ICMCTF2007 Session A2: Coatings for Use in Harsh Environments

Tuesday, April 24, 2007 1:30 PM in Room Sunrise

Tuesday Afternoon

Start	Invited?	Item
1:30 PM	Invited	A2-1 The Performance of Coating Systems Under Aggressive Deposits in Industrial Gas Turbines J.R. Nicholls, A. Encinas-Oropesa, N.J. Simms (Cranfield University, United Kingdom) Gas turbines are a critical component within combined cycle power systems that are being developed to generate electricity more cleanly and efficiently from solid fuel sources that may include coal and biomass. The use of such fuels, and the production of fuel gases, increases the potential for significant corrosion and erosion damage to gas turbine blades and vanes. This paper presents results of an experimental study of type II hot corrosion of coatings, and base alloy systems, in a simulated industrial gas turbine environment expected from burning gasified coal and biomass derived fuels. A matrix of corrosion tests have been undertaken using the ‘deposit recoat’ test procedure, with samples cooled periodically to re-apply controlled amounts of salt deposit. The deposited salt was a 4/1 mole ratio of Na₂SO₄ and K₂SO₄, with deposited fluxes of 0, 1.5, 5.0 and 15.0μmg/cm²/h. The performance of a platinum aluminide diffusion coating and an NiCoCrAlY overlay coating on CMSX4 were evaluated and compared to uncoated CMSX4.Test durations of 500h and 1000h at 700°C in a variety of gas compositions, consisting of air + 50-500vppm SO₂ + 0-500vppm HCl + 0-5vol% H₂O, have been evaluated. Section loss data has been measured, using precision optical metrology, and analysed statistically. A model to predict the section loss of CMSX4, as a function of salt deposition flux and gas composition, has been developed. The performance of current commercial diffusion and overlay coating systems are compared to this model behaviour. New coating systems, based on a functionally gradient design, are suggested that are more resistant to these aggressive deposits and gaseous environments, expected in turbines burning fuel gases derived from coal and/or biomass.
2:10 PM		A2-3 Hot Corrosion Evaluation of Various Ni-Based Diffusion Aluminide Coatings V. Deodeshmukh, B. Gleeson (Iowa State University) This study evaluates the performance of various γ-Ni+γ'-Ni₃Al and β-NiAl diffusion aluminide coatings under Type II (705°C) and Type I (900°C) hot corrosion conditions. Both types of hot corrosion conditions were simulated by depositing Na₂SO₄ salt on the coated samples and then exposing the samples in a laboratory-based furnace rig through which an O₂+SO_x atmosphere flowed. The effect of pre-oxidation treatment on these coatings was also studied. A Pt+Hf-modified γ'+γ coating exhibited superior Type I and Type II hot corrosion resistance with the pre-oxidation treatment; while Pt-modified β coating exhibited excellent Type II hot corrosion resistance without the pre-oxidation treatment. It was also found that Pt addition in β coatings considerably improved both types of hot corrosion resistance.
2:30 PM		A2-4 Hot Corrosion Behavior of HVOF Sprayed Ni-5Al Coatings on Ni and Fe-based Superalloys R.A Mahesh, R. Jayaganthan, S. Prakash (IIT Roorkee, India) Materials operating at elevated temperatures in corrosive media suffer erosion-corrosion wear, oxidation and hot corrosion. Degradation by high temperature oxidation is one of the main failure modes of hot-section components in the gas turbines. Therefore, understanding the behavior of metals and alloys at elevated temperatures, especially their their corrosion behavior and providing protective surface layers has become an object of scientific investigation. Among the various coating techniques, high velocity oxy-fuel (HVOF) spraying process is a new and rapidly developing technology, which can yield high density coatings with porosity less than 1% having high hardness, adhesion values, and good erosion, corrosion and wear resistance properties. The present investigation is concerned with the performance of HVOF sprayed Ni-5Al coatings on Ni and Fe based superalloys in molten salt (40% Na₂SO₄- 60%V₂O₅) environment at 900°C under cyclic conditions. Thermogravimetric technique was used to establish the kinetics of corrosion. X-ray diffraction, scanning electron microscopy/energy dispersive analysis and electron probe microanalysis techniques were used to characterize the as sprayed and corroded products. The bare superalloys suffered accelerated corrosion under the given environmental conditions. It was found the corrosion resistance of Ni-5Al coated Ni based superalloys was better than that of Fe- based superalloys. The mechanisms of high temperature corrosion is discussed in terms of oxidation kinetics and interdiffusion between substrate and coatings in the present work.
