KR20010019769A - Powder Composition for Simultaneous Coating of Chrome and Aluminium on Metal Surfaces and Coating Method Thereof - Google Patents

Abstract

PURPOSE: A powder composition for simultaneously coating aluminum and chromium on metal surface and a method for coating the same are provided to obtain pack cementation of aluminum and chromium by freely controlling a composition of aluminum and chromium oxide, and supply economical mixed powder by using oxide powder instead of alloy powder. CONSTITUTION: A powder composition for simultaneously coating aluminum and chromium on metal surface is a mixed powder comprising an aluminum powder, a chromium oxide powder producing chromium by reacting with the aluminum powder, an activator powder activating chromium produced by the reaction and the remaining aluminum, and a filler for preventing condensation thereof. A method for simultaneously coating aluminum and chromium on metal surface comprises the process of putting the mixed powder into a heat resistant container, coating the container with metal, and simultaneously coating chromium and aluminum on the metal by sealing the container or heating the container under the inert gas or hydrogen environment at a temperature of 800 to 1200 deg.C for 1 to 30 hours.

Description

Powder Composition for Simultaneous Coating of Chrome and Aluminum on Metal Surfaces and Coating Method Thereof}

The present invention relates to a powder composition for applying a coating layer of chromium and an aluminum alloy having high corrosion resistance and high temperature oxidation resistance to a surface of a metal part made of a metal, particularly an iron alloy or a nickel alloy, and a method of forming a coating layer on a metal surface using the powder composition. It is about. More specifically, the metal surface is composed of aluminum powder, chromium oxide powder, aluminum powder and an activator powder for activating chromium produced by the reaction of aluminum and chromium oxide, and a filler powder for preventing the powder from condensing at high temperature. The present invention relates to a powder composition for forming a coating and a method of coating aluminum and chromium on a metal surface using such a composition.

Powder coating, which increases the corrosion and oxidation resistance of metals by diffusing and penetrating elements such as aluminum and chromium on the surface of metals at a high temperature, is an element or alloy powder to be coated and an element or alloy to be coated. As an activator for activating the powder, a metal to be coated is usually placed in a mixed powder composed of a halogen salt powder and a filler powder of an inert inorganic material such as alumina or silicon carbide to prevent the powder from condensing at a high temperature. Is a method of obtaining a diffusion coating layer of several micrometers to several hundred micrometers from a metal surface when it heats for a predetermined time at high temperature of 800 degreeC-1,200 degreeC. In this process, the metal element or alloy powder element in the mixed powder reacts with the activator to be converted into metal halide. The metal halide moves into the gaseous state in the mixed powder and decomposes on the metal surface. A diffusion coating layer of constant thickness is produced.

The process of obtaining a coating layer in which only aluminum is diffused on the surface of the metal, that is, an aluminide layer by using a general powder coating method is called aluminizing, and the aluminized metal is dense aluminum oxide on the surface in a high temperature oxidizing atmosphere. That is, high temperature oxidation resistance is high because an alumina film is formed. However, alumina membranes have limited corrosion resistance in high temperature and corrosive atmospheres [N. R. Linblad, Oxid. Met., 1, 143 (1969)], [A. J. A. Mom and H. J. C. Hersbach, Mater. Sci. Eng., 87, 361 (1987)] To be used in high temperature oxidative and corrosive atmospheres such as gas turbines, the aluminide layer needs to be improved by alloying it with a highly corrosion resistant metal.

When the aluminide layer is improved by using chromium, which is a type of metal with high corrosion resistance, chromium oxide produced by chromium oxide is known to have strong corrosion resistance against sodium sulfate formed by the combination of salt and sulfur dioxide in seawater. Is widely used [R. L. McCarron, N. R. Lindblad, and D. Chatterji, Corrosion 32, 476 (1976), [E. Godlewska and K. Godlewski, Oxid. Met. 22, 117 (1984). When a simple mixed powder of aluminum and chromium is used to obtain a diffusion coating layer of aluminum and chromium on the metal surface by the conventional powder coating method, the partial pressure of chromium halide in the powder during the high temperature treatment is very low compared to the aluminum halide partial pressure. Because of this, there is a problem that the simultaneous coating is not good. Therefore, in order to obtain a diffusion coating layer of aluminum and chromium on the metal surface, the metal surface is first diffusion-coated with chromium by a powder method (Chromizing treatment), and then aluminized or [R. Mevrel, C. Duret, and R. Pichoir, Mater. Sci. Technol., 2, 201 (1986)], [E. Godlewska and K. Godlewski, Oxid. Met. 22, 117 (1984)] uses alloy powders of aluminum and chromium [F.D. Geib and R. A. Rapp, Oxid. Met., 40, 213 (1993)], [R. A. Bianco, R. A. Rapp, J. Electrochem. Soc., 140, 1181 (1993)], [R. A. Bianco, R. A. Rapp, and J. L. Smialek, J. Electrochem. Soc., 140, 1191 (1993)], [W. Dacosta. B. Gleeson and D. J. Young, J. Electrochem. Soc., 141, 2690 (1994)]. Dacosta. B. Gleeson and D. J. Young, Surf. Coat. Technol., 88, 165 (1996)].

