JPH07215782A - Corrosion resistant insulating coating method for silicon carbide heating element - Google Patents

Abstract

PURPOSE:To provide a method for coating a silicon carbide heating element by which high insulating property as well as high corrosion resistance is imparted. CONSTITUTION:The heating part and/or ends of a silicon carbide heating element are coated with a powdery mixture of 55-95wt.% zircon (ZrSiO4) with 5-45wt.% alumina (Al2O3) in 100-2,000mum thickness and the mixture is baked at 1,100-1,350 deg.C to form a coating having the corrosion resistance and insulating property of zircon and the high temp. stability of alumina. The adhesive property of the coating can be improved by heat-treating the heating element in an oxidizing atmosphere before coating.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a corrosion resistant insulating coating method for a silicon carbide heating element, and more particularly to a corrosion resistant insulating coating method for a silicon carbide heating element used in a metal melting furnace, a glass melting furnace and a ceramic firing furnace.

[0002]

2. Description of the Related Art Silicon carbide heating elements generally have a temperature of 1000.degree.
Widely used at higher temperatures. This is because silicon carbide has excellent high temperature stability and chemical stability. In particular, the high temperature stability is due to the oxide film on the surface of silicon carbide. That is, SiC + CO ₂ → SiO ₂ +
The oxide film formed on the surface by CO ₂ ↑ serves as a self-protection film and suppresses further oxidation. However, when used in metal melting furnaces, glass melting furnaces, ceramic firing furnaces, etc., the metal vapor generated from the object to be heated, the basic substance, etc. react with the protective film on the above surface, and the life of the silicon carbide heating element Becomes extremely short. Conventionally, in order to protect a silicon carbide heating element, various corrosion resistant coatings have been proposed on the heating element surface. For example, ceramic powder other than silicon carbide, such as molybdenum silicide, on the surface of the silicon carbide heating element,
There are a method of applying alumina, zirconia, glass and the like, and a method of forming a dense silicon carbide film on the surface of the silicon carbide heating element by a CVD method. Further, in order to positively utilize the self-protecting film on the surface of silicon carbide, there is known a method of increasing the thickness of the self-protecting film by heat treatment before actually using it. However, metal vapors, basic substances, etc. generated from the object to be heated react with the furnace material, and the electrical insulation of the furnace material decreases,
Breakage of silicon carbide heating element due to electric leakage. In the worst case, not only the silicon carbide heating element but also the power supply equipment will be burned. Therefore, the corrosion-resistant coating of the silicon carbide heating element used in a metal melting furnace, a glass melting furnace, a ceramic firing furnace, etc. is required to have corrosion resistance against metal vapor, a basic substance and the like as well as insulation. However, in the above-mentioned conventional technique, not only the corrosion resistance is insufficient, but also the insulating property is not taken into consideration. Here, the insulation is represented by insulation = specific resistance of coating substance × film thickness. In particular, the film thickness needs to be at least 100 μm or more. For example, silicon carbide by CVD, molybdenum silicide, glass, and a self-protection film by heat treatment have low specific resistance and almost no insulation. Alumina and zirconia are originally insulating materials, but since the coefficient of thermal expansion differs greatly from that of silicon carbide, the coating layer thickness of 100 μm or more required to ensure sufficient insulation cannot be obtained. Therefore, it cannot be said that the insulating property is high. Furthermore, metal vapor generated from the object to be heated,
Corrosion resistance to basic substances is not sufficient. Corrosion resistance, specific resistance of various conventional coating materials or protective film,
The film thickness and the determination result are shown in Table 1.

[0003]

SUMMARY OF THE INVENTION It is an object of the present invention to provide a method for coating a silicon carbide heating element which solves the problems of the prior art and has high corrosion resistance and high insulation.

[0004]

That is, the corrosion-resistant insulating coating method according to the present invention is a method of forming a zircon (ZrSiO ₄ ) on the heat generating portion and / or the end portion of a silicon carbide heating element.
It is characterized in that a mixed powder having a composition of 55 to 95% by weight and alumina (Al ₂ O ₃ 5 to 45% by weight) is applied to 100 to 2000 μm and baked at 1100 to 1350 ° C. Further, the above mixed powder is applied. It is preferable that the silicon carbide heating element is heat-treated in an oxidizing atmosphere to form an oxide film of 5 to 50 μm on the surface of the silicon carbide particles before the heating.

[0005]