2:50 PM	Invited	A2-5 High Temperature Chlorine Corrosion as a Result of Incineration of Contaminated Fuel - Reasons, Mechanisms and Solutions R. Warnecke, C. Deuerling (GKS-Gemeinschaftskraftwerk Schweinfurt GmbH, Germany); F. Haider, S. Horn (University of Augsburg, Germany); J. Maguhn (GSF, Germany); V. Mueller (GKS-Gemeinschaftskraftwerk Schweinfurt GmbH, Germany); H. Nordsieck (BIFA, Germany); B. Waldmann (University of Augsburg, Germany); R. Zimmermann (GSF, Germany) For the application of protection layers on structures, i.e. tubes, on the one hands side it should be known everything about the application itself and on the other hands side about the environment and its attack onto the structure. For a waste-to-energy plant there were made investigations of the chlorine transporting gas and aerosol phase. Measurements of the particle size and containing chemical elements over the passes of the boiler quantifies the sulfutation along the way of the flue gas. Especially both the depositing of the particles and the reactions in the deposits of the tubes can be quantified now. Furthermore the electrochemical current of the corrosion is measured and interpreted by the process parameters. All these informations will lead to a better understanding of the environmental attack and at least to optimise the protecting layers.
3:50 PM		A2-8 Coatings for Corrosion Protection of Gasifier Components J. Perez-Mariano, K.H. Lau, E. Alvarez, R. Malhotra, G. Krishnan, A. Sanjurjo (SRI International, Menlo Park) Heat-exchangers, particle filters, turbines, and other components in integrated coal gasification combined cycle system must withstand the highly sulfiding conditions of the high-temperature coal gas over an extended period of time. The performance of components degrades significantly with time unless expensive high alloy materials are used. Deposition of a suitable coating on a low-cost alloy may improve its resistance to such sulfidation attack, and decrease capital and operating costs. In this study, deposition of several coatings on ferrous substrates by Chemical Vapor Deposition in a Fluidized Bed was studied. These coatings included Ti- and Al-based metal diffusion coatings, TiN-based composite ceramic coatings and ceramic/metal multilayer coatings. Coated substrates were exposed to simulated coal gas at temperatures in the range of 300°C to 900°C, in order to study their corrosion resistance under conditions that mimic different parts of a coal gasification system. Corrosion mechanisms observed in some of the samples were studied, with special emphasis in the formation of surface sulfides after migration of steel elements to the sample surface. Protective coatings were developed with potential use in some components of advanced coal gasification systems.
4:10 PM	Invited	A2-10 FeAl-Coatings as Corrosion Protection in Waste and Biomass Fired Electric Stations M. Spiegel (Max Planck Institute for Iron Research, Germany); R. Warnecke (GKS-Gemeinschaftskraftwerk Schweinfurt GmbH, Germany) Iron aluminides are light weighed and cheap materials, providing sufficient high temperature oxidation strength due to the formation of an alumina scale. In comparison to expensive nickel-based alloys used nowadays for service as heat exchangers in thermal power plants, this alloys could show a reasonable compromise between reduced costs and service life. Oxidation experiments in air showed that alloys Fe-(15-40)Al exhibit the formation of an alumina scale even at 700°C, necessary for corrosion protection. Alloying third elements like Ti, Nb and Ta in order to achieve a sufficient high temperature creep strength does not influence oxidation significantly i.e. an alumina scale is formed. Further experiments were conducted on the corrosion properties in HCl containing gases as well as beneath chloride melts, showing satisfactory corrosion rates. Based on this results it is intended to prepare iron aluminium alloys as thermal spray coatings for in-plant tests.
4:50 PM		A2-12 Comparison of Phase Transformations in Cryomilled and Unmilled Ni-343 Powder Investigated by Differential Thermal Analysis F. Tang, K. Ma, L. Ajdelsztajn, A. Maich, J. Schoenung (University of California) Ni-343 (Ni-22Cr-10Al-1Y (wt.%)) powder is commonly used to deposit the bond coat layer in thermal barrier coatings (TBCs). In previous work, we achieved a three-fold increase in TBC thermal cycle lifetime by cryomilling the Ni-343 powder prior to deposition of the bond coat. In the present study, we further compare the unmilled and cryomilled Ni-343 powder by investigating the phase transformation in these powders by means of differential thermal analysis (DTA). For the unmilled Ni-343 powder, an endothermic peak was observed at approximately 100°C during heating at a heating rate of 10°C/min. During cooling, an exothermic peak was observed at approximately 94°C, indicating that this phase transformation is reversible. For the cryomilled Ni-343 powder, both the endothermic and exothermic peaks appeared at temperatures approximately 4°C lower than those for the unmilled powder. This reversible phase transformation causes kinks in the thermal expansion/temperature curves when a bulk Ni-343 material is heated up and cooled down. Therefore, the lowering in the phase transformation temperature can change the thermal-mechanical behavior of the cryomilled Ni-343 bond coat, and may thus affect the thermal cycle lifetime in the TBCs. The phase constitution before and after the phase transformation was investigated through X-ray diffraction (XRD) and microscopy. The activation energy for the phase transformation was estimated by measuring the shift in the DTA peak temperature at various heating and cooling rates.