WO2022243036A1

WO2022243036A1 - Anti-corrosion layer for surfaces of low-aluminum alloys

Info

Publication number: WO2022243036A1
Application number: PCT/EP2022/061936
Authority: WO
Inventors: Leszek Niewolak; Thomas Stöcker; Ilka Verena Luck; Mike Löffert
Original assignee: Heraeus Deutschland GmbH & Co. KG
Priority date: 2021-05-20
Filing date: 2022-05-04
Publication date: 2022-11-24
Also published as: EP4092158A1

Abstract

The invention relates to a method for depositing an aluminum oxide layer on the surface of a metal body, the metal of the metal body containing aluminum in an amount of 0.3-6 wt.%. The deposition is carried out by spraying an aerosol onto the surface of the metal body using the aerosol deposition method while an aluminum oxide layer is deposited. The aluminum oxide layer can serve as an anti-corrosion layer. The invention also relates to a metal body comprising an aluminum oxide layer which is produced according to the method.

Description

Anti-corrosion layer for surfaces made of low-aluminum alloys

The present invention relates to a method for producing an aluminum oxide layer on the surface of a metal body, the metal of the metal body containing aluminum in an amount of 0.3-6% by weight. Furthermore, the invention relates to a metal body having an aluminum oxide layer produced according to the invention.

Despite various strategies to reduce and avoid corrosion of metals, this remains a challenge. This is particularly the case when metals are exposed to a corrosive atmosphere at high temperatures.

Special alloys have been developed to improve corrosion resistance. A known class of high-temperature resistant alloys are so-called MCrAlY alloys. Here, M stands for an element from the group consisting of iron, cobalt and nickel. The alloys also contain chromium (Cr), aluminum (Al) and yttrium (Y). FeCrAlY alloys are mainly used for the manufacture of components that are only exposed to low mechanical loads, such as heating elements, resistors or catalyst carriers. Ni(Co)CrAIY are mainly suitable for the production of components that are subject to higher mechanical loads, such as boilers, heat exchangers, valves and pumps, chemical reactors, jet engines, gas turbines, fasteners. MCrAlY alloys are particularly interesting because they can form a passivating aluminum oxide layer on their surface. In this case, the aluminum required for the passivation layer comes from the alloy itself. The formation of the passivation layer can deplete the alloy of aluminum, which can lead to a change in the mechanical properties.

But even these MCrAlY alloys have their limits. Components made of MCrAlY alloys with a low aluminum content (also known as marginal alumina formers) are used in turbine construction because they have advantageous mechanical properties. To protect these low-aluminum alloys from corrosion, they are coated with a layer of high-aluminum MCrAIY, since high-aluminum alloys can form dense passivation layers themselves. A disadvantage of alloys with a high aluminum content is the comparatively low mechanical stability of these alloys. Therefore, barrier layers of porous aluminum oxide can be placed on these alloy layers with a high aluminum content in order to additionally protect them from high temperatures and oxidation. Aluminum oxide layers are known from JP2009280854A, which were produced by means of aerosol deposition of particles with a diameter of <1 μm. These prior art layers are porous to help you keep heat away from the underlying MCrAlY alloys. Other methods by which aluminum oxide can be applied are also known from the prior art. However, these coating methods, such as thermal spraying, sputtering and PVD, are only able to produce amorphous or metastable Al2O3 layers, which, however, do not provide sufficient protection at the operating temperatures of MCrAlY alloys, for example around 900°C, and one or more phase transitions exhibit. This leads to a change in volume, combined with significant crack formation, so that the protective effect of the oxide layer for the above-mentioned alloys is lost.

The additional application of an aluminum-rich MCrAlY layer on an aluminum-poor MCrAlY alloy is also disadvantageous, since it has to be applied in a separate work step and has to be covered with a thermal protection layer (TBC) for optimal protection. It would be desirable if an anti-corrosion layer could be applied directly to mechanically stable MCrAlY alloys.

