CA1188078A - Method of anti-corrosive protection of silicon carbide products - Google Patents
Method of anti-corrosive protection of silicon carbide productsInfo
- Publication number
- CA1188078A CA1188078A CA000453202A CA453202A CA1188078A CA 1188078 A CA1188078 A CA 1188078A CA 000453202 A CA000453202 A CA 000453202A CA 453202 A CA453202 A CA 453202A CA 1188078 A CA1188078 A CA 1188078A
- Authority
- CA
- Canada
- Prior art keywords
- product
- silicon carbide
- corrosion
- channels
- oxidizing atmosphere
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Expired
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Abstract
ABSTRACT OF THE DISCLOSURE
The disclosure describes a method for the pro-tection of a silicon carbide product against corrosion at elevated temperatures, the corrosion resulting from the utilization of a silicon carbide product in an oxidizing atmosphere. The surface of the product is in contact with oxidizing atmosphere of a heating chamber, which atmosphere has free access to the surface and the interior of the product. According to this method, free access to the surface and the interior of the product is made possible by increasing the porosity of the product by at least 10%
or by providing channels in the product. The total volume of the channels is at least 10% of the volume of the product and the channels are open to gases or air.
The disclosure describes a method for the pro-tection of a silicon carbide product against corrosion at elevated temperatures, the corrosion resulting from the utilization of a silicon carbide product in an oxidizing atmosphere. The surface of the product is in contact with oxidizing atmosphere of a heating chamber, which atmosphere has free access to the surface and the interior of the product. According to this method, free access to the surface and the interior of the product is made possible by increasing the porosity of the product by at least 10%
or by providing channels in the product. The total volume of the channels is at least 10% of the volume of the product and the channels are open to gases or air.
Description
This invention relates to a method for the anticorrosive protection of silicon carhide products which are used in an oxidizing atmosphere at elevated temperatures against a special kind of internal corrosion~
which takes place under conditions of limited access of oxygen to the pores inside ~he product and is ca~sed by carbon monoxide formed in the pores.
There are many methods of preventing or 5lowirlg down the corrosion of silicon carbide products which are used at elevated ox very high temperatures in an oxygen containing atmosphere. The corrosion mentioned above proceeds accordin~
to the equation.
SiC + 2 ~2 ~ SiO~ + C02 In cases where for technological reasons products made of silicon carbide are to be used, methods of pxeventing or slowing down the corrosion by various coatings or shields are applied. The prospectus of the Morganite International Limited Company recommends to protect elements made of silicon carbide with coatings of D or L type glazes. A coating with a glaze of the entire surface of various ceramic articles which are used at high temperatures as parts of gas turbines and the like is described in US Patent ~o. 4,159,357.
US Patent No. 2,003,592 applies as coat a layer of fine silicon ~arbide crystals bound with a binding agent. US Patent No. 4,226,914 discloses a tightly adherent coating on metallic or ceramic substrates. made by plasma spraying of a mixture of finely divided silicon carbide and silicon to be used for rocket nozzles and in other areas where extremely high temperatures are present. According to GRF Patent No. 2,533,895 the method consists in impregnating the products with a refractory material, preferably containing molybdenum and silicon bisilicideO The porosity of the product is decreased from 30 per cent to about S per cent, ensuring a certain degree of resistance to corrosion. GDR
Patent No. 136,611 includes a two-step impregnation of silicon carbide heating elements. US Patent No. 2,943,008 concerns batts supporting ceramic ware in kilns during burning, made among o~hers from silicon carbide, wherein the upper or both the upper and bottom surfaces are covered with a protecti~e ceramic layer to prevent rapid oxidation.
All these methods of protecting silicon carbide products aim to prevent or slow down the process of corrosion by oxygen, of porous ceramic materials, such as silicon carbide among others, which are used at elevated temperatures.
That kind of corrosion occurs in the whole porous body of unprotected products made from silicon car~ide. The protective coats and shields substantially limit the oxygen penetration into the pores inside the product.
