EP0167102A1 - Process for the production of a protective oxide layer on a moulded article made from high-temperature material - Google Patents

Abstract

1. A process for producing a protective oxide layer on a shaped article made from high-temperature material with no or little aluminium content, in which a thin alloy layer containing aluminium is produced in the untarnished surface and the surface thus treated is subjected to a diffusion annealing and oxidizing operation, characterised by first (a) annealing the surface containing aluminium in hydrogen for at least ten hours at a temperature not less than 900 degrees C but at which the material of the shaped article is not yet altered, and after cooling (b) to a temperature below 700 degrees C treating it for not less than 5 hours in a mixture of steam-containing inert gas and some vpm oxygen, and finally (c) annealing it at 900-950 degrees C for at least 24 hours in a mixture of steam-containing inert gas and hydrogen in a volume ratio of H2 : H2 O of about 1 : 1.

Description

The invention relates to a method for producing an oxidic protective layer on a molded body made of high-temperature material, which contains little or no aluminum, in which a thin aluminum-containing alloy layer is produced in the metallic bare surface and the surface treated in this way is subjected to a diffusion annealing and oxidation process.

It is known to protect metallic materials against corrosion and in particular oxidation, especially at high temperatures, by covering oxide layers, which are either formed by oxidation of the base material itself or - as stated in US Pat. No. 3,129,069 - by alloying the surface with an oxide layer former how especially aluminum and then oxidation are generated.

The latter process was developed especially for nickel-based alloys, which themselves contain too little aluminum to form a dense covering aluminum oxide layer.

According to the above-mentioned US patent, the workpiece to be protected is briefly melted Dipped aluminum and then freed of any aluminum residues, after which the thin aluminum-containing alloy layer of the surface forms an oxide protective layer on the nickel-based alloy when the workpiece is heated in an oxygen-containing environment.

The protective layer formation described above was developed in particular for turbine blades which are used in an oxidizing environment.

It has now been found that workpieces protected in this way undergo changes in a non-oxidizing atmosphere, in particular in gases containing hydrogen, since the oxide protective films formed have a certain permeability to hydrogen.

The aim of the invention is therefore a surface protection which better protects the base material against the access of permeating hydrogen. The basic materials used are, in particular, aluminum alloys with low-temperature high-temperature materials as defined in DIN standard 17006, sheet 3.

This object is achieved according to the invention by forming an oxide protective layer of the type mentioned at the outset, which is characterized in that the aluminum-containing surface is initially at least ten hours at a temperature of at least 900 ° C. in which the material of the shaped body has not yet changed, glows in hydrogen and, after cooling to a temperature below 700 ° C., is treated for at least 5 to 10 hours in a mixture of inert gas containing steam and with a few vpm of oxygen and finally at 900-950 ° C. a mixture of water vapor-containing inert gas with hydrogen with an H ₂ : H ₂ 0 ratio of about 1: 1 glows for at least one day.

The formation of the oxide layer in the manner mentioned surprisingly results in a significantly increased barrier effect of the layer against hydrogen permeation. In the case of the oxide layer formation mentioned, argon in particular serves as the inert gas.

• 1) ein Glühen in Wasserstoff;
• 2) eine sog. "Tieftemperaturoxidation" und
• 3) eine "Hochtemperaturoxidation".

The oxide layer formation according to the invention essentially comprises three sections:

• 1) a glow in hydrogen;
• 2) a so-called "low temperature oxidation" and
• 3) "high temperature oxidation".

In the first stage (with chromium-containing alloys) there is a certain amount of chromium in the surface and at the same time a removal of remaining scale layers and oxides. For this purpose, the temperature is at least 900 ° C, but the alloy should not change, which means that the temperature is limited upwards. With Hastelloy X ( ^R ) the temperature of the workpiece should be below 1000 ° C. stay. The usual temperatures are between 900 and 950 ° C.

The desired removal of residual oxides and enrichment of chromium in the surface requires a certain reaction time, which should in particular be at least 10 hours. The workpieces provided with an aluminum-containing surface layer are preferably annealed in hydrogen for about 30 hours.

In the second phase, oxygen attacks the surface at a relatively low temperature. In this second phase, the temperature is below 700 ° C down to temperatures of about 300 ° C, below which the reaction and diffusion rates are greatly reduced. The water vapor content of the gas corresponds approximately to the vapor pressure of warm water with temperatures around 50 ° C. The oxygen content is low and is around 10–6 bar at a few vpm 0 ₂ or on the order of magnitude. This "nucleating treatment" takes place - depending on the temperature - for at least 5 hours, the time period being "unlimited" and only being controlled by practicality considerations. In this second stage of the process, an approximately 0.1 μm “seed layer” is formed.

The "high temperature oxidation" in stage 3 takes place at the highest possible temperature, which with the Structural stability of the base material is compatible. Temperatures between 900 and 950 ° C. are expediently used. In this phase, the cations diffuse through the oxide layer, the growth of which can be described by a parabolic time law, with the formation of protective layers about 1 - 2 µm thick in a hydrogen / water vapor mixture in the region of 1: 1, some of which Percentages are present in inert gas. Mixtures of argon with about 5% hydrogen, which are saturated with water vapor at ambient temperature, are useful, for example. The treatment time is about 30-70 hours and gives layer thicknesses in the range of 1-2 µm. The hydrogen content in this mixture determines the oxidation potential of the gas, which is in the range of 10 ^-16 . Under these conditions, iron and nickel are not oxidized and only Cr ₂ 0 ₃ and Al ₂ 0 _{3 are} formed.