The object of the present invention was to overcome at least one disadvantage of the prior art. In particular, the object of the invention was to provide a method with which metal bodies, in particular containing an MCrAlY alloy, can be better protected against corrosion, in particular at temperatures of at least 900.degree. In particular, the oxidation protection of metal bodies made of MCrAIY with a low aluminum content of at most 6% by weight should be improved within the scope of the invention.

The object is achieved by a method for producing an aluminum oxide layer on the surface of a metal body, having the steps: a. Providing a metal body, wherein the metal of the metal body contains aluminum in an amount of 0.3-6% by weight and wherein the metal body has a surface, b. Providing an aluminum oxide powder having a particle size distribution in the form of an aerosol, c. Spraying the aerosol onto the surface of the metal body using the aerosol deposition method, depositing an aluminum oxide layer with an average thickness in the range of 0.5 μm - 20 μm.

The method according to the invention allows a particularly dense aluminum oxide layer to be applied directly to a metal body containing a metal according to the invention. This aluminum oxide layer gives the metal body a particularly high level of corrosion resistance.

In a first aspect, the invention relates to a method for producing an aluminum oxide layer. This aluminum oxide layer is particularly suitable to protect metal surfaces at temperatures above 400°C in corrosive atmospheres. Corrosion can be understood, for example, as oxidation in an oxygen-containing environment.

In step a), a metal body with a surface is provided. The metal body to which the aluminum oxide layer is applied comprises a metal, with the metal according to the invention containing aluminum in an amount of 0.3-6% by weight. The metal of the metal body preferably contains a metal alloy or consists of it. In a preferred embodiment of the invention, the metal of the metal body contains aluminum in an amount of 0.5-4% by weight. The metal alloy is preferably an alloy of the type MCrAIY (MCrAlY alloy). Here M is an element selected from the group consisting of iron, cobalt and nickel and combinations of these elements. Furthermore, MCrAlY alloys contain at least chromium (Cr), aluminum (Al) and yttrium (Y). Optionally, the MCrAIY alloy can also contain at least one element selected from the group consisting of Mn, Si, Ti, Zr, Cu, La, Ce, Hf, C, P and S and combinations of these elements. Metals, in particular MCrAlY alloys, with the aluminum content according to the invention have the advantage that they have improved mechanical properties compared to such MCrAlY alloys with a higher aluminum content of more than 6% by weight.

The metal body can have any shape. Foils, metal sheets and massive molded parts are preferred. In one possible embodiment, the shape of the molded part is designed in such a way that the entire surface is accessible to a spraying process. For example, the metal body can be a component of a turbine or a high-temperature furnace.

The metal body has a metallic surface if possible. This means that, apart from unavoidable impurities, there are preferably no intentionally applied substances on the surface, in particular no metal oxides. For example, the metal surface can be stored in air. In a preferred embodiment, the surface of the metal body on which the aluminum oxide layer is produced has no bond coat layer, in particular no layer containing an aluminum-rich MCrAlY alloy with more than 6% by weight aluminum. In a preferred embodiment, the metallic surface has no alloyed elements, such as platinum, which form a bond coat layer. Platinum is often used in the prior art to enable better bonding of an aluminum oxide layer and is typically introduced into an MCrAlY alloy in a separate step. In the present invention, this step can be omitted. In one possible embodiment of the invention, the aluminum oxide layer according to the invention can be used as a bond coat/ayer.

In step b) an aluminum oxide powder is provided. The aluminum oxide powder preferably has a particle size distribution dso of at least 1 μm, in particular at least 10 μm and particularly preferably at least 20 μm and more preferably at least 30 μm. Furthermore, the aluminum oxide powder preferably has a particle size distribution dso of at most 150 μm, in particular at most 125 μm, very particularly preferably at most 100 μm and even more preferably at most 80 μm. The aluminum oxide powder preferably has a particle size distribution with a dso value in the range from 0.1 μm to 150 μm. Furthermore, the particle size distribution can have a dgo value of 125 μm or less. The particle size distribution within the scope of the invention can be determined, for example, by means of laser diffraction in accordance with ISO 13320:2009.