It has been experienced that under conditions of limited penetration of oxygen into the pores of a silicon carbide product, caused by the above mentioned and not perfectly gas tigh~coatings and shields, another ~ind of corrosion of silicon carbide products takes p]ace inside the body of the product at places distant from the surface.
This different kind of corrosion of silicon carbide product appears at much lower temperatures than the previous one.
It leads to a very rapid destruction of the product and is caused by carbon monoxide formed in the pores under conditions of a very small o~yyen concen~ration.
3~ Thus, the application of protective coatings and shields, which are not perfectly gas tight or have at least a weak point, or become fissured with time, is extremely detrimental to silicon carbide products.
Presented below is the process of corrosion of silicon carbide products in an oxidizing atmosphere and under conditions of a limited penetration of oxygen into the pores of the product, that is in the case of such an installation of the product that a portion of the total outer surface of the product has been isolated from oxygen or air, as e.g. in a crucible for melting metals the outer face of which is heated, in a thermocoupl~ shield immersed in metal or in a zinc ~ectification column of a New Jersey system or in the side linings of alumina reduction cells and many other applications.
Under conditions of a limited penetration of oxygen into the pores of the product, carbon mono~ide is produced as a result of the action of the small concentration of o~gen on trace amounts o~ carbon always present in silicon carbide products, thus leading to the formation o~
a strongly reducing atmosphere.
The carbon monoxide reacts with silicon carbide according to ~he following formulae C + 1/2 2 ~ CO
SiC + C0 ) SiO + 2 C
SiC + lJ2 2--~ SiO + C
which takes place under conditions of limited access of oxygen to the pores inside ~he product and is ca~sed by carbon monoxide formed in the pores.
There are many methods of preventing or 5lowirlg down the corrosion of silicon carbide products which are used at elevated ox very high temperatures in an oxygen containing atmosphere. The corrosion mentioned above proceeds accordin~
to the equation.
SiC + 2 ~2 ~ SiO~ + C02 In cases where for technological reasons products made of silicon carbide are to be used, methods of pxeventing or slowing down the corrosion by various coatings or shields are applied. The prospectus of the Morganite International Limited Company recommends to protect elements made of silicon carbide with coatings of D or L type glazes. A coating with a glaze of the entire surface of various ceramic articles which are used at high temperatures as parts of gas turbines and the like is described in US Patent ~o. 4,159,357.
US Patent No. 2,003,592 applies as coat a layer of fine silicon ~arbide crystals bound with a binding agent. US Patent No. 4,226,914 discloses a tightly adherent coating on metallic or ceramic substrates. made by plasma spraying of a mixture of finely divided silicon carbide and silicon to be used for rocket nozzles and in other areas where extremely high temperatures are present. According to GRF Patent No. 2,533,895 the method consists in impregnating the products with a refractory material, preferably containing molybdenum and silicon bisilicideO The porosity of the product is decreased from 30 per cent to about S per cent, ensuring a certain degree of resistance to corrosion. GDR
Patent No. 136,611 includes a two-step impregnation of silicon carbide heating elements. US Patent No. 2,943,008 concerns batts supporting ceramic ware in kilns during burning, made among o~hers from silicon carbide, wherein the upper or both the upper and bottom surfaces are covered with a protecti~e ceramic layer to prevent rapid oxidation.
All these methods of protecting silicon carbide products aim to prevent or slow down the process of corrosion by oxygen, of porous ceramic materials, such as silicon carbide among others, which are used at elevated temperatures.
That kind of corrosion occurs in the whole porous body of unprotected products made from silicon car~ide. The protective coats and shields substantially limit the oxygen penetration into the pores inside the product.
It has been experienced that under conditions of limited penetration of oxygen into the pores of a silicon carbide product, caused by the above mentioned and not perfectly gas tigh~coatings and shields, another ~ind of corrosion of silicon carbide products takes p]ace inside the body of the product at places distant from the surface.
This different kind of corrosion of silicon carbide product appears at much lower temperatures than the previous one.
It leads to a very rapid destruction of the product and is caused by carbon monoxide formed in the pores under conditions of a very small o~yyen concen~ration.
