Field of the Invention
The invention relates to a method and a coating for protecting a chromium steel substrate of a component of a turbomachine against corrosive and erosive attacks at temperatures up to approximately 500.degree. C., wherein the protective coating contains aluminum.
More particularly, the invention relates to substrates on components for all types of turbomachines, especially turbocompressors, regardless of how they are driven, and to gas and steam turbines, with particular reference to components of turbomachines of the kind that are supposed to be operated at temperatures of up to approximately 500.degree. C. An especially important field to which the invention applies is the protection of compressor blades and other components which are stressed like them, in the turbocompressors of gas turbines.
Some possibilities for protecting a substrate of a component of a turbomachine against corrosive and erosive attacks at temperatures up to 450.degree. C. are disclosed in Patent European Application 0 379 699 A1 (corresp. to U.S. Pat. No. 5,120,613). In that disclosure, blades for turbine machines, which are predominantly made of ferritic and/or ferritic/martensitic basic materials, are provided with protective coatings of aluminum alloys, especially aluminum alloys containing from 6 to 15 weight % silicon. Such aluminum alloys are to be applied to the blades by a high-speed spraying process.
The phenomenon of vibration-induced corrosion cracking on coated compressor blades for turbomachines has been addressed in detail in the article by H. Hoffmann, W. Magin, M. Schemmer and F. Schmitz entitled "Schwingungsri.beta.korrosion beschichteter Verdichterschaufel-Werkstoffe" [Corrosion Fatigue in Coated Compressor Blade Materials], in Zeitschrift f ur Werkstofftechnik [Journal for Materials Science] 17 (1986) 413. The compressor blades mentioned in that article have protective coatings made of aluminum pigments dispersed in chromate/phosphate binders, on substrates of chromium steels. Protective coatings of nickel or nickel-cadmium alloys are also mentioned.
The problem of erosive attacks, to which compressor blades and the like are exposed, is discussed in detail in the article by K. G. Schmitt-Thomas, T. Happle and P. Steppe entitled "Untersuchung der Strahlverschlei.beta.best andigkeit von Werkstoffen und Beschichtungen mit Hilfe eines Wirbelbett-Testverfahrens" [Investigation into the Blasting Wear Resistance of Materials and Coatings Using Fluidized-Bed Process], in Werkstoffe und Korrosion [Materials and Corrosion] 41 (1990) 623. That article also addresses the interaction of erosion and corrosion in turbomachine blades, since abrasion in a protective coating occurring due to erosion finally lays bare the substrate of a blade which has a material that is typically essentially optimized only for mechanical properties, and does not have adequately good resistance to erosion and corrosion. The mechanisms of erosion, which depend especially on the angles at which eroding particles strike a component, are discussed at length and the dependency of the effect of the erosion on the type of material being exposed to the erosion is also addressed. Erosion and corrosion problems of compressor blades, especially compressor blades with inorganically bound aluminum pigment coatings, which may possibly be provided with inorganic or organic cover coatings, are described in detail.
The book entitled "Praxis der Kraftwerk-Chemie" [Power Plant Chemistry in Practice], published by Hans-G unther Heitmann, Vulkan-Verlag, Essen, 1986, and especially the article in it entitled "Gasturbinen-Anlagen" [Gas Turbine Systems] by F. Schmitz, pp. 574 ff., also provide important information on the problems of corrosive and erosive attacks in the compressors of gas turbine systems. Details on the erosive and corrosive attacks, and especially on vibration-induced corrosion cracking, and on the problems that occur when typical high-temperature-lacquer protective coatings are used, are also discussed. In that connection, corrosion phenomena, which begin at pores in the protective coatings and can cause damage to the basic materials beneath protective coatings that appear superficially to be more or less intact, should be mentioned.
The article entitled "Korrosionverhalten von anodisch oxidierten Aluminium-Werkstoffen" [Corrosive Behavior of Anodically Oxidized Aluminum Materials] by W. Paatsch, Metalloberfl ache [Metal Surface] 45 (1991) 8, provides information on corrosion phenomena in aluminum surfaces that have been anodically oxidized. Anodic oxidation of aluminum is known in many fields in the industry, although not in connection with turbomachines, for forming sturdy, decorative surfaces. That article is silent on the problems of erosion and load-bearing ability of an aluminum surface at elevated temperature.