EP0663964B1 - Protection of chromium-steel substrates against corrosive and erosive attack at temperatures up to about 500 degrees celsius - Google Patents

Abstract

The invention concerns a method of protecting chromium-steel substrates against corrosion and erosion at temperatures up to about 500 °C. A protective layer containing aluminium is formed on the substrate. The invention calls for this to be done by first depositing a metallic layer containing aluminium and then hardening or annealing at least the surface of the protective layer. The invention enables highly effective protection to be provided, using simple techniques, against corrosion and erosion, particularly for turbine blades, and in particular turbocompressor blades.

Description

Protection against corrosive and erosive attacks at temperatures up to about 500 ° C for a substrate made of chrome steel.

The invention relates to protection against corrosive and erosive attacks at temperatures up to approximately 500 ° C. for a substrate of a component of a turbomachine consisting of chromium steel by means of a protective layer which contains aluminum.

The invention relates to substrates on components for all types of turbomachinery, in particular turbocompressors regardless of the type of their drive, and to gas and steam turbines, particular reference being made to components of such turbomachinery which are to be operated at temperatures of up to approximately 500.degree . A particularly important field of application of the invention is the protection of compressor blades and other components loaded in this way in the turbocompressors of gas turbines.

EP 0 379 699 A1 provides options for protecting a substrate on a component of a turbomachine against corrosive and erosive attacks at temperatures up to 450 ° C. According to this document, blades for turbomachinery, which mainly consist of ferritic and / or ferritic-martensitic base materials, are provided with protective layers made of aluminum alloys, in particular of aluminum alloys with 6 to 15% by weight of silicon. Such aluminum alloys are to be applied to the blades using a high-speed spraying process.

The phenomenon of vibration crack corrosion on coated compressor blades for turbomachinery is in the article "Vibration crack corrosion of coated compressor blade materials" by H. Hoffmann, W. Magin, M. Schemmer and F. Schmitz, Zeitschrift für Werkstofftechnik 17 (1986) 413, explained in detail. The compressor blades mentioned in this article have protective layers made of aluminum pigments dispersed in chromate / phosphate binders on substrates made of chromium steel. Protective layers made of nickel or nickel-cadmium alloys are also mentioned.

The problem of erosive attacks, to which compressor blades and the like are exposed, is explained in detail in the article "Investigation of the jet wear resistance of materials and coatings with the aid of a fluidized bed test method" by KG Schmitt-Thomas, T. Happle and P. Steppe, Materials and Korrosion 41 (1990) 623. This article also deals with the interaction of erosion and corrosion on blades for turbomachinery, since erosion of a protective layer finally exposes the substrate of a blade, the material of which is usually essentially optimized only for mechanical properties and none has sufficient resistance to erosion and corrosion. The mechanisms of erosion, which depend in particular on the angles at which eroding particles strike a component, are explained in detail; the dependence of the effect of erosion on the type of material exposed to erosion is also explained. Erosion and corrosion problems of compressor blades, in particular of compressor blades with inorganically bound aluminum pigment coatings, which are possibly provided with inorganic or organic cover layers, are described in detail.

The book "Praxis der Kraftwerk-Chemie", published by Hans-Günter Heitmann, Vulkan-Verlag, Essen, 1986, in particular the article "Gas Turbine Systems" by F. Schmitz, pages 574 ff., Also contains important information on the problem of corrosive and erosive attacks in the compressors of gas turbine systems. Details on the erosive and corrosive attacks, in particular on vibration crack corrosion, and on the problems that occur when using conventional high-temperature lacquer protective layers are also explained. In this regard, reference should be made to corrosion phenomena that can result from pores in the protective layers and damage to the base materials under protective layers that appear more or less intact from the outside.

The article "Corrosion Behavior of Anodically Oxidized Aluminum Materials" by W. Paatsch, Metallfläche 45 (1991) 8, provides information on corrosion phenomena on aluminum surfaces that have been anodically oxidized. The anodic oxidation of aluminum is known in many areas of technology, but not in connection with turbomachinery, to form robust, decorative surfaces. The article is silent on the problem of erosion and the resilience of an aluminum surface at elevated temperatures.

In view of the problems of the protective layers hitherto provided for the formation of protection against corrosive and erosive attacks at temperatures of up to approximately 500 ° C. for a component of a turbomachine, the object of the invention is to provide a substantially improved protection for a substrate of a component of a turbomachine consisting of chromium steel to achieve, the cost of achieving the protection being kept low, possibly even reduced, shall be.

The method according to the invention for achieving protection against a corrosive and / or erosive attack at a temperature of up to about 500 ° C. for a substrate of a component of a turbomachine consisting of chromium steel, wherein a protective layer is formed on the substrate which contains aluminum that an aluminum-containing metal layer is applied to the substrate and hardened or cured to form the protective layer at least on its surface.