3~ Thus, the application of protective coatings and shields, which are not perfectly gas tight or have at least a weak point, or become fissured with time, is extremely detrimental to silicon carbide products.
Presented below is the process of corrosion of silicon carbide products in an oxidizing atmosphere and under conditions of a limited penetration of oxygen into the pores of the product, that is in the case of such an installation of the product that a portion of the total outer surface of the product has been isolated from oxygen or air, as e.g. in a crucible for melting metals the outer face of which is heated, in a thermocoupl~ shield immersed in metal or in a zinc ~ectification column of a New Jersey system or in the side linings of alumina reduction cells and many other applications.
Under conditions of a limited penetration of oxygen into the pores of the product, carbon mono~ide is produced as a result of the action of the small concentration of o~gen on trace amounts o~ carbon always present in silicon carbide products, thus leading to the formation o~
a strongly reducing atmosphere.
The carbon monoxide reacts with silicon carbide according to ~he following formulae C + 1/2 2 ~ CO
SiC + C0 ) SiO + 2 C
SiC + lJ2 2--~ SiO + C
2 SiO ---t SiO2 ~ Si As a result of above reactions silicon dioxide and silicon are produced from a portion of the silicon monoxide and the remaining silicon monoxide is transfor~ed into a solid phase of a brown colour.
7~
The liberated carbon reacts in turn with existing oxygen at low concentration producing further carbon monoxide, which in turn Eorms silicon monoxide and liberates carbon.
A mixture of the listed compounds remains partially in the pores of the product, whereas the rernaining portion is deposited on the outer faces of the product. The process leads to a very rapid destruction of the product by fissures and cracks.
The rate of corrosion depends first of all on how the installation of the product is effecked and on its average temperature of use.
According to this pattern the corrosion of silicon carbide products starts already at a temperaturQ of about The invention deals with a kind of corrosion of silicon carbide product hitherto not considered but e~per-ienced by the inventors, which occurs already at moderate temperatures of use in an oxidizing atmosphere caused by carbon monoxide formed in the pores of ~he product under conditions of limited penetration o~ oxygen into the pores depending on the manner in which the product is installed, as well as effective and easy to be used protection methods against this kind of corrosion.
This is achieved by shielding a part of the surface of the silicon carbide product which is exposed to an oxygen containing atmosphere thus preventing the formation of carbon monoxide in the pores of the product with appropriate perfectly gas tight shields or a neutral gas atmosphere.
This is also achieved in cases where a part surface of a silicon carbide product or its entire surface is exposed 7~
to an atmosphere containing oxygen at low concentration, by enhancing the penetration of oxygen into the pores o~ the product to prevent the formation of carbon monoxide, which is being diluted with an excess of oxygen. This can be made possible by increasing the number of open pores in the product an~/or by providing uniformly distributed channels in the whole volume of the product.
In the first case the shield can be made from porcelain, steatite or other known gas tight suitable ceramic materials, or in some cases from metal, particularly alloyed steel. Also substances in molten state such as salts or glasses can be used for shielding silicon carbide products against an oxygen containing atmosphere.
In the second case we have found that an increased number of open pores in the product giving an open porosity exceeding a~out 40 per cent is sufficient to prevent the mentioned C0-type corrosion. Also when the product has a lower porosity we found that if the volume of uniformly distributed channels of various shape in the whole volume of the silicon carbide product is in excess of about 10 per cent of ~he total volume of the product, a sufficient protection against this type of corrosion is reached.
A positive experience has been made with a protective atmosphere of nitrogen.
It is obvious that for any particular case the most economic and suitable protection method should be selected on an experimental basis.
The requirements for a suitable protection in any particular case can be defined upon following: no chemical interaction between the protection and other constituents of the system, perfect gas tightness of the shield or coat, sufficient thermal and mechanical resistance, suitable for any particular thermal or/and electrical conductivity and other properties.
Obviously a universal recipe does not exist.