However, the former powder coating method is disadvantageous because it is processed in a two-step process, and it is not easy to obtain an alloy coating of a desired composition. The latter method of using an alloy powder of chromium and aluminum cannot freely select the composition of the powder. It is expensive in itself and has a disadvantage in that it is difficult to regenerate the powder once used.

The present invention provides a mixed powder for diffusion coating that simultaneously coats aluminum and chromium in a single process as a means to solve the above problems, as well as freely select the composition of aluminum and chromium, aluminum using a low-cost mixed powder It is an object to provide a powder coating method of and chromium.

1 is a composition profile of chromium and aluminum in specimens A1 and A2 after the pack cementation treatment of Example 1;

FIG. 2 shows the component profiles of chromium and aluminum in specimens B1 and B2 after the diffusion coating process of Example 2. FIG.

FIG. 3 shows the component profiles of chromium and aluminum in specimens C1 and C2 after the diffusion coating process of Example 3. FIG.

The powder composition of the present invention is a chromium oxide (Cr ₂ O ₃ ) that provides chromium, aluminum that reduces the chromium oxide to chromium in a high temperature inert atmosphere, and an activator that activates reduced chromium and aluminum. Mixed powder mixed with 5-50 wt%: 5-30 wt%: 2-5 wt%: 15-85 wt% of ammonium chloride (NH ₄ Cl) and non-reactive filler such as SiC or alumina, respectively It consists of.

Meanwhile, the coating method of aluminum and chromium on the metal surface using the mixed powder is as follows.

Into the heat-resistant container, a metal part such as a gas turbine blade to be coated with aluminum and chromium is added, and the above-mentioned mixed powder is added to the container, and the container is sealed or an inert gas, preferably argon gas, to prevent oxygen infiltration. B under a hydrogen atmosphere, heated at a temperature of 800 ℃ to 1,200 ℃ for 1 to 30 hours. On the other hand, the metal is an iron-based alloy or a nickel-based alloy, which is 80% by weight of iron and 70% by weight or more of nickel, respectively. The formation reaction of chromium oxide and chromium in aluminum is as follows.

Cr ₂ O ₃ + 2Al → 2Cr + Al ₂ O ₃ (1)

In the above Reaction Formula (1), 1 mol (152 g) of Cr ₂ O ₃ and 2 mol (54 g) of aluminum react to obtain 2 mol (104 g) of chromium. When the maximum chromium content is calculated by the reaction (1), the weight is 50.5%. Therefore, the addition of the filler can lower the chromium content, and by adding extra luminium, the weight ratio of aluminum and chromium in the pack powder can be controlled. Reaction Equation (1) proceeds faster as the temperature increases, so that the composition and thickness of the aluminum and chromium diffusion coating layers of the metal parts can be controlled. Hereinafter, the present invention will be described in detail with reference to the following preferred examples.

<Example 1>

The powder having the following composition was mechanically mixed and ground in a ball mill to have an average particle size of 325 mesh or less.

Cr ₂ O ₃ : 10%, Al: 10%, NH ₄ Cl: 2%, NaCl: 4%, Al ₂ O ₃ : remainder (weight ratio)

When all of Cr ₂ O ₃ is substituted with chromium in the powder mixed with the above components, the molar ratio of chromium and aluminum is about 0.55. About 12 g of this powder was placed in a container made of alumina and embedded therein with a 10 mm × 10 mm × 2 mm pure iron (Fe) specimen. The lid made of alumina was covered and sealed, placed in an electric furnace, and treated under the following conditions. Specimens treated with the mixed powder are of two types and the treatment conditions are as follows.

-Specimen A1: 1,050 ℃, 16 hours

-Specimen A2: 950 ℃, 8 hours + 1,050 ℃, 8 hours

After quenching, the metal specimens in the alumina vessel were taken out, washed, cut and mounted. Mounted specimens were polished and the components of the diffusion coating layer were analyzed using a scanning electron microscope equipped with Energy Dispersive X-ray (EDS). FIG. 1 shows a constituent profile of aluminum and chromium of a specimen on which a diffusion coating layer is formed, and it can be seen that chromium and aluminum are simultaneously coated with chromium and aluminum up to 300 μm of an electrolytic iron specimen treated under the above conditions. In addition, it can be seen that the concentration of chromium and aluminum in the A2 specimens treated under the two-step temperature condition was high, but the surface concentration of chromium was not satisfactory at about 5 at.%.

<Example 2>

The powder having the following composition was mechanically mixed and ground by a ball mill so that the average particle size was 325 mesh or less.

Cr ₂ O ₃ : 20%, Al: 10%, NH ₄ Cl: 2%, NaCl: 4%, Al ₂ O ₃ : remainder (weight ratio)

When all of Cr ₂ O ₃ is substituted with chromium in the powder mixed with the above components, the molar ratio of chromium and aluminum is about 2.45. About 12 g of this powder was placed in a container made of alumina, and embedded therein were electrolytic iron (Fe) specimens having a size of 10 mm × 10 mm × 2 mm. The lid made of alumina was covered and sealed, placed in an electric furnace, and treated under the following conditions, followed by furnace cooling. Specimens treated with the mixed powder are of two types and the treatment conditions are as follows.