The aluminum oxide powder preferably has aluminum oxide in an amount of at least 85% by weight, preferably at least 90% by weight and particularly preferably at least 99% by weight. In a particularly preferred embodiment of the invention, the alumina powder comprises alpha alumina. In particular, the aluminum oxide powder has alpha aluminum oxide in an amount of at least 85% by weight, preferably at least 90% by weight and particularly preferably at least 99% by weight.

In addition to alpha alumina, the alumina may optionally also contain other phases such as beta, theta and gamma alumina, particularly in unavoidable traces. Preferably the total amount of alumina not present as alpha-alumina is less than 15 wt%, or less than 5 wt%, or less than 1 wt%, especially less than 0.1 wt% or all more preferably less than 0.001% by weight. The various phases of alumina and the amounts of each can be determined using XRD as described herein. A particularly high corrosion stability can be achieved, in particular at temperatures of 400° C. or more in oxygenated environments. In addition to aluminum oxide, in particular alpha aluminum oxide, the aluminum oxide powder can optionally contain at least one further component. The at least one further component can be, for example, an impurity or a specifically added component.

The at least one further component can be selected, for example, from the group consisting of MgO, MgAhCU, CaO, Y2O3, YAG (yttrium aluminum garnet), YAP (yttrium aluminum perovskite), La ₂ 0 ₃ , LaAI0 ₃ , ZrÜ2, Hf0 ₂ , TiO _x and CeO _x , and combinations thereof.

In a further possible embodiment, the aluminum oxide powder can contain at least one further component which is selected from the group consisting of S1O2, CaO, SrO, BaO, Na2Ü, K2O, Fe oxides, Cr oxides, Mn oxides, Ni oxides, Cu Oxides and Co-oxides.

The at least one further component in the aluminum oxide powder can be present in a total amount of less than 15% by weight, in particular less than 5% by weight or less than 1% by weight. The amount of the at least one further component is particularly advantageously present in an amount of at most 0.5 or even at most 0.1% by weight. Optionally, the aluminum oxide powder does not contain any other component apart from aluminum oxide within the scope of the measurement inaccuracy.

According to the invention, the aluminum oxide powder is provided in the form of an aerosol.

For this purpose, the aluminum oxide powder used is placed in an aerosol generator.

By introducing a process gas, the aluminum oxide powder is converted into an aerosol and finely distributed in the process gas. The process gas can be an inert gas, oxygen or mixtures thereof. The inert gas can be selected from the group consisting of helium, argon and nitrogen and mixtures of these gases.

In step c), the aerosol is sprayed onto the surface of the metal body to obtain an aluminum oxide layer with an average thickness in the range from 0.5 μm-20 μm, in particular in the range from 1 μm-5 μm. The aluminum oxide layer preferably has a thickness of at least 0.5 μm, in particular at least 1 μm and very particularly preferably at least 2 μm. Furthermore, the aluminum oxide layer preferably has a thickness of at most 20 μm, in particular at most 10 μm and very particularly preferably at most 5 μm. In a particularly preferred embodiment, the aluminum oxide layer has an average thickness which is at most 50%, in particular at most 25%, of the dso value of the particle size distribution of the aluminum oxide powder. Particularly advantageous aluminum oxide layers can be obtained if these layers essentially no longer have any primary particles from the aerosol. The layer thicknesses of the present invention are determined using a mechanical stylus instrument at one edge of the layer. The aluminum oxide layer preferably contains crystallites with a maximum diameter in the range of 15 nm-150 nm. Based on the number, at least 90% of the crystallites, in particular at least 95% of the crystallites, are preferably in this diameter range. The number of crystallites and their size can be determined using image analysis in an SEM image.