The invention ensures a considerable extension of the service life of various silicon carbide products, used among others in the non ferrous metals industry as furnace parts, shelves for rectification columns in New Jersey system, shields for thermocouples, fittings for burners as well as in other industries. The invention also enables to increase the scope of utilization of silicon carbide products in the industry, e.g. for side linings of alumina reduction cells, tapping holes and spouts of furnaces and other uses, as well as channel shapes for induction furnaces.
The method according to the invention will be illustrated by means of a non-restrictive example by means of th~ accompanying drawing, in which:
Figure 1 represents a method for the anticorrosive protection of a cru~ible made of silicon carbide and installed in an electric crucible furnace for melting aluminium, Figure 2 shows the anticorrosion protection of a heating element made of silicon carbide and mounted in an electric resistance furnace for melting zinc with direct heating of the zinc bath~
The embodiment of the invention shown in Figure 1 relates to a crucible furnace for melting aluminium. The furnace consists of a casing 1, cover 2, crucible 3, metallic heating elements 4, gas supply system 5, melted aluminium 6, and a heating room 7 wherein the crucible 3 made of silicon carbide has been protectea against corrosion by supplying pure nitrogen to the heating room 7 via gas supply system 5 to provide a protecting atmosphere for the silicon carbide of which the crucible 3 has been made.
The embodiment shown in Figure 2 relates to a heating element used in electric resistance furnaces for melting zine by direct heating of the zinc bath and consisting of a ceramie weight 11, a poreelain ring 12l a crueible 13 made of silicon carbide, current conductors 14 and 15, contaet metal. 16, as well as a zine bath 17, wherein the erueible 13 has been proteeted against eorrosion by means of a poreelain ring 12 defining an air-tight shield protruding from the zine bath over the crueible rim 13 because the bottom edges of the said porcelain ring 12 are immersed in the contact metal 16 and the zine bath 17.
This application is a division of Canadian Patent Applieation No. 355~576 filed July 7, 1980.
7~
The liberated carbon reacts in turn with existing oxygen at low concentration producing further carbon monoxide, which in turn Eorms silicon monoxide and liberates carbon.
A mixture of the listed compounds remains partially in the pores of the product, whereas the rernaining portion is deposited on the outer faces of the product. The process leads to a very rapid destruction of the product by fissures and cracks.
The rate of corrosion depends first of all on how the installation of the product is effecked and on its average temperature of use.
According to this pattern the corrosion of silicon carbide products starts already at a temperaturQ of about The invention deals with a kind of corrosion of silicon carbide product hitherto not considered but e~per-ienced by the inventors, which occurs already at moderate temperatures of use in an oxidizing atmosphere caused by carbon monoxide formed in the pores of ~he product under conditions of limited penetration o~ oxygen into the pores depending on the manner in which the product is installed, as well as effective and easy to be used protection methods against this kind of corrosion.
This is achieved by shielding a part of the surface of the silicon carbide product which is exposed to an oxygen containing atmosphere thus preventing the formation of carbon monoxide in the pores of the product with appropriate perfectly gas tight shields or a neutral gas atmosphere.
This is also achieved in cases where a part surface of a silicon carbide product or its entire surface is exposed 7~
to an atmosphere containing oxygen at low concentration, by enhancing the penetration of oxygen into the pores o~ the product to prevent the formation of carbon monoxide, which is being diluted with an excess of oxygen. This can be made possible by increasing the number of open pores in the product an~/or by providing uniformly distributed channels in the whole volume of the product.
In the first case the shield can be made from porcelain, steatite or other known gas tight suitable ceramic materials, or in some cases from metal, particularly alloyed steel. Also substances in molten state such as salts or glasses can be used for shielding silicon carbide products against an oxygen containing atmosphere.
In the second case we have found that an increased number of open pores in the product giving an open porosity exceeding a~out 40 per cent is sufficient to prevent the mentioned C0-type corrosion. Also when the product has a lower porosity we found that if the volume of uniformly distributed channels of various shape in the whole volume of the silicon carbide product is in excess of about 10 per cent of ~he total volume of the product, a sufficient protection against this type of corrosion is reached.
