DE102004052104B3 - Pre-oxidation of articles made from aluminum alloys comprises heating to form stable alpha-aluminum coating in atmosphere comprising e.g. water vapor with specified free oxygen content - Google Patents

Abstract

Method for pre-oxidation of articles made from aluminum alloys or with an aluminum-containing coating comprises heating them above 1,050[deg]C to form a stable alpha-aluminum coating. An oxygen-poor atmosphere is used which at room temperature comprises either: (A) water vapor with a free oxygen content below 100 ppm; (B) a mixture of water vapor and hydrogen with a free oxygen content below that of hydrogen; or (C) a mixture of carbon dioxide and carbon monoxide with a free oxygen content below that of carbon monoxide.

Description

The The invention relates to a method for the treatment of components comprising aluminum-containing alloys and / or aluminum-containing Protective layers, which is the formation of metastable oxide layers reduced or suppressed.

alloys based on Fe-Al, Ni-Al, Ni-Cr-Al or Fe-Cr-Al are characterized by an excellent oxidation resistance up to very high operating temperatures of about 1400 ° C. The Oxidation resistance is due to the formation of a dense and slow growing alumina layer, resulting in high temperature use on the material surfaces forms. This protective Topcoat based on a selective oxidation of the alloying element Aluminum occurs, occurs only if the aluminum content in the alloy is sufficiently high, z. B. at least about 8 mass% in Fe-Al or Ni-Al alloys and about 3 mass% in Fe-Cr-Al or Ni-Cr-Al alloys.

The formation of the alumina-based capping layer consumes the alloying element aluminum present in the alloy. The consumption per unit time is proportional to the oxide growth rate and therefore increases with increasing temperature because the oxide growth rate (k in cm ² per second) increases with increasing temperature. The overall existing in a respective component aluminum reservoir increases in proportion to the component wall thickness. In the case of a layer or film as a component, the thickness typically corresponds to the layer thickness, for a wire as a component, for example, to the diameter.

Becomes while a long-term use of the aluminum-containing component by the Formation of the aluminum oxide layer on the surface of the aluminum content so far reduced that a critical aluminum concentration fell below so there can be no more protective aluminum oxide layer train more. this leads to to a very fast and so-called "breakaway oxidation", in which the main alloying elements Chromium, nickel and iron oxidize. Because these elements are bad-protective oxides form the time at which "breakaway oxidation" occurs, also regularly the End of life of the component.

Out the above shows that the life with increasing Oxide growth rate and decreasing component wall thickness decreases.

Some examples of typical times (t _B ) to end of life of components of FeCrAl alloys (commercial designations eg KANHAL AF or ALUCHROM YHF) as a function of temperature and wall thickness are known from the literature. For example - for 1 mm wall thickness at 1200 ° C, about 10,000 h, - for 0.05 mm wall thickness at 1100 ° C about 700 h, - for 0.05 mm wall thickness at 1200 ° C about 80 h.

From theoretical considerations, it can be deduced that the lifetime decreases by a factor of about 10 with a temperature increase of 100 ° C. The temperature dependence of t _B results from the known temperature dependence of the oxide growth rate k. This is defined as follows: k = k O e -Q / RT (Equation 1) with Q = activation energy for diffusion processes in the oxide layer, T = temperature and R = general gas constant.

The dependence of the time to the end of life (t _B ) of the component wall thickness (d) is approximately the following for most applications: t B proportional to d 3 , (Equation 2)

This makes it clear that the time to end of life is greatly reduced by a reduction in the component wall thickness. For very thin-walled components of the abovementioned alloys, as present, for example, in support materials of passenger vehicle catalysts (typical film thicknesses 0.02 to 0.1 mm), in fiber-based gas burners or filters (typical fiber diameter 0.015 to 0.1 mm), Therefore, the required operating times of a few thousand hours can only be achieved if the operating temperatures of 900 ° C, that is kept relatively low. The same applies to very thin coatings based on FeCrAl or Ni (Co) CrAl.

