WO1989007159A1

WO1989007159A1 - Metallic object, in particular gas turbine blade with protective coating

Info

Publication number: WO1989007159A1
Application number: PCT/DE1989/000023
Authority: WO
Inventors: Friedhelm Schmitz; Norbert Czech; Bruno Deblon
Original assignee: Siemens Aktiengesellschaft
Priority date: 1988-02-05
Filing date: 1989-01-19
Publication date: 1989-08-10
Also published as: IN171444B; EP0397731A1; JPH03503184A; DE58904084D1; EP0397731B1

Abstract

The present invention concerns multi layer protective coatings for metallic objects (1), in particular gas turbine blades. Given the existence of two different corrosion mecanisms which determine the service life of such objects, two superimposed protective layers are applied. The inner layer (2) protects against attacks by corrosion at temperatures between 600°C and 800°C (HCTII), and the outer layer (3) affords optimal protection againt corrosion at temperatures between 800°C and 900°C (HTCI). In addition, an outermost coating layer forming a thermal barrier (6) can be provided. The first coating layer (2) is preferably a diffusion layer with a chromium content greater than 50 % and an iron and/or manganese content greater than 10 %, and the second coating layer (3) is preferably a MCrAlY layer, containing for example approximately 75 % chromium, approximately 7 % aluminium and approximately 0,7 % yttrium, applied by low-pressure plasma spraying.

Description

Metal object, in particular gas turbine blade with protective coating

The present invention relates to a metal object, in particular a component of a gas turbine plant, for. B. A blade, according to the preamble of claim 1. Many components which are exposed to hot gas, in particular in gas turbines, are subject to thermal, mechanical and erosive stresses and are also to a large extent corrosive influences. Coverings that form from salts, the origin of which can be attributed to fuel and air pollution, together with some gaseous substances lead to corrosive damage by high-temperature corrosion (HTK). The corrosion exciters can be very heterogeneous. On the one hand, the type and origin of the fuels and, on the other hand, the composition of the combustion air determine the different forms of attack that come about through different chemical mechanisms. In the case of fuels, varying sulfur contents in natural gases and petroleum, vanadium fractions in heavy oil, heavy metals in blast furnace gas and heavy metals and chlorides in coal gas can have an influence. The liquid and solid aerosols contained in it play a decisive role in the composition of the combustion air, depending on the location of the plant, the combustion air may contain heavy metals, alkalis and / or chlorides.

Various coating layers, including multiple coatings for components exposed to hot gas, are known from the literature in large numbers for different purposes. In particular, it is known from DE-C-28 26 910 to provide metal objects with a graduated coating. The innermost layer is a diffusion layer mainly containing chromium. The graded coating described is generally intended to protect the metal object against heat corrosion, in which case corrosion tests at temperatures of about 925 ° C. are described.

DE-C-28 26 909 discloses a further double layer for metal objects subjected to such loads, an inner partial layer containing portions of the elements aluminum, chromium and yttrium. US Pat. No. 3,649,225 also describes double layers which are intended to prevent high-temperature corrosion. In most known double layers, the lower, generally thin layer does not itself serve as protection against external attack, but only improves the durability and adhesion of the upper layer.

The known layer systems protect a component against oxidation and corrosion at very high temperatures, but intensive studies have shown that the known layers do not simultaneously protect against a different type of corrosion attack at temperatures between 600 "and 800" C. As can be seen from FIG. 1 of the drawing, there are 2 different types of attack for high-temperature corrosion after investigations which have become known.

1 shows that in addition to the high-temperature corrosion already mentioned in the range of approximately 850 ° C. (hereinafter referred to as HTKI), whereas the known protective layers are designed, there is another strong corrosion mechanism which has a maximum in the range of approximately 700 ^* C. Fig. 1 shows a diagram in which the rate of corrosion versus temperature is plotted.

In certain modes of operation of gas turbine systems, in particular in cases in which the turbine operates in the partial load range over longer periods of time, the corrosion mechanism at 700 ^* C (hereinafter referred to as HTKII) now plays a decisive role in the life of components ^* . It has been shown that this type of corrosion during part-load operation gradually the protective layers suitable against attacks at higher temperatures are destroyed, so that subsequently the components are exposed unprotected to the remaining attack mechanisms under full load operation and again at higher temperatures.

The object of the present invention is therefore to create a combination of protective layers which makes a metal object resistant to both known attack techniques eπ, HTKI and HTKII and thus increases the service life of the component.