The present invention will be described in detail. Zircon (ZrSi
O ₄ ) has low wettability with metals and is a very excellent material that has been used in the metal industry such as a metal ladle. It also has high corrosion resistance to basic substances. Further, as an insulating material, it has been conventionally used as an insulating glass. Furthermore, since the coefficient of thermal expansion of zircon at 1300 ° C or less is almost the same as that of silicon carbide, when used as a coating material with a film thickness of 100 μm or more, which is necessary to ensure sufficient insulation, peeling due to the difference in thermal expansion does not occur. Does not happen. However, zircon exhibits anomalous thermal expansion above 1300 ° C.
It is also a difficult material to start decomposing above 500 ° C.
Alumina, though inferior to zircon, is known as a corrosion resistant insulating material. It is also a material that can be used stably up to 1700 ° C or higher. However, since the coefficient of thermal expansion is different from that of silicon carbide, it is 100% necessary to ensure sufficient insulation.
Coating with a film thickness of μm or more was impossible. As a result of researching these two materials, the present inventors have found that in order to maintain the corrosion resistance, insulation, and thermal expansion of zircon and to add the high temperature stability of alumina, zircon (ZrSiO ₄ ) 5
The inventors have found that the optimum composition is 5 to 95% by weight and alumina (Al ₂ O ₃ ) 5 to 45% by weight, and have reached the present invention.
That is, if the content of zircon is less than 55% by weight, the difference in thermal expansion from that of silicon carbide becomes large and coating cannot be performed. On the other hand, if it is 95% by weight or more, the high temperature stability is poor and it is not suitable as a coating for a silicon carbide heating element. Further, in order to improve the adhesion with silicon carbide, the silicon carbide heating element is heat-treated in an oxidizing atmosphere before applying the mixed powder to form an oxide film of 5 to 50 μm on the surface of the silicon carbide particles. It is preferably formed.

[0006]

EXAMPLES The present invention will be described below with reference to Examples and Comparative Examples, and the results are shown in Table 2.

Example 1 Zircon 1 having an average particle size of 3 μm
20 g and 80 g of alumina having an average particle size of 1 μm were weighed, 3 g of methyl cellulose and 80 g of pure water were added as a mixing aid, and mixed in a pot mill for 2 hours. Spray the mixture onto the silicon carbide heating element to 200
μm was applied. After fully drying at 120 ℃, 1150 ℃
Baked for 10 minutes. When this silicon carbide heating element was used in a reflection type aluminum melting furnace and a glass melting furnace,
Although the life was 3 months, the coating did not peel off and there was no defective insulation.

Example 2 Zircon 1 having an average particle size of 3 μm
80 g and 20 g of alumina having an average particle size of 1 μm were weighed, 3 g of methyl cellulose and 80 g of pure water were added as a mixing aid, and mixed in a pot mill for 2 hours. The mixture is sprayed onto the silicon carbide heating element at 100 and 100
0 μm was applied. After fully drying at 120 ° C, 1300
Baking for 10 minutes at ° C. When this silicon carbide heating element was used in a reflection type aluminum melting furnace and a glass melting furnace, although the life was 4 months, the coating was partially peeled off, but there was no insulation failure.

[Embodiment 3] A silicon carbide heating element is placed in the atmosphere 1
Coating was performed by the same method as in Example 2 except that heat treatment was performed at 300 ° C. for 1 hour and an oxide film of 10 μm was attached to the surface. When this silicon carbide heating element was used in a reflection type aluminum melting furnace and a glass melting furnace, the life was 4 months, but the coating did not peel off and there was no insulation failure.

Comparative Example 1 200 g of zircon having an average particle size of 3 μm was weighed, 3 g of methyl cellulose and 80 g of pure water were added as a mixing aid, and they were mixed in a pot mill for 2 hours. The mixture was sprayed onto the heating portion and the end portion of the silicon carbide heating element to apply 1500 μm. After sufficiently drying at 120 ° C, baking was performed at 1300 ° C for 10 minutes. When this silicon carbide heating element was used in a reflection type aluminum melting furnace and a glass melting furnace, the life was 0.5 months.
In addition, the ammeter of the furnace became unstable during use and a leakage phenomenon was observed. After use, the coating was almost decomposed.

Comparative Example 2 Alumina 2 having an average particle size of 2 μm
Weigh 00g, and use methyl cellulose 3 as a mixing aid.
g and 80 g of pure water were added and mixed in a pot mill for 2 hours. The mixture was spray-applied to the heating portion and the end portion of the silicon carbide heating element at 1000 μm. After sufficiently drying at 120 ° C, baking was performed at 1300 ° C for 10 minutes. When this silicon carbide heating element was used in a reflection type aluminum melting furnace and a glass melting furnace, the life was one month. In addition, the ammeter of the furnace became unstable during use and a leakage phenomenon was observed. The coating was almost peeled off after use.

[0012]

[Table 1]

[0013]

[Table 2]

[0014]

As described above, the present invention can inexpensively provide a method for coating a silicon carbide heating element having high corrosion resistance and high insulation, and particularly, a metal melting furnace, a glass melting furnace, and a ceramic firing furnace. It is most suitable as a protective film for a silicon carbide heating element used for.

Claims (2)

[Claims] 1. A mixed powder having a composition of 55 to 95% by weight of zircon (ZrSiO ₄ ) and 5 to 45% by weight of alumina (Al ₂ O ₃ ) is added to the heating portion and / or the end portion of the silicon carbide heating element. ~ 2000μm coated, 1100
A method for corrosion-resistant insulation coating of a silicon carbide heating element, characterized by baking at ˜1350 ° C. 2. A silicon carbide heating element is heat-treated in an oxidizing atmosphere before applying the mixed powder to form an oxide film of 5 to 50 μm on the surface of the silicon carbide particles. A method for corrosion-resistant insulation coating of a silicon carbide heating element as described.