The invention is described below using an example, namely a Hastelloy X ^(R) sample was provided with an oxide layer which offers considerable resistance to the permeation of hydrogen.

The aluminum alloy layer to be subjected to the three-stage oxide layer formation is obtained in the following way: An aluminum bath is used for the immersion treatment, the temperature of which is only slightly above the melting point lies. The cleaned, not preheated workpiece is briefly immersed in this bath, on average for about 30 to 40 seconds. During this time, only the areas close to the surface are accessible to the aluminum, since the diffusion rate of the aluminum in the solid material is so slow at about 700 ° C that it does not penetrate deeper. The diffusion layer thickness achieved will generally be about 5 to 10 microns.

The workpiece is cleaned before diving. In particular, pretreatment of the surface with medium-fine emery paper (in particular 220 grit; grain size of about 100 μm) ensures a "disturbed" surface layer which can quickly absorb aluminum. Before the immersion treatment, the surface is freed of all impurities and, in particular, degreased, for which a treatment in an ultrasonic bath, which consists of a fat-dissolving solvent or solvent mixture, has proven useful.

The immersion treatment in the molten aluminum bath creates an aluminum coating in addition to the aluminum-containing diffusion layer adjacent to the surface. The oxidation of this aluminum coating would generally result in an aluminum oxide surface layer. However, it has been shown that a protective layer is formed in this way, but this protective layer prevents complete oxidation of "foreign aluminum" so that adjacent to the protective layer there are areas of the workpiece which are near the surface and which may or may show undesired changes, in particular embrittlement.

For this reason, the aluminum layer produced is removed again by etching treatment, which is only intended to remove the surface application of aluminum. Dipping into warm 20 to 25% sodium hydroxide solution (depending on the thickness of the aluminum coating for about 10 to 15 minutes) with subsequent treatment in approximately half-concentrated nitric acid with intermediate rinsing and subsequent thorough removal of the etching agents has proven effective. A time limitation of the etching process largely prevents the etching agents from penetrating into the workpiece surface and removing the diffused aluminum.

The invention is described below using an example.

example

Disc-shaped alloy samples of Hastelloy _X ( ^R ) with a diameter of 45 mm and a thickness of 2.5 mm were cleaned and treated with emery paper (220 grit) to achieve a smooth sanded surface. These disks were then placed in an ultrasonic bath with an acetone / methanol mixture (1: 1) and cleaned for approximately 60 seconds. These samples were immersed in a molten aluminum bath of pure aluminum (99.8%) at approximately 700 ° C. for approximately 40 seconds. This immersion treatment leads, as metallographic follow-up examinations have shown, to a sample surface with a two-layer layer, the outer zone of which consists almost exclusively of aluminum, while a second zone in the form of an aluminum diffusion layer is formed adjacent to the sample surface, the thickness of which is approximately 1 to 5 μm .

The cooled samples were chemically etched to remove the aluminum coating and expose the aluminum diffusion layer.

For this purpose, the samples were first immersed in warm (80 ° C) 20% sodium hydroxide solution for 10 minutes, which removes the aluminum layer. The intermediate rinsed samples were then immersed in a nitric acid bath made of 1: 1 65% nitric acid diluted with water. This aftertreatment is used to remove a dark coating that remains in sodium hydroxide solution after the immersion treatment.

The samples treated in this way were heated to 950 ° C. in a hydrogen atmosphere in 10 hours and left there for 30 hours. After cooling to 650 ° C within 13 hours, the hydrogen was replaced by a water vapor-containing argon / oxygen mixture, which 2.5 vpm 0 ₂ and 180 mbar H ₂ 0 contained. The samples were left in this atmosphere for 29 hours, after which the oxygen content of the argon was replaced by 5% hydrogen and the water vapor content was reduced to 30 mbar. In this atmosphere, the samples were heated to 950 ° C. within 10 hours and kept at this temperature for 72 hours. The samples in the argon / hydrogen / water vapor mixture were then cooled to room temperature in about 12 hours.

The hydrogen inhibition factors of the samples treated in this way were between 1800 and 2200, while untreated comparative samples usually give values between 400 and 600.

Claims (5)

1. A process for producing an oxidic protective layer on a molded body made of high-temperature material, which contains little or no aluminum, in which a thin aluminum-containing alloy layer is produced in the bare metal surface and the surface treated in this way is subjected to a diffusion annealing and oxidation process,
characterized in that the aluminum-containing surface (a) is initially annealed in hydrogen for at least ten hours at a temperature of at least 900 ° C, at which the material of the shaped body does not change, however, and after cooling (b) to a temperature below 700 ° C for at least 5 to 10 hours in a mixture of water vapor-containing inert gas with some vpm oxygen and finally (c) at 900 - 950 ° C in a mixture of water vapor-containing inert gas with hydrogen with an H ₂ : H ₂ 0 ratio of about 1: 1 glows for at least a day. 2. The method according to claim 1,
characterized in that argon is used as the inert gas. 3. The method according to claim 1 or 2,
characterized, that the hydrogen annealing treatment (a) is carried out at 900-950 ° C. for 30 hours. 4. The method according to any one of the preceding claims, characterized in that the low-temperature oxidation (b) at 625 ° C for about a day in a 2-3 vpm 0 ₂ containing mixture of argon with about 10 - 20% hydrogen. 5. The method according to at least one of the preceding claims,
characterized in that the high-temperature oxidation (c) is carried out for 30 to 70 hours in a mixture of argon with 5% hydrogen saturated with water vapor at room temperature.