V205) environment, in actual degrading conditions prevailing in a coal fired boiler of a thermal plant, erosion in simulated coal-fired boiler environment and simulated marine environment. Three types of boiler steel substrate materials namely "ASTM-SA210-Grade A-1," "ASTM-SA213-T-11" and steel "ASTM-SA213-T-22" and a Fe-based superalloy having Midhani Grade Superfer 800H have been selected for the present study as the substrate materials. The nanostructured thin TiAIN and AICrN coatings; with thickness around 4i.tm, were deposited on the substrates at Oerlikon Balzers Coatings India Limited, Gurgaon, India. The conventional thick TiAI and AICr coatings were deposited on the substrates by Plasma Spraying. The coating work was carried out at a commercial firm namely Anod Plasma Limited, Kanpur, India. The gas nitriding of the plasma sprayed conventional thick Ti-Al and Al-Cr coatings was done in the lab. The as-coated specimens were characterized by metallography, SEM, AFM, Porosity analysis, EDAX and X-ray mapping. Microhardness, bond strength and Surface roughness have also been measured. All the coatings were dense, uniform, continuous and adherent. The thickness of coating was in the range of 5.2-6.3 pm, 4.2-5.5 pm, 140-174 pm and 122-166 pm for the nanostructured TiAIN, nanostructured AICrN, conventional TiAIN and conventional AICrN coatings respectively. The nanostructured thin TiAIN and AICrN coatings exhibited negligible porosity values for as coated; which were less than 0.5 %. The conventional ii TiAIN and AICrN coatings showed; higher porosity values (1.90-4.30%) for as sprayed conventional Ti-Al and Al-Cr coatings; which after gas nitriding were found to be less than 0.65 %. The grain size (calculated by Scherrer formula from XRD plot) for nanostructured thin TiAIN and AICrN coatings was less than 22 and 28 nm respectively which was further verified by AFM analysis. A good adhesion of the conventional thick TiAIN and AICrN coatings was evident from bond test results. Average bond strength of 68.74 MPa and 54.69 MPa was observed in case of conventional TiAIN and AICrN coatings respectively. The behavior of uncoated and coated alloys has been studied in the air, molten salt and actual industrial environment. Air and molten salt studies 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. 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 250°C.',The coated as well as bare alloy specimens were exposed to the platen super-heater zone of the coal fired boiler of Stage-II at Guru Nanak Dev Thermal Plant, Bathinda, Punjab (India). The specimens were exposed to the combustion environment for 10 cycles. Each cycle consisted of 100 hours heating followed by 1 hour cooling at ambient conditions. The temperature was measured at regular intervals during the studs& and the average temperature was about 900°C with variation of ± 10°C. At the end of each cycle the samples were critically examined regarding the colour, lustre, tendency to spall and adherence of scale and then subjected to weight change measurements. XRD and SEM/EDAX techniques were used to identify the phases obtained and the elemental analysis of the surface scale. These corroded samples were then cut across the cross-sections and mounted to study the cross-sectional details by X-Ray mapping. Based on the overall weight gain after 50 cycles in air and in an aggressive Na2SO4-60%V205 (molten salt) environment at 900°C temperature, the oxidation resistance of the bare Fe-based alloys studied in the present investigation has been found to be in the following order: AIR ENVIRONMENT S.F 800H superalloy > Grade A-1 boiler steel > T-11 boiler steel > T-22 boiler steel 111 Na2SO4-60%V205 (MOLTEN SALT) ENVIRONMENT S.F 800H superalloy > T-11 boiler steel > Grade A-1 boiler steel > T-22 boiler steel The superior oxidation resistance of the Superfer 800H superalloy may be attributed to the development of homogeneous and continuous scale consisting of oxides of Ni and Cr due to higher percentage of Ni and Cr in the Superfer 800H. The higher weigh gain and spalling as observed in case of T-11 and T-22 boiler steels may be attributed to the presence of molybdenum in the steels. During initial oxidation; Fe is oxidized and the oxide scale is protective in nature. With progress of oxidation molybdenum becomes enriched at the alloy interface, leading to the formation of an inner layer of molten MoO3 (m. p. 795°C) which penetrates along the alloy-scale interface. This liquid oxide disrupts and dissolves the protective oYic de scale, causing the alloy to suffer catastrophic oxidation. Based on the overall weight gain after 50 cycles in air and in molten salt environment at 900°C temperature, the oxidation resistance of the coating on the Fe-based