A positive experience has been made with a protective atmosphere of nitrogen.
It is obvious that for any particular case the most economic and suitable protection method should be selected on an experimental basis.
The requirements for a suitable protection in any particular case can be defined upon following: no chemical interaction between the protection and other constituents of the system, perfect gas tightness of the shield or coat, sufficient thermal and mechanical resistance, suitable for any particular thermal or/and electrical conductivity and other properties.
Obviously a universal recipe does not exist.
The invention ensures a considerable extension of the service life of various silicon carbide products, used among others in the non ferrous metals industry as furnace parts, shelves for rectification columns in New Jersey system, shields for thermocouples, fittings for burners as well as in other industries. The invention also enables to increase the scope of utilization of silicon carbide products in the industry, e.g. for side linings of alumina reduction cells, tapping holes and spouts of furnaces and other uses, as well as channel shapes for induction furnaces.
The method according to the invention will be illustrated by means of a non-restrictive example by means of th~ accompanying drawing, in which:
Figure 1 represents a method for the anticorrosive protection of a cru~ible made of silicon carbide and installed in an electric crucible furnace for melting aluminium, Figure 2 shows the anticorrosion protection of a heating element made of silicon carbide and mounted in an electric resistance furnace for melting zinc with direct heating of the zinc bath~
The embodiment of the invention shown in Figure 1 relates to a crucible furnace for melting aluminium. The furnace consists of a casing 1, cover 2, crucible 3, metallic heating elements 4, gas supply system 5, melted aluminium 6, and a heating room 7 wherein the crucible 3 made of silicon carbide has been protectea against corrosion by supplying pure nitrogen to the heating room 7 via gas supply system 5 to provide a protecting atmosphere for the silicon carbide of which the crucible 3 has been made.
The embodiment shown in Figure 2 relates to a heating element used in electric resistance furnaces for melting zine by direct heating of the zinc bath and consisting of a ceramie weight 11, a poreelain ring 12l a crueible 13 made of silicon carbide, current conductors 14 and 15, contaet metal. 16, as well as a zine bath 17, wherein the erueible 13 has been proteeted against eorrosion by means of a poreelain ring 12 defining an air-tight shield protruding from the zine bath over the crueible rim 13 because the bottom edges of the said porcelain ring 12 are immersed in the contact metal 16 and the zine bath 17.
This application is a division of Canadian Patent Applieation No. 355~576 filed July 7, 1980.
Claims
1. In a method for the protection of a silicon carbide product against corrosion at elevated temperatures, said corrosion resulting from utilization of said silicon carbide product in an oxidizing atmosphere, wherein the surface of said product is in contact with said oxidizing atmosphere in a heating chamber, said oxidizing atmosphere having free access to said surface and the interior of said product, the improve-ment wherein said free access to said surface and interior of said product is made possible by increasing the porosity of said product by at least 10% or by providing channels in said product, the total volume of said channels being at least 10%
of the volume of said product, said channels being open to gases or air.
of the volume of said product, said channels being open to gases or air.
Applications Claiming Priority (3)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| PL21696479A PL216964A1 (en) | 1979-07-07 | 1979-07-07 | |
| PLP-216964 | 1979-07-07 | ||
| CA000355576A CA1177228A (en) | 1979-07-07 | 1980-07-07 | Method of anti-corrosive protection of silicon carbide products |
Related Parent Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA000355576A Division CA1177228A (en) | 1979-07-07 | 1980-07-07 | Method of anti-corrosive protection of silicon carbide products |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1188078A true CA1188078A (en) | 1985-06-04 |
Family
ID=25669113
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA000453202A Expired CA1188078A (en) | 1979-07-07 | 1984-04-30 | Method of anti-corrosive protection of silicon carbide products |
Country Status (1)
| Country | Link |
|---|---|
| CA (1) | CA1188078A (en) |
-
1984
- 1984-04-30 CA CA000453202A patent/CA1188078A/en not_active Expired
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Legal Events
| Date | Code | Title | Description |
|---|---|---|---|
| MKEX | Expiry |