-Specimen B1: 1050 ℃, 16 hours

-Specimen B2: 950 ℃, 8 hours + 1050 ℃, 8 hours

After the furnace was cooled as in Example 1, the metal specimens in the alumina container were taken out, washed, cut and mounted. The mounted specimens were polished and the components of the diffusion coating layer were analyzed using a scanning electron microscope with EDS. 2 shows that the content of chromium as a component profile of aluminum and chromium of the specimen on which the diffusion coating layer is formed is about three times higher than that of Example 1. In addition, it can be seen that the concentration of chromium in the B1 specimen treated under the temperature condition of one step is higher than that of the B2 specimen treated under the two-step temperature condition and diffusion-coated to 300 µm in the electrolytic iron. The concentration of aluminum did not show much difference between the two specimens and the aluminum content was very low, about 1 at.%.

<Example 3>

The powder having the following composition was mechanically mixed and ground by a ball mill so that the average particle size was 325 mesh or less.

Cr ₂ O ₃ : 15%, Al: 10%, NH ₄ Cl: 2%, NaCl: 4%, Al ₂ O ₃ : remainder (weight ratio)

When all of Cr ₂ O ₃ is substituted with chromium in the powder mixed with the above components, the molar ratio of chromium and aluminum is about 1.14. About 12 g of this powder was placed in a container made of alumina, and two electrolytic iron (pure iron) specimens having a size of 10 mm x 10 mm x 2 mm were buried therein. The lid made of alumina was covered and sealed, placed in an electric furnace, and treated under the following conditions, followed by furnace cooling. Specimens treated with the mixed powder are of two types and the treatment conditions are as follows.

-Specimen C1: 950 ℃, 8 hours + 1050 ℃, 8 hours

-Specimen C2: 950 ℃, 8 hours

The metal specimens in the alumina container cooled as in Example 1 were taken out, washed, cut and mounted. The mounted specimens were polished and the components of the diffusion coating layer were analyzed using a scanning electron microscope with EDS. 3 is a constituent profile of aluminum and chromium of the specimen on which the Al / Cr diffusion coating layer is formed, and in the case of specimen C1 treated at two temperature conditions, the concentration of chromium on the surface is 15at.% Or more, similar to that of Example 2. The aluminum concentration on the surface is about 5 at.% Or more, achieving the goal of co-diffusion coating of chromium and aluminum. On the other hand, in the case of specimen C2 treated only to see the effect of the first half 950 ℃, 8 hours at the temperature treatment conditions of the two stages, the concentrations of chromium and aluminum are in contrast to the specimen C1. That is, the surface aluminum concentration is about 20 at.%, While the surface concentration of chromium is about 5 at.%. Therefore, it can be seen that by changing the concentration and processing conditions of the powder in the present invention can control the depth and concentration of the diffusion coating layer.

The surface diffusion mixed powder of the present invention can freely adjust the composition of chromium oxide and aluminum to obtain a diffusion coating of aluminum and chromium, and provides an economically mixed powder using oxide powder instead of alloy powder. In addition, it is possible to obtain a diffusion coating of aluminum and chromium in a single process using the mixed powder.

Claims (7)

In coating chromium and aluminum on the metal surface at the same time, the powder for diffusion coating is aluminum powder, chromium oxide powder which reacts with aluminum powder to form chromium, and an activator powder which activates chromium and extra aluminum produced by the above reaction. And powder composition for the simultaneous coating of aluminum and chromium on the metal surface, characterized in that the mixed powder consisting of a filler for preventing the condensation. According to claim 1, wherein the aluminum (Al) is 5 to 30% by weight, the chromium oxide (Cr ₂ O ₃ ) is 5 to 50% by weight of the aluminum and chromium simultaneously coating powder composition. According to claim 1, wherein the active agent is a salt of a halide (halide), specifically for ammonium chloride or sodium chloride or a mixture thereof 2 to 5% by weight for the simultaneous coating of aluminum and chromium on a metal surface Powder composition. The method of claim 1, wherein the filler is alumina (Al ₂ O ₃ ) or silicon carbide (SiC) of 15 to 85% by weight, characterized in that the powder composition for the simultaneous coating of aluminum and chromium on the metal surface. The mixed powder composed of the composition of claim 1 is placed in a heat resistant container and the metal to be coated is buried, and then the container is sealed or heated to a temperature of 800 ° C. to 1,200 ° C. in an inert gas or hydrogen atmosphere for 1 to 30 hours. Simultaneous coating of aluminum and chromium on a metal surface, characterized by coating chromium and aluminum at the same time. The method of claim 5, wherein the metal is an iron-based alloy or a nickel-based alloy, each of which is 80% by weight of iron and 70% by weight of nickel. The method of claim 5, wherein the inert gas is argon (Ar).