alloys has been found to be in the following order: AIR ENVIRONMENT Substrate ASTM-SA210-Grade A-1 Boiler Steel: Conventional AICrN > Conventional TiAIN > Nanost. AICrN > Nanost. TiAIN > Bare Grade A-1 Substrate ASTM-SA213-T-11 Boiler Steel: Conventional AICrN > Conventional TiAIN > Nanost. AICrN > Nanost. TiAIN > Bare T-11 Substrate ASTM-SA213-T-22 Boiler Steel: Conventional AICrN > Nanost. AICrN > Conventional TiAIN > Bare T-22 > Nanost. TiAIN Substrate Superfer 800H Superalloy: Nanost. AICrN > Nanost. TiAIN > Bare S.F 800H > Conventional TiAIN > Conventional AICrN &AgaZons:60%V05 (MOLTEN SALT) ENVIRONMENT Substrate ASTM-SA210-Grade A-1 Boiler Steel: Conventional AICrN > Conventional TiAIN > Nanost. TiAIN > Nanost. AICrN > Bare Grade A-1 Substrate ASTM-SA213-T-11 Boiler Steel: Conventional AICrN > Nanost. AICrN > Nanost. TiAIN > Conventional TiAIN > Bare T-11 Substrate ASTM-SA213-T-22 Boiler Steel: Conventional AICrN > Nanost. TiAIN> Conventional TiAIN > Nanost. AICrN > Bare T-22 Substrate Superfer 800H Superalloy: Nanost. AICrN > Nanost. TiAIN > Conventional TiAIN > Conventional AICrN> Bare S.F 800H iv All the coatings have shown protection to the substrate based on the overall weight gain after 50 cycles in air environment at 900°C temperature except nanostructured TiAIN coated T-22 boiler steel. The nanaostructured TiAIN and AICrN coatings has shown resistance to oxidation to some extent as the overall weight gain is less, but failed to sustain during the course of oxidation study. This might be due to the formation of oxide scale which is composed of a porous oxide mixture of TiO2 and A1203, with the domination of TiO2. The plasma sprayed gas nitride conventional thick coatings i.e. TiAIN and AICrN when subjected to cyclic oxidation studies at 900°C for 50 cycles developed a protective scale mainly consisting on A1203 along with some amount of TiO2 (in case of conventional TiAIN coating) and Cr2O3 (conventional AICrN coating). In case of T-11 and T-22 boiler steels; the conventional TiAIN coating failed to sustain during the course of study. The weight change plots for the uncoated and coated alloys indicated that the oxidation behavior has shown conformance to parabolic rate law. Development of some minor cracks near or along the edges of the coated specimens may Joe attributed to the thermal shocks due to differences in the heat expansion coefficients of the oxides, coatings and the substrate. In case of specimens exposed to actual industrial environment; based on the materials depth affected by corrosion or corrosion rate in mils per year (mpy) after 1000 hours of exposure, the corrosion resistance of the bare Fe-based alloys studied iruthe present investigation has been found to be in the following order: S.F 800H superalloy > Grade A-1 boiler steel > T-22 boiler steel > T-11 boiler steel Based on the materials depth effected by corrosion or corrosion rate in mils per year (mpy) after 1000 hours of exposure, the corrosion resistance of the uncoated and coated Fe-based alloys studied in the present investigation has been found to be in the following order: Substrate ASTM-SA210-Grade A-1 Boiler Steel: Conventional TiAIN > Conventional AICrN > Nanost. TiAIN > Bare Grade A-1> Nanost. AICrN Substrate ASTM-SA213-T-11 Boiler Steel: Conventional TiAIN > Conventional AICrN > Nanost. TiAIN > Nanost. AICrN > Bare T-11 Substrate ASTM-SA213-T-22 Boiler Steel: Conventional TiAIN > Conventional AICrN > Nanost. AICrN > Conventional TiAIN > Bare T-22 Substrate Superfer 800H Superalloy: Conventional TiAIN and AICrN > Nanost. TiAIN > Nanost. AICrN > Bare S.F 800H The coated as well as uncoated boiler steels have shown higher corrosion rate as compare to Superfer 800H supeallloy. In case of boiler steels; maximum corrosion rate has been observed in bare T-11 and minimum in case of conventional TiAIN coated Grade A-1 boiler steel. All the coatings have shown resistance to corrosion in terms of corrosion rate when compared with respective bare alloy except nanostructured AICrN coated Grade A-1. The conventional coatings have shown good resistance to the corrosive environment as the oxygen penetration is limited to very less thickness as compare to the uncoated and nanostructured coated boiler steels. All the coated as well as uncoated boiler steels have shown ash deposition on the surface. Thus final thickness is contributed by scale formation, erosion and ash deposition. 