In this temperature range, in particular between 750 and 950 ° C, however, the growth rate (k) of the oxide layer disadvantageously shows a significant deviation from the aforementioned temperature dependence. This deviation occurs in particular in the initial stage, in particular in the first 100 h of the oxidation stress. The reason for this deviation lies in the fact that at temperatures in the range 700-950 ° C metastable Al ₂ O ₃ modifications, in particular θ-, δ- or γ-Al ₂ O ₃ occur, and not at high temperatures, that is, at and above 1000 ° C, the stable a-Al ₂ O ₃ (hexagonal structure, corundum lattice) is formed.

These metastable oxide modifications are characterized by significantly higher growth rates than the α-Al ₂ O ₃ . They generally occur only in the early stages of oxidation. After long periods, a transition to stable α-Al ₂ O ₃ with the corresponding low growth rate occurs. The time at which this transition occurs decreases with increasing temperature. For a FeCrAl alloy, the transition occurs at 950 ° C z. B. after about 15 hours, at 800 ° C, however, the time can be several hundred hours.

The Lifetime of a component at temperatures in the range 750-950 ° C can be thus generally not known from the higher temperatures Extrapolate oxide growth rates. For thick-walled components with for example, 1 to 2 mm wall thickness This is usually not problematic as the aluminum reservoir in the alloy is so high that the initial high growth rate at temperatures around 900 ° C, due to the metastable oxide modifications, no significant Reduction of the entire aluminum reservoir causes.

For very thin components, such as 0.03 to 0.1 mm thin films, however, can be exhausted by the initial high growth rate of the oxide layer, the existing, very small aluminum reservoir adversely already in a few hours. This regularly leads to complete destruction of the component in a very short time. The actual lifetime is thus orders of magnitude smaller than one would expect due to the extrapolation of the growth rates of the α-Al ₂ O ₃ layers at high temperatures (about 1000 to 1200 ° C).

The The aforementioned alloys and layers are therefore suitable for use in the mentioned thin-walled Components such as car catalysts, gas burners or Filter systems, as well as in very thin Protective layers of z. B. layer thickness of 20-50 microns not suitable.

The Formation of fast-growing, metastable alumina layers while the use of thin-walled Alloys or protective coatings in the temperature range 700-950 ° C may periodically thereby prevents the alloy or applied protective layer is pre-oxidized at high temperatures, in particular at 1050 ° C or higher.

Becomes, however, this pre-oxidation in, as commonly used, air or an oxygen-containing gas, such as pure oxygen or a mixture of oxygen and inert gas, the α-alumina layer grows relatively quickly. This makes the aluminum reservoir for very thin components in the alloy or the protective layer already during the Pre-oxidation to a large extent or even completely exhausted.

Out Studies on the mass growth behavior of aluminum-containing Alloys during heating and subsequent isostatic oxidation at 1200 ° C shows that the pre-oxidation times are very short and extreme would have to be respected exactly to "breakaway oxidation", or unreasonably high To avoid aluminum consumption.

However, this is difficult to keep in practice, since especially for larger components or large quantities in a furnace, regularly longer times of a few hours are needed to ensure a uniform temperature distribution throughout the oven, ie in all components. In addition, at the high temperature of z. B. 1200 ° C usually longer times of about one to two hours required to completely transform metastable oxides that have formed during the heating phase in the stable α-modification of Al ₂ O ₃ .

Out DE 698 23 468 T2 is, for example, a method for pre-oxidation of substrates or Be known coatings comprising aluminum. During the process for applying aluminum oxide layers, an inert or non-reactive gas is introduced into the region surrounding the substrate. By using an amount of inert or non-reactive gas equal to or greater than the amount of oxygen deposited later in the cycle, it is achieved that the rate of growth of the thermally grown oxide layers (TGO) during the preheat cycle can be substantially reduced ,

Task and solution

The The object of the invention is to provide a method available in which components comprising aluminum-containing alloys and aluminum-containing protective layers when used in the temperature range 750-950 ° C a clear have improved long-term behavior.

The The object of the invention is achieved by a method of treating aluminum-containing alloys and aluminum-containing layers for high temperature use according to the main claim. Advantageous embodiments of the method can be found in the dependent claims again.