The invention is based on the knowledge that the hardenability or hardenability of the aluminum itself or of the aluminum base materials can advantageously be used to form a protection of the type mentioned. The metal layer containing aluminum can be hardened, for example, chemically, in particular by oxidation, or mechanically, in particular by rolling. Curing is understood to mean, for example, a structural change in the metal layer caused by heat treatment, in particular precipitation hardening. The hardening or hardening need not necessarily cover the entire metal layer; it can be advantageous to restrict the hardening or hardening to a part near the surface and thus to obtain a so-called "duplex layer". The hard layer formed according to the invention advantageously has a Vickers hardness HV 0.025 of more than about 200, considerably more than HV 0.025 of a conventional high-temperature lacquer layer, where HV 0.025 is usually at most 120.

The metal layer to be applied to the substrate to be protected advantageously consists mainly of aluminum and is accordingly in particular an aluminum-based alloy, for example with the addition of at least one of the elements magnesium, copper and zinc. Silicon, manganese and titanium can also be used as additives.

The hardening or hardening of the metal layer takes place with particular advantage in such a way that the metal layer is converted into a hard layer at least on its surface. As already indicated, the hard layer can be produced by numerous different methods that may be combined with one another, in particular mechanical strengthening, chemical or thermal treatment. It is particularly favorable if part of the metal layer remains under the hard layer, so that the protective layer is a duplex layer which comprises the metal layer and the hard layer. In view of the erosion attack depending on the orientation of the attacked areas of the substrate to the direction of flight of eroding particles, a duplex layer, which comprises a rather hard layer on the one hand and a rather ductile metal layer on the other hand, is particularly favorable since hard layers and ductile layers each have different types Resist erosion: hard layers are suitable as protection against an erosion attack by particles that strike grazing to approximately oblique, ductile metal layers are advantageous for protection against erosion by particles hitting at large angles, in particular obliquely to approximately vertically. The duplex layer can therefore provide protection against eroding particles regardless of their angle of incidence, although removal of the hard layer must initially be expected in areas of the component where the particles meet approximately vertically until which is exposed to erosion resistant to large impact angles, ductile metal layer.

In any case, it is particularly favorable to form a hard layer by at least partially oxidizing the metal layer; the oxidizing is preferably an anodizing, in particular anodizing. Following anodic oxidation, the hard layer obtained can be additionally compacted by treating it with boiling water or a boiling, aqueous salt solution. Details of this are known in the field of anodic oxidation of aluminum and do not require any further explanation here. Any oxidation of an aluminum-containing layer produces a surface layer which contains aluminum oxide or corundum, one of the hardest minerals, as an essential component. In order to achieve a particularly thick, dense and hard layer, anodic oxidation is particularly suitable. It should be noted that not only layers of essentially pure aluminum can be used for anodic oxidation, but in particular also layers of aluminum-magnesium alloys. In particular, aluminum-based alloys with the addition of magnesium in a weight fraction between 0.5% and 5%, in particular between 1% and 4%, possibly with further small proportions of silicon, iron, copper, chromium, zinc and / or titanium in the usual range, suitable.

An alternative method of forming a hard layer on a metal layer is to use a hardenable alloy to form the metal layer followed by hardening. The hardening can be limited to a region of the metal layer near the surface be achieved by curing, for example, by irradiation with laser light; it can also capture the entire metal layer, for which the component provided with the metal layer can be heat-treated in a conventional manner in an oven. An aluminum-based alloy with additions of magnesium and copper or zinc is particularly suitable as the hardenable alloy. Advantageously, an aluminum-based alloy is used with magnesium parts by weight between 0.4 and 2% and copper between 3.5 and 5%, with usual impurities and possibly further admixtures, as mentioned above. Also possible is an aluminum-based alloy with a weight proportion of zinc between 1% and 5%, in particular between 4% and 5%, and magnesium up to 2%, in particular between 1% and 1.5%, also with usual impurities and possible further admixtures.

In general, it is advantageous to apply a metal layer with a thickness of between 15 μm and 200 μm, preferably between 40 μm and 100 μm, to form protection against corrosive and erosive attacks at temperatures up to about 500 ° C.

The metal layer is applied with particular advantage electrochemically, in particular by electroplating, in any configuration of the method. Electroplating produces a particularly uniform and dense layer with an extremely low porosity, which accordingly suppresses the occurrence of pitting corrosion. Pitting corrosion occurs when an electrically conductive liquid, for example a drop of water with salt or ash content, enters the pore of the protective layer and with the protective layer and the like Substrate forms a galvanic element. The decomposition processes occurring in such an element can, starting from the pore, spread into the boundary layer between the protective layer and the substrate and destroy the substrate under the externally intact protective layer. For this reason, the electrochemical application of the metal layer is particularly preferred since it avoids pores.

It is particularly advantageous to apply the protective layer of any configuration directly to the substrate, that is to say without inserting any intermediate layers. As a result, the effort involved in achieving protection is kept to a minimum.

According to the invention, a protective layer is also specified on a substrate of a component of a turbomachine made of chromium steel, which protective layer offers protection against corrosive and erosive attacks at temperatures up to approximately 500 ° C. and by at least superficial hardening or hardening of a metal layer containing aluminum applied to the substrate was formed by the inventive method.