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. The samples were cleaned in acetone, dried, weighed to an accuracy of 1x10-5 g using an electronic balance, eroded in the test rig for 3 hours and then weighed again to determine weight loss. In the present study standard alumina 50 micron was used as erodent. The two temperatures were taken for the test, sample temperature 400°C and air/erodent temperature 900°C sim ulated to service conditions of boiler tubes in which sample temperature and flow gas temperature correspond to the inner and outer temperature of water wall pipes. Erosion resistance was measured in terms of volume loss after the erosion testing. All the specimens subjected to erosion wear were analyzed for the characterization of erosion products. The specimens were analyzed using surface SEM, EDAX and measurement of surface profiles using optical profilometer. All the uncoated and nanostructured TiAIN and AICrN coated alloys have shown higher erosion rate at oblique impact (at 30°) than at normal impact (at 90°), which indicate ductile behavior as proposed by Murthy et al. (2001). The nanostructured thin TiAIN and AICrN coatings were removed by the continuous strikes of the eroding particles on the surface of the coatings in most of the cases. The plasma sprayed conventional TiAIN coatings have successfully protected the substrates. All the conventional thick TiAIN coated alloys have shown higher erosion rate at oblique vi impact (at 30°) than at normal impact (at 90°) thus indicating ductile behavior except in case of Grade A-1 boiler steel. The plasma sprayed conventional AICrN coatings have successfully protected the substrates at both impact angles except in case of T-11 boiler steel. The conventional AICrN coating gets removed by the continuous strikes of the eroding particles on the surface of the coating in case of T-11 boiler steel at both impact angles. All the conventional thick AICrN coated alloys have shown higher erosion rate at normal impact (at 90°) than at oblique impact (at 30°) except in case of Superfer 800H superalloy. In order to evaluate the corrosion behavior of the substrates and coatings in simulated marine environment; linear polarization resistance (LPR) and potentiodynamic polarization tests were conducted in an aerated 3 wt% NaCI solution at room temperature. The initial corrosion current density and LPR (Rp) was measured by LPR test. It can be inferred from the corrosion parameters i.e. corrosion current densities (icon) obtained in LPR test that all the coatings are protecting the substrates except conventional coatings on T-11 boiler steel and conventional TiAIN coated Superfer 800H superalloy. In case of uncoated alloys; bare T-22 boiler steel has shown maximum corrosion current density (108.60 I.AA/cm2) and Supefer 800H has shown best corrosion resistance at initial stage on the basis of corrosion current density (05.37 [tA/cm2) and polarization resistance values obtained in the test. In Potentiodynamic polarization test; the corrosion current densities of the coatings were found much lower than that of the substrate steel except for nanostructured and conventional TiAIN coated Superfer 800H superalloy. Also, the corrosion current densities of the substrate and the coatings were found much lower as compared to the LPR test (at initial stage) results. A protective oxide layer may have formed which has blocked further corrosion. The corrosion product formed may have reduced the passage of the electrolyte to attack the samples, and hence providing protection. The ASTM B117 Salt Fog test was used to evaluate the performance of the uncoated and nanostructured thin TiAIN and AICrN coatings. In the 8117 test, the samples are exposed to a salt fog generated from a 5% sodium chloride solution with a pH between 6.5 and 7.2 in salt fog testing set up (HSK 1000, Heraeus Votsch, Germany). All the samples were placed in the salt fog chamber for 24 Hrs, 48 Hrs and 72 Hrs. After exposure; samples were monitored and analyzed by using XRD and SEM/EDAX vii techniques. The uncoated boiler steels have shown higher weight loss per unit area in all three test conditions i.e. 24 Hrs, 48 Hrs and 72 Hrs tests; as compared to their coated counterparts. The uncoated as well as nanostructured TiAIN and AICrN coated Superfer 800H superal.loy have performed well as these specimens have shown no weight change during exposure for 24 Hrs, 48 Hrs and 72 Hrs to salt fog tests. Both the coatings (nanostructured TiAIN and AICrN) have shown good protection to the substrate in terms of weight loss per unit area. Also, the weight loss per unit area increases with the duration of the test in case of bare and coated boilers steels. In case of uncoated boiler steel; the T-11 and T-22 have shown higher weight loss than Grade A-1 for 24 Hrs test duration. The bare Grade A-1 boilers steel undergoes higher weight loss during 48 Hrs and 72 Hrs test studies as compared to T-11 and T-22 boiler steels. XRD diffractograms for uncoated boiler steels have also indicated Fe304 is the main phases present in the oxide scale. viii