To achieve this object, a metal object with the coatings according to the characterizing features of claim 1 is proposed. Then the component is first provided with a layer which, owing to its thickness or composition, has great resistance to attack mechanisms at temperatures of 600 ° to 800 ° C. Furthermore, a second coating layer is applied, which is particularly resistant to attacks at 800 "to 900" C. This structure is based on the knowledge that components which are exposed to hot gas are generally cooled internally, so that there is a temperature gradient from the outermost layer to the interior of the component inner layer against the attack mechanism designed at lower temperatures, while the outer layer should protect against corrosion at high temperatures.

It should also be noted that a component does not in principle have to be completely provided with both layers, provided that the temperature load of individual areas is different. The invention should therefore of course also include the double coating only in partial areas of the metal objects. However, the proposed arrangement of the layers has the advantage that the service life of a component is increased in any case, even if the average prevailing attack mechanism is different at different locations on the component and is not necessarily known. If, for example, a special one well-cooled part of the component is mainly in the temperature range around 700 ° C even during full load operation, so the outermost protective layer, which is not optimized for this type of attack, is gradually destroyed, but subsequently protects the layer underneath.

2 shows an example of the effects of the double layer on the operating time in a diagram. In this diagram, the consumption is plotted against the operating time and typical consumption curves for different temperature stresses of different parts of a component are shown.

3 shows the effect of a thermal barrier layer over a corrosion protection layer in an internally cooled component. The diagram shows two typical temperature profiles inside and outside the component and the protective layers.

Advantageous embodiments of the invention are proposed in claims 2 to 10.

According to claim 2, a diffusion layer applied to the metal object with a chromium content greater than 50% is suitable as the first coating layer. Such diffusion layers are known per se according to the prior art, in particular from DE-C-28 26 910. However, their beneficial effect compared to HTKII when combined with a second coating layer against HTKI was not recognized. An additional amount of iron or manganese, e.g. B. 10 to 30% (all the following data are percentages by weight), the thickness of such a diffusion layer can be increased to over 0.130 mm, the possible layer thickness also increasing with increasing proportion of iron or manganese, which of course reduces the service life HTKII conditions increased.

Instead of a first coating layer as a diffusion layer, a support layer can also be provided. which can be applied, for example, by low-pressure plasma spraying. This layer should contain 30 to 55%, preferably about 40%, chromium and 0.5 to 2%, preferably about 1%, of at least one of the elements from the group of rare earths, yttrium, scandium, hafnium, zirconium, Niobium, tantalum, silicon. If at all, aluminum should only be present in small amounts, namely less than 5%, preferably even less than 3. The rest of the layer consists of one or a mixture of the elements iron, cobalt, nickel, in which case production-related impurities can be added.

If one does not use cobalt, but only one or a mixture of the elements made of iron and nickel for the first coating layer, the chromium content can be chosen lower, namely between 15 and 50%, to achieve an equally good effect. , preferably between about 20 and 30%.

Claim 5 teaches that the second coating layer should belong to the MCrAlY type. Such layers are also known per se according to the prior art, e.g. B. again from DE-C 28 26 910. The knowledge, however, not only to optimize such a coating layer against HTKI, but also to use it in combination with an underlying layer, which is optimized against HTKII, is not to be taken from the prior art remove. However, this combination in particular results in a particularly long service life of the metal objects at different temperatures. According to the invention, the second coating layer should have the following composition: 15 to 40% chromium, preferably approximately 20 to 30%; 3 to 15% aluminum, preferably about 7 to 12%; 0.2 to 3% of at least one element from the group of rare earths, yttrium, tantalum, hafnium, scandium, zirconium, niobium, rhenium, silicon, preferably about 0.7%, rest of at least one of the elements made of cobalt, nickel, and manufacturing-related impurities. According to claim 6, it is proposed that the second coating layer should be applied by plasma spraying, in particular low-pressure plasma spraying. In principle, various coating methods come into consideration, as have already been described in DE-C-28 26 910, but low-pressure plasma spraying permits the application of particularly adhesive and oxide-free layers of greater layer thickness. Accordingly, it may be that the outer coating layer has a greater layer thickness than the inner one.

In contrast to the prior art, in which all different coating layers are to be connected by diffusion to the metal object and to one another, it can be important for the layers optimized according to the invention and their durability that diffusion processes between the layers by a Diffusion barrier layer can be prevented. This is the subject of claim 7. In the case of layers which have been optimized very precisely for certain conditions, it is undesirable for the concentrations of individual constituents, such as, for example, chromium or aluminum, to be equalized by diffusion, since the specific properties of the individual layers can be lost in the process. Here, a diffusion barrier layer can significantly increase the service life. Such a layer can consist, for example, of titanium nitride or titanium carbide.