The aluminum oxide layer produced preferably has a relative density of 96% or more, in particular of 98% or more and very particularly preferably of 99% or more. For example, the aluminum oxide layer has a density of around 99.5%. The relative density refers to the theoretically achievable material density under standard conditions.

The spraying of the aerosol generated in step b) takes place in step c) using the aerosol deposition method (ADM) with deposition of an aluminum oxide layer with an average thickness in the range of 0.5 μm-20 μm. The composition of the alumina layer preferably has the same characteristics as the alumina powder, i. H. During the spraying process, there are no chemical or physical conversion processes or additional contamination.

An ADM coating system is used for the aerosol deposition method. This coating installation contains an aerosol generator in which the aluminum oxide powder used is converted into an aerosol as described in step b). In addition, the ADM coating system contains a coating chamber in which the coating process takes place. A vacuum in the preferred range of 60 mbar to 1066 mbar prevails in the aerosol generator, particularly during the process. A pressure in the preferred range of 0.2 mbar to 20 mbar prevails in the coating chamber, particularly during the process. The pressure in the coating chamber is lower than the pressure in the aerosol generator. Typically, the pressure difference between the aerosol generator and the coating chamber is in the range of 200 mbar - 500 mbar. The aluminum oxide powder is transported from the aerosol generator through a nozzle into the coating chamber via a process gas. The particles are accelerated due to the resulting pressure difference between the aerosol generator and the coating chamber, in particular to a speed in the range of 100 m/s - 600 m/s, and are deposited on the surface of the metal body. As a result of the impact, the aluminum oxide particles break up into fragments, particularly in the nm range, and form a dense and well-adhering layer. More preferably, substantially all of the alumina particles fracture upon impact. The process gas used can be selected from inert gases, oxygen, air or combinations thereof. The inert gas can preferably be at least one gas selected from the group consisting of helium (He), argon (Ar) and nitrogen (N2) or combinations thereof.

The metal body to be coated is preferably moved with an XY table. Alternatively or additionally, it is possible for a nozzle to be moved over the metal body to be coated. A combination of both, i.e. a movement of the table and a nozzle in the XY direction against each other, is also possible.

ADM is a cold coating process. Since the particles of the aluminum oxide powder and the resulting aluminum oxide layer are already essentially in the a-phase, there is no phase change when the aluminum oxide layer is exposed to temperature. In this way, leaks in the layer produced due to cracks and pores are avoided.

The method according to the invention makes it possible, among other things, to apply aluminum oxide layers to metals according to the invention, which themselves are not capable of forming a stable passivation layer. In this way, metal bodies with good mechanical properties of a low-aluminum alloy can be equipped with high corrosion resistance, which can usually only be achieved with high-aluminum alloys.

In a second aspect, the invention relates to a metal body comprising a metal containing aluminum in an amount of 0.3-6% by weight, with an aluminum oxide layer having a thickness of 0.5 μm-20 μm and a relative density of 96% or more. The relative density is preferably at least 98% and in particular at least 99%. The density of a layer produced by means of aerosol deposition can be measured using a densimeter, for example.

The hardness of an aluminum oxide layer according to the invention using the ADM method is preferably 6 GPa or more, in particular 8 GPa or more.

The features that were described in the first aspect of the invention for the method also apply, where applicable, to the metal body according to the invention. In particular, the features for the composition of the aluminum oxide powder also apply to the composition of the aluminum oxide layer.

The deposited aluminum oxide layer can have one or more beneficial effects. Firstly, the aluminum oxide layer can indicate the depletion of the alloy Reduce aluminum by high temperature corrosion, which can lead to increased life of the metal body.

Furthermore, the aluminum oxide layer produced according to the invention can prevent metastable aluminum oxides such as theta-alumina from being formed on the surface of an MCrAlY alloy, or if the aluminum oxide layer according to the invention has alpha-alumina, this can have the formation of further alpha-alumina and a cause high density.

