ICMCTF2015 Session A1-2: Coatings to Resist High Temperature Oxidation, Corrosion and Fouling

Detrimental effect of molten corrosive salts on life of thermal barrier coating (TBC) systems are widely known. Corrosive species such as vanadium oxide and sodium sulphate leach yttria from YSZ and form t – YVO₄. Leaching of yttria causes conversion of metastable t’ – ZrO₂ to m – ZrO₂ , resulting in volume change which will damage the top coat thereby the whole TBC system. Formation of t- YVO₄ induces growth stresses in the coating which also comprimises the mechanical integrity of the top coat. Yttria leaching and YVO₄ formation, which happen simultaneously, depend on the concentration of the corrosive salt, exposure time and temperature. The extent of the top coat damage by these corrsive salts also depend on the thickness and the density of the coating. Understanding the influence of these factors is important for development of more durable coatings. In the present work, a thick TBC coating, 750µm, and a standard TBC coating, 300µm, were subjected to hot corrosion tests involving vanadium pentoxide and sodium sulphate as the corrosive salt. The tests were conducted at 750^oC, 900^oC and 1000^oC where at each temperature the salt concentration was varied from 4 mg/cm², 10 mg/cm² to 20 mg/cm². Results show that the concentration of the salt has stronger influence on coating life than the time and temperature. Thick denser coatings seemed to show lower resistance to corrosion than thinner, denser coatings at a given temperature and concentration.

For large land-based turbines, directionally-solidified (DS) superalloy components with advanced thermal barrier coatings (TBC) to lower the metal operating temperature can replace more expensive single crystal superalloys. In order to assess relative TBC performance, furnace cyclic testing was used with superalloys 1483, X4 and Hf-rich DS 247 substrates and high velocity oxygen fuel (HVOF)-NiCoCrAlYHfSi bond coatings at 1100 °C with 1-h cycles in 10% H2O. There was no statistically significant effect of substrate alloy on the average lifetime of the air plasma sprayed (APS) yttria-stabilized zirconia (YSZ) top coatings for these conditions. The residual compressive stress in the α-Al2O3 scale underneath the YSZ top coating and on a bare bond coating was measured using photo-stimulated luminescence spectroscopy (PSLS) and was similar for all three substrates. Two-dimensional maps collected from the same region at regular cycling intervals revealed delaminations occurring at the YSZ/bond coating interface across all three coating systems at roughly the same rate and frequency. X-ray fluorescence (XRF) measurements collected from the bare bond coating surface revealed higher levels of Ti interdiffusion occurring between the 1483 substrate and the bond coating, which was expected because it contained the highest Ti content.

Research sponsored by the U. S. Department of Energy, Office of Fossil Energy, Coal and Power R&D.

Turbine efficiency can be improved by increasing the operating temperature, but also by reducing the gas path flow clearance to minimize the leakage and back flow of combustion gases. In general, air plasma sprayed (APS) abradable coating materials are used in the turbine hot section to reduce the stator-rotor gap. The layered structure of these seal coating materials assists with the desired removal of material in thin layers when the turbine blades sweep through the coating; the compliant sacrificial coating structure protects the mechanical integrity of turbine blade. To lower CO₂ emissions, the use of Integrated Gasification Combined Cycle (IGCC) technology is being explored, where the turbines are powered on high H₂ content (lower carbon) fuel that may affect the composition, microstructure, abradability and durability of the coatings at turbine operational temperatures.

The presence of high water vapor in the combustion chamber of the turbine may lead to accelerated degradation of the abradable coating materials, and an associated reduction in the life expectancy of turbine hot-section components. These coating materials are comprised of a metal or ceramic matrix, generally incorporate second phase designed to enhance machinability, and are sprayed with polymer pore formers to enhance the open porosity of the coatings (in part to enhance abradability). In this work Zirconia based composite materials with varying machinability phases and varied porosity have been used to study the thermal conductivity, chemical stability, mechanical stability and abradable characteristics of baseline abradable coating systems as a function of emerging IGCC combustion environments. Investigation of the mechanisms that control the removal of materials and performance of the abradables through the use of laboratory thermo-mechanical test methods will be discussed. Particular attention is placed on the application of a test intended to simulate the effect of the motion of turbine blades cutting into the abradable seals.

Metal dusting is a severe form of corrosion, which occurs in high carbon activity atmospheres at temperatures of 400-900°C (e.g. in reformer or coal gasification plants). Steels and nickel base alloys are strongly affected because of the catalytic effect of nickel and iron on the dissociation of carbon containing gas species, followed by carbon uptake and subsequent graphite nucleation inside the material. Conventional protection is given either by dense oxide scales on coatings or on alloys with high amounts of oxide scale forming elements.

Sulphur in the gas is known to suppress Metal Dusting by a catalytical surface modification. A similar catalytic surface effect was shown to be also effective on a new functional Ni-Sn coating, which was developed at the DECHEMA-Forschungsinstitut, DFI. A combination of an electrochemical nickel plating step with a subsequent tin powder pack cementation led to the formation of an intermetallic Ni-Sn layer. In this work this approach was further improved by modification with oxide forming aluminium. While the Ni-Sn coating is prone to oxidizing environments, the enrichment of the coating with oxide scale forming elements extends the application range for such coatings significantly. These novel coatings can provide sufficient protection e.g. even in steam containing Metal Dusting atmospheres by a dual protection: the classical oxide barrier (conventional approach) and the catalytic inhibition.

Al oxide/Cr thin films produced by reactive magnetron sputtering have proven to be promising coatings for protection of ferrous materials in carburizing atmospheres at high temperature up to 20h of exposure¹. The present work includes the results of a 304L coated and uncoated substrates after an extended exposure of 50h to a CH₄+H₂+residual O₂ atmosphere at 800°C. The results were compared with those of 20h to study the evolution of both the substrate and the film in such conditions. Even after 50h, the coated 304L steel remains largely unaffected by the carburizing atmosphere, however both its structure and that of the protective coating undergo some changes due to diffusion of the various components. The results from the uncoated exposed sample indicate that the small amount of oxygen in the carburizing atmosphere still plays an important after 50h of exposure.

1. E, Uribe et al., Surf. Eng. 2014, DOI: http://dx.doi.org/10.1179/1743294414Y.0000000322