Subject of the invention

It has been found that the long-term stability of components comprising aluminum-containing alloys and protective layers can be significantly improved when used in the temperature range of 750-950 ° C, especially in the initial stage of the oxidation of the component instead of the metastable oxide modifications a predominantly from α-Al ₂ O. ₃ constructed oxidic cover layer is formed. Components here are to be understood as meaning both films or wires consisting of an Al alloy and components provided with an Al-containing protective layer, for example gas turbine blades. Typical Al-containing alloys are alloys based on Fe-Al, Ni-Al, Ni-Cr-Al or Fe-Cr-Al, as already mentioned at the outset.

• – Wasserdampf
• – Wasserstoff mit Wasserdampf
• – Kohlendioxid mit Kohlenmonoxid
• – Gemisch aus einem Inertgas (z.B. Argon oder ggf. Stickstoff) und Wasserdampf
• – Gemisch aus einem Inertgas (z.B. Argon oder ggf. Stickstoff) mit Wasserdampf und Wasserstoff
• – Gemisch aus einem Inertgas (z.B. Argon oder ggf. Stickstoff) mit Kohlendioxid und Kohlenmonoxid.

In contrast to the prior art, in the process according to the invention, the pre-oxidation is not carried out in an atmosphere containing oxygen as an oxidizing component (such as pure oxygen or air) but in an atmosphere containing carbon monoxide as the oxidizing gas component Contains carbon dioxide or water vapor with a very low proportion of free oxygen. Examples of such gases are in particular:

• - Steam
• - Hydrogen with water vapor
• - Carbon dioxide with carbon monoxide
• - Mixture of an inert gas (eg argon or possibly nitrogen) and water vapor
• - Mixture of an inert gas (eg argon or possibly nitrogen) with water vapor and hydrogen
• - Mixture of an inert gas (eg argon or possibly nitrogen) with carbon dioxide and carbon monoxide.

In these atmospheres, the growth rate of the α-Al ₂ O ₃ layer and thus the consumption of aluminum in the alloy is significantly lower than when oxidized in atmospheres, the oxygen as a significant oxidizing component, such. B. have air. Thus, given a pre-oxidation time of, for example, one hour in one of these atmospheres, the amount of Al consumed in the alloy is significantly less than if the preoxidation had been carried out in an oxygen-based atmosphere during the same time. Thus, the aluminum reservoir in the component which has been pretreated by the new process is substantially larger, and thus the life much longer, than in a component which has been pretreated in an oxygen-based atmosphere.

To be particularly advantageous for carrying out the process according to the invention, the atmospheres described in detail below have been found, which introduced so at room temperature in an oven at the corresponding temperature of the pre-oxidation forms an atmosphere, the k value for the α-Al ₂ O. ₃ education significantly lowers.

It has been found within the scope of the invention that the proportion of free oxygen alone is not critical for the process according to the invention. Thus, for example, in a pure oxygen atmosphere at a vacuum of 10 ^-6 bar, the desired α-Al ₂ O ₃ oxide layers do not regularly form on the surface of the Al-containing components. This is explained by the fact that in such an atmosphere the reservoir of oxygen is absolutely too low to allow the desired layer formation.

• a) Wasserdampf Bei Zugabe von reinem Wasserdampf als oxidierender Atmosphäre sollte der Gehalt an freiem Sauerstoff nicht mehr als 100 ppm betragen.
• b) Wasserstoff mit Wasserdampf Bei Verwendung einer Wasserstoff/Wasserdampf Atmosphäre kann das Verhältnis zwischen 10/1 bis 1/1000 frei gewählt werden. Für die Wirkung des Verfahrens ist jedoch entscheidend, dass der freie Sauerstoffgehalt geringer ist, als der entsprechende Wasserstoffgehalt (O₂ < H₂).
• c) Kohlendioxid mit Kohlenmonoxid Hier gilt ähnliches, wie für die Mischung Wasserstoff mit Wasserdampf. Bei Verwendung einer Kohlendoxid/Kohlenmonoxid Atmosphäre kann das Verhältnis ebenfalls zwischen 10/1 bis 1/1000 frei gewählt werden. Der freie Sauerstoffgehalt sollte ebenfalls geringer sein, als der entsprechende Gehalt an Kohlenmonoxid (O₂ < CO).
• d) Gemisch aus einem Inertgas (z.B. Argon oder ggf. Stickstoff) und einer unter a) bis c) genannten Atmosphäre Das verwendete Inertgas ist bei technischen Gasen häufig Träger von sauerstoffhaltigen Verunreinigungen. Hier gilt für den freien Sauerstoffgehalt der gesamten Mischung jeweils analog das unter a) bis c) gesagte.