The invention accordingly also relates to a substrate which is provided with a protective layer according to the invention as protection against a corrosive and / or erosive attack at a temperature up to about 500 ° C. Such a substrate can in particular belong to a blade of a turbomachine such as a turbocompressor, be it a rotor blade or a guide blade. The blade can have a foot part for fastening the component and a blade part, which is the effective part in the thermodynamic process in the turbomachine, and at least one of which is a gas, in particular air, gas turbine exhaust gas or steam, exposed sheet part has a substrate protected according to the invention.

• 0,1 bis 0,3 % Kohlenstoff
• 11 bis 17 % Chrom
• 0 bis 6 % Nickel
• 0 bis 1,5 % Molybdän
• 0 bis 1 % Vanadium
• 0 bis 1 % Silizium
• 0 bis 1 % Mangan

Rest Eisen mit herstellungsbedingten Verunreinigungen.The substrate preferably consists of a chromium steel with the following proportions, the proportions being given in percentages by weight:

• 0.1 to 0.3% carbon
• 11 to 17% chromium
• 0 to 6% nickel
• 0 to 1.5% molybdenum
• 0 to 1% vanadium
• 0 to 1% silicon
• 0 to 1% manganese

Remainder iron with production-related impurities.

The substrate protected according to the invention preferably has, at least in part, a ferritic or martensitic structure.

Examples of chromium steels which are suitable for substrates to be protected according to the invention are the chromium steels X20 Cr 13, X20 CrMoV 12 1, X20 CrNiMo 15 5 1, X12 CrNiMo 12. The chromium steel X20 Cr 13 is regarded as particularly preferred.

The invention relates to the attainment of protection for a substrate, in particular a substrate on a turbine or compressor blade of a turbomachine, against one Corrosive and / or erosive attack at a temperature up to about 500 ° C. A protective layer is formed on the substrate, which contains aluminum. According to the invention, a metal layer containing aluminum is first applied and hardened or cured at least on its surface to form the protective layer. Within the scope of the invention, highly effective protection against corrosion and erosion can be obtained with simple means.

Claims (18)

1. Process for achieving protection against a corrosive and/or erosive attack at a temperature up to about 500°C for a substrate, comprising chrome steel, of a component of a turbo machine, a protective coating, which contains aluminium, being formed on the substrate, characterized in that an aluminium-containing metal coating is applied to the substrate and, to form the protective coating, is hardened or age-hardened at least at its surface.
2. Process according to Claim 1, in which a metal coating is applied which principally comprises aluminium, is preferably an aluminium-based alloy, in particular having an addition of at least one of the elements magnesium, copper and zinc.
3. Process according to Claim 1 or 2, in which, to form the protective coating, the metal coating is converted at least at its surface into a hard layer.
4. Process according to Claim 3, in which the metal coating is converted at least in places essentially completely into the hard layer.
5. Process according to Claim 3 or 4, in which at least in places a part of the metal coating remains beneath a hard layer.
6. Process according to Claim 3, 4 or 5, in which the hard layer is formed by oxidizing the metal coating.
7. Process according to Claim 6, in which the oxidation is an anodic oxidation, in particular anodization.
8. Process according to Claim 7, in which the hard layer is sealed, preferably by treatment with boiling water or boiling aqueous salt solutions.
9. Process according to one of Claims 3 to 5, in which the metal coating is formed from an age-hardenable alloy and is age-hardened to form the hard layer.
10. Process according to Claim 9, in which the alloy is an aluminium-based alloy, preferably having additions of magnesium and copper or zinc.
11. Process according to one of the preceding claims, in which a metal coating is formed having a thickness which is between 15 µm and 200 mm, preferably between 40 µm and 100 µm.
12. Process according to one of the preceding claims, in which the application of the metal coating is carried out electrochemically, preferably by galvanizing.
13. Process according to one of the preceding claims, in which the protective coating is applied directly to the substrate.
14. Protective coating on a substrate, comprising chrome steel, of a component of a turbo machine, which offers protection against a corrosive and/or erosive attack at a temperature up to about 500°C, and contains aluminium, characterized by a metal coating which is applied to the substrate, contains aluminium and is hardened or age-hardened at its surface.
15. Protective coating on a substrate according to Claim 14, which belongs to a blade, preferably a rotor blade or a guide vane, for a turbo machine, preferably a turbo compressor.
16. Protective coating on a substrate according to Claim 15, the blade having a root part and an aerodynamic vane part and the substrate belonging to the aerodynamic vane part.
17. Protective coating on a substrate according to one of Claims 14 to 16, which comprises a chrome steel having the following proportions (values in percentages by weight):

    0.1 to 0.3% carbon

    11 to 17% chromium

    0 to 6% nickel

    0 to 1.5% molybdenum

    0 to 1% vanadium

    0 to 1% silicon

    0 to 1% manganese

    the remainder being iron with production-related impurities.
18. Protective coating on a substrate according to one of Claims 14 to 17, which has at least in part a ferritic or martensitic structure.