In the case of internally cooled metal objects in particular, one possibility of protection against particularly high temperatures is to prevent the temperatures from reaching the metallic layers. This can be achieved by thermal barrier layers on the outside of the metal object. These layers have the effect that the underlying metallic layers only have temperatures against which they are designed. In order to prevent the thermal barrier layer from flaking off, it can be advantageous according to claim 9 to oxidize the surface of the second coating layer before the thermal barrier layer is applied. As a result of the coating of a component according to the invention, a total coating thickness of over 0.3 mm can be achieved.

An embodiment of the invention is shown schematically in FIG. 4. A component 1 has a first metallic coating layer 2, which is optimized against HTKII or is resistant due to its thickness. Above it is a second coating layer 3, which is resistant to HTKI. If necessary, there may be diffusion barrier layers 4, 5 between the base material 1 and the first coating layer 2 and / or between the first coating layer 2 and the second coating layer 3, which hinder the concentration compensation of individual elements by diffusion. Finally, a thermal barrier layer 6, which protects against particularly high temperatures, can also be attached on the very outside.

The multiple coatings according to the invention are particularly suitable for blades and parts of gas turbines in which individual, e.g. B. cooled components, locally different temperatures occur, such as in systems that are operated at times at full load and sometimes only at partial load. According to the invention, thick multiple protective layers can be applied to such components, which usually consist of nickel-based alloys, which considerably extend the service life of the components, in particular of the blades. In special cases, the coatings described can, as far as this is economically justifiable, be supplemented by further similar or different coatings.

Claims

1. Metal object, in particular a gas turbine blade, made of a base material (1) based on nickel, the base material being provided with at least two superimposed coating layers (2, 3) to improve the high-temperature corrosion resistance, the following features:

a) a first of the coating layers (2) has a thickness and / or a composition which provides effective protection against corrosion at temperatures of 600 ° to 800 ° C. (HTKII);

b) over this first coating layer (2) is a second coating layer (3) which is optimized with regard to corrosion resistance at temperatures of 800 "to 900" C (HTKI).

2. Metal object according to claim 1, dadurchge¬ indicates that the first coating layer (2) is an applied to the base material diffusion layer with a thickness greater than 0.130 mm, which consists mainly of chromium and additionally at least 10% (weight percent) of at least one of the Elements made of iron, containing manganese, preferably about 20 to 30% iron.

3. Metal object according to claim 1, so that the first coating layer (2) is a support layer, which is preferably applied by low-pressure plasma spraying, with the following composition (percentages by weight):

Chromium 30 to 55%, preferably about 40%;

Aluminum less than 5%, preferably less than 3%;

0.5 to 2%, preferably about 1%, of at least one rare earth element, yttrium, scandium, hafnium,

Zirconium, niobium, tantalum, silicon;

Balance of at least one of the elements made of iron, cobalt, nickel, and manufacturing-related impurities.

4. Metal object according to claim 1, so that the first coating layer (2) is an application layer with the following composition (weight percent):

Chromium 15 to 50%, preferably about 20 to 30%;

Aluminum less than 5%, preferably less than 3%;

0.5 to 2%, preferably approximately 1%, of at least one element from the group of rare earths, yttrium, scandium, hafnium,

Zirconium, niobium, tantalum, silicon;

Remainder of at least one of the elements made of iron, nickel, as well as production-related impurities.

5. Metal object according to one of the preceding claims, that the second coating layer (3) is a support layer with the following composition (percentages by weight):

15 to 40% chromium, preferably about 20 to 30%; 3 to 15% aluminum, preferably about 7 to 12%; 0.2 to 3% of at least one element from the group of rare earths, yttrium, tantalum, hafnium, scandium, zirconium, niobium, rhenium, silicon, preferably about 0.7%; The rest of at least one of the elements made of cobalt, nickel, as well as production-related impurities.

6. Metal object according to one of the preceding claims, d a d u r c h g e k e n n z e i c h n e t that at least the second coating layer is applied by plasma spraying, in particular low-pressure plasma spraying.

7. Metal object according to one of the preceding claims, characterized in that between the base material (1) and the first coating layer (2) and / or between the first coating layer (2) and the second coating layer (3), a diffusion barrier layer (4 or 5th ), e.g. B. from titanium nitride, to reduce diffusion processes between the different material compositions is available.

8. A metal object according to one of the preceding claims, that a ceramic thermo-barrier layer (6) with low thermal conductivity is present on the second coating layer (3), preferably made of zirconium oxide with the addition of yttrium oxide.

9. A metal object according to claim 8, so that the second coating layer (3) is pre-oxidized on its surface to form the thermal barrier layer (6).

10. Metal object according to one of the preceding claims, d a d u r c h g e k e n n z e i c h n e t that the total thickness of the coating layers is greater than 0.3 mm.