By contrast, the atmosphere used according to the invention is a gas or a gas mixture which can potentially provide sufficient oxygen through its components water vapor, carbon monoxide or carbon dioxide, even if the free oxygen content is very low, for example below 100 ppm. Advantageous atmospheres to be used are:

• a) Water vapor When pure steam is added as the oxidizing atmosphere, the content of free oxygen should not be more than 100 ppm.
• b) Hydrogen with water vapor When using a hydrogen / steam atmosphere, the ratio between 10/1 and 1/1000 can be freely selected. However, it is crucial for the effect of the process that the free oxygen content is lower than the corresponding hydrogen content (O ₂ <H ₂ ).
• c) Carbon dioxide with carbon monoxide The same applies here as for the mixture of hydrogen with water vapor. When using a carbon dioxide / carbon monoxide atmosphere, the ratio can also be freely selected between 10/1 to 1/1000. The free oxygen content should also be lower than the corresponding content of carbon monoxide (O ₂ <CO).
• d) Mixture of an inert gas (eg argon or possibly nitrogen) and an atmosphere mentioned under a) to c) The inert gas used is often the carrier of oxygen-containing impurities in industrial gases. Here, the same applies to the free oxygen content of the entire mixture as in a) to c).

As a result of the abovementioned atmospheres, the k value of the oxidation at the temperatures of the preoxidation is advantageously reduced, so that on the one hand complete formation of the stable α-Al ₂ O ₃ cover layer is achieved, but on the other hand the aluminum depletion occurring in the alloy or the layer is advantageously reduced. As a result, the life of these pretreated alloys, or layers can be significantly extended.

Special description part

following the subject matter of the invention is based on figures and tables explained in more detail, without that the subject of the invention is limited thereby.

The Oxide growth rate, which differs in the aforementioned atmospheres to the rates in an atmosphere based on oxygen can quantitatively derived from the following findings.

In general, it is assumed that the dependence of the thickness x (eg in μm) α-Al ₂ O ₃ -layer on the time t (in hours) results from the following law: x = p · t n (Equation 3)

As oxide formation correlates with oxygen uptake, the mass of the component increases during the oxidation process. The surface-specific mass increase (Δm in mg per cm ² ) is proportional to the oxide layer thickness x. Therefore, in practice, the oxidation rate is not described by equation (3) but by: Δm = k · t n (Equation 4)

For pure Al ₂ O ₃ formation, a mass change of 1 mg per cm ² corresponds to an oxide layer thickness of approximately 5 micrometers

Which Influence the temperature and equilibrium oxygen partial pressure the atmosphere while the pre-oxidation on the oxidation factor k, is from the following figures clearly.

The value of the oxidation factor k is strongly temperature-dependent. It increases strongly with increasing temperature. Typical values of k during oxidation in an oxygen-based atmosphere are in 1 shown. The 1 shows, as an example, the experimentally determined temperature dependence of the k value for the formation of an α-Al ₂ O ₃ layer in an atmosphere B containing water vapor as the oxidizing component (dashed line). In this case, the atmosphere used consists of a mixture of an inert gas Ar with 4% H ₂ and 2% H ₂ O. For comparison, the temperature dependence of the k value for an atmosphere A with a gas mixture of argon with 20% oxygen is plotted ( solid line).

The exponent n is hardly dependent on the temperature; it is about 0.35 for most α-Al ₂ O ₃ -forming alloys.

The dependence of the oxidation factor k on the oxygen partial pressure is for three relevant temperatures (1200 ° C, 1150 ° C, 1100 ° C) in 2 shown. The k value decreases significantly with lower temperature. However, temperatures of 1050 ° C for the pre-oxidation should not be undercut, because otherwise a complete conversion of the metastable oxidation states into the stable α-Al ₂ O ₃ can not be guaranteed regularly.