The metal body can be used in various applications, for example as a part in turbine construction. In particular, the metal body can be a turbine blade. Here, the use of the aluminum oxide layer according to the invention can mean that an aluminum-rich MCrAlY layer can be dispensed with as a bond coat layer.

Furthermore, the metal body according to the invention can be used, for example, for high-temperature applications, in particular for applications in which the metal body is exposed to temperatures of >400°C or >600°C, in particular temperatures in the range of 600-900°C. Preferably, corrosion can be reduced or suppressed by the invention both in the case of static and dynamically changing temperature loads.

Other typical applications in which metal bodies according to the invention can be used include heating elements, resistors, catalyst supports, boilers, heat exchangers, valves, pumps, chemical reactors, high-temperature furnaces, jet engines, gas turbines and fastening parts.

measurement methods

particle size distribution

The particle size distribution can be determined by laser diffraction according to ISO 13320:2009 using the "Helos BR/R3" device (Sympatec GmbH, Germany). The measuring range is in the range of 0.9 - 875 pm. The dry dispersing system RODODS/M (Sympatec GmbH, Germany) with vibrating channel dosing unit VI BRI (with Venturi nozzle) can be used to disperse the powder particles. The amount of sample is 5 g. The wavelength of the laser radiation used is 632.8 nm. The evaluation can be carried out using the Mie theory. The particle sizes are obtained as a volume distribution, ie within the scope of the present invention the particle size distribution is determined in the form of a volume distribution cumulative curve. From the Particle size distribution (volume distribution) measured by laser diffraction can be calculated as described in ISO 9276-2:2014, dso values.

X-ray diffraction (XRD)

The XRD measurements are carried out in accordance with DIN EN 13925-1:2003-07 and DIN EN 13925-2:2003-07. The measurement is carried out directly on the layer produced. The general measurement details used are summarized as follows: Diffraction: Bragg-Brentano; Detector: scintillation counter; Radiation: CuKa 1.5406 Å;

Source: 40kV, 25mA; Measurement method: reflection. As an internal reference, an empty sample holder is first measured to determine the background signal. This background measurement is subtracted from all subsequent measurements of the samples to be examined.

Discrete diffraction signals in the diffractogram can be evaluated according to the Debye-Scherrer method using the Bragg equation. From this, the phase of alumina present can be determined.

layer thickness

The layer thickness is determined using a mechanical stylus device (perthometer). For this purpose, part of the component to be coated is masked and the layer to be examined is deposited on this masked substrate. After removing the masking, the layer thickness can be determined by measuring a layer step with the perthometer probe. The MarSurf XR1 device from Mahr GmbH can be used for this.

density measurement

The relative density is calculated as follows:

(actual density / theoretical density) ^c 100%).

The actual density can be measured with a densimeter using the Archimedes method according to DIN EN 623-2:1993-11. The AlfaMirage SD200L device can be used for this.

hardness measurement

The hardness of a layer produced using ADM can be measured using nanoindentation, for example. The nano test platform controls the movement of a diamond in contact with the sample. A change in current at a coil results in an applied force (electromagnetic force actuator principle) and causes a change in the position of the diamond. During the hardness measurement, the diamond tip continuously penetrates the sample. This change in path is documented as a function of the load via the change in capacitance of a capacitor. Consequently, by exact calibration of the applied coil current and by measuring the change in path, both the penetration depth and the applied load can be determined. The measurement is carried out in accordance with DIN EN ISO 14577:2015-11.

examples

Example 1:

In Example 1, an aluminum oxide powder was deposited on a FeCrAlY metal plate using the aerosol deposition method. Oxygen was used as the process gas and the pressure in the coating chamber was <20 mbar during the coating process, with the pressure in the aerosol generator being about 200 mbar higher than in the coating chamber.

Using the aerosol deposition method, an aluminum oxide layer was deposited directly on the surface of a metal plate made of an FeCrAlY alloy with an edge length of 20x20x1 mm. The alumina powder used had particle sizes in the range of 0.1 and 5 pm (dgo < 125 pm) and oxygen was used as the carrier gas.

