Corrosion And Coatings For Protection
Initial superalloys were intended to have both strength and adequate oxidation resistance, and this was accomplished with superalloys which contained upward of 20 wt % chromium. Oxidation resistance up to temperatures around 982∘C (1800∘F) was excellent. However, as suggested earlier, in order to increase the design flexibility of nickel based superalloys, chromium con-tent was reduced so that more hardener could be added. Concurrently, some superalloys were put into service in environments (e.g., marine salts) that intensified oxidation or ion-induced attack such as hot corrosion. Also, the operating temperatures (surface environment tempera tures) to which the alloys are exposed in the most demanding high-temperature conditions have increased with the strength capability increases of the available alloys.
At higher temperatures, the chromium oxide formed during prior heat treatment is less protective and does not regenerate with exposure to high temperatures. General oxidation and intergranular oxidation along the grain boundaries of superalloys are a problem with chromium-protected superalloys, However, the problem is not as great as initially anticipated owing to the protective nature of aluminum oxide formed in greater amounts by the higher aluminum values of second- and third-generation gamma-prime-hardened superalloys. Neverthe less, general oxidation still occurs and causes reduced cross sections, thus effectively increasing stresses on the remaining material. Grain boundary oxidation creates notches. The combination of these events is of concern, and to protect against them, coatings are applied to superalloys.
The early coatings were diffusion coatings produced by pack aluminizing or slurry application. Chemistry of the coating was determined by the chemistry of the alloy. Later coatings were produced by overlaying a specific chemistry of a protective nature on the surface of the component using physical vapor deposition. Diffusion-type coatings can coat internal (non-line-of-sight) surfaces while the overlay coatings can only coat external surfaces that can be seen by the coating apparatus. Diffusion coatings are cheaper and, for a given thickness, probably nearly as protective as overlay coatings. Diffusion coatings are used to coat internal cooling passages in hot-section airfoils. Some commercial diffusion coating processes are available, but most overlay coating processes are proprietary, having been developed by superalloy users such as the aircraft gas turbine manufacturers. However, overlay coatings can be made nearly twice as thick as the diffusion coatings so overlay coatings often are the coatings of choice for turbine airfoil applications. Overlay coatings also have the advantage of the ability to alloy the coating with various elements that can enhance oxidation behavior. The added thickness of overlay coatings lends itself to better oxidation performance but reduces the component thermal fatigue life.
Overlay coatings are generally more expensive than diffusion coatings. Some diffusion coatings are deposited in conjunction with precious metals such as gold or platinum. There are significant benefits to this incorporation of the noble metals in the coating if the application can justify the increased cost. Overlay coatings can be applied by various processes such as electron beam physical vapor deposition (EBPVD) and plasma spray (PS). In EBPVD, a container of alloy powder or an alloy ingot is vaporized by an electron beam while the part to be coated is suspended above the vapor source. The thickness of coating is determined by the time of part exposure and the temperature of the vessel. In PS, alloy powder is transformed into high-temperature plasma by an electrical arc and propelled toward the component to be coated by a carrier gas. This PS process uses less material than EBPVD. These processes can also be used to apply thermal barrier ceramic coatings which can act as a heat shield onto metallic overlay coatings. Owing to the physics of the deposition processes, the chemistry of the coating ingot or powder may differ from the final deposited coating chemistry.
Coating and corrosion technology are complex and do not lend themselves to a simple overall description and formula for protection. Hot-corrosion phenomena are found below a nominal temperature of 927∘C (1700∘F). Coatings and higher chromium content in an alloy.
A Guide to Engineering Selection of Superalloys for Design
inhibit surface attack. Coatings, in general, preserve the surface so that a component may be reused by removing and then restoring the coating.
Coating selection is based on knowledge of oxidation/corrosion behavior in laboratory, pilot plant, and field tests. Attributes that are required for successful coating selection include:
• High resistance to oxidation and/or hot corrosion
• Ductility sufficient to provide adequate resistance to TMF
• Compatibility with the base alloys
• Low rate of interdiffusion with the base alloy
• Ease of application and low cost relative to improvement in component life
• Ability to be stripped and reapplied without significant reduction of base-metal dimensions or degradation of base-metal properties.
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