It can be clearly recognized by which factor the k value is advantageously reduced, especially at relatively high temperatures, if an atmosphere used according to the invention containing carbon monoxide, carbon dioxide or water vapor as oxidizing component is chosen instead of a base atmosphere containing much oxygen, and thus a free equilibrium oxygen partial pressure of regularly less than 10 ^-4 , in particular less than 10 ^-6 and particularly advantageously less than 10 ^-8 in bar.

The 3 shows experimental measurements of the oxide growth of an α-Al ₂ O ₃ -forming FeCrAl alloy during heating to 1200 ° C with a heating rate of 90 degrees per minute (Phase I) and the subsequent isothermal phase (II) at 1200 ° C in one Oxygen base atmosphere (air) and in the above-mentioned Ar / H ₂ / H ₂ O atmosphere (see also 1 ). The weight gain was determined by thermogravimetry. Shown is the mass change due to oxygen uptake due to oxide formation as a function of the experimental time.

Also are entered calculated mass change values (ie oxide layer thicknesses) where the aluminum from a typical thin-walled component with different Geometries completely depleted would. there It was assumed that the component consists of a FeCrAl alloy made with a typical Al content of 5 mass%. Of the with a) designated limit corresponds to the total consumption of aluminum in a film with 20 microns Layer thickness. The lower limit (b) corresponds analogously to complete Aluminum consumption in a foil with only 15 μm layer thickness or alternatively one Wire with a diameter of 30 μm.

It becomes clear that with small layer thicknesses or wire diameters in the usual pre-oxidation in an oxygen-containing atmosphere, the life of the alloy or the layer will be exhausted after just a few hours can. As a typical pre-oxidation of about 2 hours sought will, would be in this case, an alloy with a thickness of only 15 microns thick disadvantageous the "breakaway Oxidation "already reached.

Tabelle 2: Zeiten (einschließlich Aufheizzeit) bis zu einem Al-Verbrauch von 70 % in Komponenten unterschiedlicher Geometrie gefertigt aus einer FeCrAl-Legierung mit 5 Mass.-% Al.

Table 1 summarizes for this example what percentage of the aluminum in the different component geometries would be consumed after a total oxidation time of one hour in the different atmospheres. It is also listed after which oxidation times in the two atmospheres 70% of the aluminum from the alloy is consumed. Table 1: Percentage Al consumption after one hour of pretreatment (including heat-up time) of components of different geometry made of FeCrAl alloy with 5 mass% Al. Data derived from FIG. 3.

Table 2: Times (including heating time) up to an Al consumption of 70% in components of different geometry made from a FeCrAl alloy with 5 mass% Al.

For other oxidation temperatures and other gas mixtures, analogous quantitative data from Equation 4 can be obtained using the data in 1 and 2 be derived.

Claims (5)

A method for pre-oxidation of components of an Al alloy or with an aluminum-containing coating, wherein the component for forming a stable α-Al ₂ O ₃ cover layer are heated to temperatures above 1050 ° C, characterized in that a low-oxygen atmosphere is used which has at room temperature either - water vapor with a free oxygen content [O ₂ ] of less than 100 ppm, or - has a mixture of water vapor and hydrogen, the content of free oxygen being less than the content of hydrogen ([O ₂ ] <[H ₂ ]), or - a mixture of carbon monoxide and carbon dioxide, wherein the content of free oxygen is less than the content of carbon monoxide ([O ₂ ] <[CO]). The method of claim 1, wherein the oxygen-poor the atmosphere additionally having an inert gas. Method according to the preceding claim 2, wherein Argon and / or nitrogen used as an additional inert gas become. Method according to one of the preceding claims 1 to 3, in which a mixture of water vapor and hydrogen in the ratio 10 to 1 to 1 to 1000 is used. Method according to one of the preceding claims 1 to 3, in which a mixture of carbon monoxide and carbon dioxide in the ratio 10 to 1 to 1 to 1000 is used.