The sample with the applied layer was statically treated in air at 900° C. for 100 h. The sample was then examined under the scanning electron microscope (SEM). The sample shows a regular alpha-ALOs oxide layer with no spalling and no theta -AI2O3 formation. Figures 1B and 1D show SEM images taken at different scales. It can be seen here that the layers are uniform and closed.

For comparison, the same metal plaque without an aluminum oxide coating was treated under the same temperature conditions. The comparison sample without the aluminum oxide layer shows irregular oxidation with spalling, as can be seen in Figures 1A and 1C.

Example 2:

In Example 2, an aluminum oxide powder analogous to Example 1 (dgo <125 μm) was deposited on a small metal plate using the aerosol deposition method. An aluminum oxide layer was deposited directly on the surface of a small metal plate made of a NiCrAIY alloy (Alloy 602 CA, (NiCr25FeAIY)) with an edge length of 20×20×1 mm. Oxygen was used as the process gas and the pressure in the coating chamber was <20 mbar during the coating process, with the pressure in the aerosol generator being about 200 mbar higher than in the coating chamber. The sample produced in this way was statically heated in air at 1000° C. for 50 h. After the temperature treatment, the sample was examined by SEM. The sample shows a regular protective alpha-alumina layer with no apparent defects.

For comparison, the same metal body without an aluminum oxide protective layer was heated in air at 1000° C. for 50 h. Subsequently, the surface showed an irregular oxide layer in an SEM analysis, which could be due to the formation of Cr,Ni oxides.

The change in weight of the uncoated sample after 50 hours of oxidation was about 1 mg/cm ² . In contrast, the weight of the small metal plate with the aluminum oxide layer deposited remained unchanged within the framework of the weighing inaccuracy. From this it can be deduced that the sample is corroded due to the missing protective layer.

Claims

patent claims

1) Process for depositing an aluminum oxide layer on the surface of a metal body, the metal of the metal body containing aluminum in an amount of 0.3-6% by weight, comprising the steps: a. providing a metal body having a surface, b. providing an alumina powder in the form of an aerosol and c. Spraying the aerosol onto the surface of the metal body using the aerosol deposition method, depositing an aluminum oxide layer with an average thickness in the range of 0.5 μm - 20 μm.

2) The method according to claim 1, wherein the aluminum oxide powder has a particle size distribution with a dso value in the range of 0.1 pm - 150 pm.

3) The method according to claim 1 or 2, wherein the metal of the metal body comprises aluminum in an amount of 0.5 to 4% by weight.

4) The method according to any one of claims 1-3, wherein the metal body comprises or consists of an alloy of the type MCrAIY, where M stands for an element selected from the group consisting of cobalt, nickel and iron or combinations of these elements.

5) The method according to any one of claims 1 - 4, wherein the aluminum oxide layer has a relative density of 96% or more.

6) The method according to any one of claims 1 - 5, wherein the aluminum oxide layer has a relative density of 99% or more.

7) The method according to any one of claims 1-6, wherein the aluminum oxide layer comprises or consists of alpha aluminum oxide.

8) The method according to any one of claims 1 - 7, wherein the aluminum oxide layer comprises at least 85% by weight a/p/7a aluminum oxide.

9) Metal body containing aluminum in an amount of 0.3 - 6% by weight, with an aluminum oxide layer with a thickness of 0.5 µm - 20 µm and a relative density of 96% or more being deposited directly on the surface of the metal body.

10) Metal body according to claim 9, wherein the metal of the metal body is an alloy of the type MCrAIY. 11) Metal body according to claim 10, characterized in that the aluminum oxide layer is deposited using a method according to any one of claims 1-8.

12) Use of an aluminum oxide layer with a thickness of 0.5 pm - 20 pm and a relative density of 96% or more to improve the corrosion stability of a metal body comprising an alloy of the MCrAIY type, the alloy having an aluminum content of 0.3 - 6 wt. -% having.