High-temperature coatings such as MCrAlY, aluminide coatings, and thermal barrier coatings are used to protect metallic substrates from corrosion in various corrosive applications like gas turbines and incinerators. Recent efforts to reduce operational costs by using less expensive fuels in marine gas turbines may seriously impact the performance of current and anticipated high-temperature protective coatings. The increasing desire to raise temperatures in different high-temperature applications may require the development and implementation of alternative, more corrosion-resistant coatings.
The performance of high temperature coatings is dependent on: (1) the environment, (2) temperature, (3) quality control of the coating application, (4) coating structure and chemistry, (5) fuel contaminants, and (6) alloy substrate.
This paper will examine some of the factors that influence the interaction of high-temperature protective coatings with several corrosive species found in gas turbine and/or waste incinerator environments and offer some suggestions to improve the hot corrosion-resistance of protective, barrier coatings.
High temperature coatings provide a protective oxide barrier from corrosive or highly oxidative environments that may degrade the substrate alloy. To provide optimum performance coatings need to have good adherence to the substrate, ductility to resist thermal cycling, avoid defects that may provide easy pathways to deleterious gases, and maintain corrosion resistance over the design life of the high temperature component. There are three major classes of high-temperature coatings: (1) aluminides, (2) overlay coatings, and (3) thermal barrier coatings.
Aluminides
Aluminide coatings on transition metals and alloys such as iron, nickel and cobalt are â-FeAl, â- NiAl, and â-CoAl, respectively. Diffusion aluminide coatings are typically processed via pack cementation, slurry-diffusion or chemical vapor deposition. Subsequent heat treatments help develop the proper mechanical properties and trigger further diffusion within the coating. The principal protective oxide generated by the high-temperature oxidation of these aluminide intermetallics is Al2O3 which causes subsequent improved oxidation and corrosion resistance of aluminide coatings although less protective oxides can form if other alloying elements are present in the substrate, either in solution or as precipitated phases.
Modified aluminide coatings are altered by the addition of a secondary element like chromium, platinum, a reactive element, or a combination of these elements. These coatings are generated by either: (1) incorporating the secondary element into the coating by pack codeposition, or (2) deposition of the modifying element prior to an aluminization process by electroplating, electrophoresis sputtering, chemical vapor deposition, or pack cementation.
Chromium-modified aluminide coatings result in a microstructure that contains layers of chromium and NiAl. One commercial chrome-modified aluminide coating displayed a 3-zone structure after thermal treatment: an outer zone consisting of a fine dispersion of á-Cr in a hyperstoichiometric â-NiAl matrix, a middle zone containing stoichiometric â-NiAl, and an inner zone with á-Cr in a hypo-stoichiometric â-NiAl.
Platinum, a relatively inert metal, is added to improve the hot corrosion resistance of an aluminide coating. This effect is strongly influenced by processing procedures for the coatings. Platinum has shown significant benefits on the oxidation resistance of both low activity and high activity PtAl coatings. Platinum promotes the formation of thinner and purer Al2O3 scales during
