It is well known that in the manufacturing process of graphite products, the pyrolysis and polycondensation of organic carbon materials make the artificial graphite materials form porous properties. Generally, the porosity is about 20-30, and most of them have 121 pores, which leads to the increase of diffusion speed and diffusion depth of oxidizing gas, and the oxidation resistance is poor. Generally, oxidation begins at about 400 ℃ in the air. One of the main measures to prevent the oxidation of graphite material is to reduce the contact area between graphite material and oxygen. In essence, the oxidation-resistant material is used to cover the pores or active center of graphite material, so that its surface is not directly exposed to the air. At present, there are three kinds of oxidation resistance methods for graphite materials: one is surface coating. The second is vapor deposition; the third is the impregnation port, in which the impregnation method has the advantages of simple equipment, easy operation, remarkable effect and good economy, which has been paid more and more attention.
Surface coating method:
Coating method is to spray a layer of oxidation resistant coating on the surface of graphite material. Generally, the coating varies according to the coating materials, the coating process is different, and the characteristics of the products are also different. (a) Spraying metals, carbides, silicides, etc. spraying materials include metals, carbides, silicides and mixtures of the above materials. A layer of aluminum based cermet is sprayed or fused on the surface of graphite electrode. The commonly used coating process is "alternate spraying sintering method" of aluminum and refractory on the electrode surface ”The oxidation resistant coating graphite electrode must meet the following requirements: 1) it can withstand high temperature without melting, and the decomposition temperature of the coating is above 1850 "C; 2) it has good bonding with the electrode surface and has similar thermal expansion coefficient; 3) its conductivity is higher than that of graphite electrode substrate; 4) it should have certain mechanical strength; 5) it has no effect on metallurgical operation and molten steel quality Adverse effects. Compared with the electrode of the same quality, the graphite electrode with anti-oxidation coating can reduce the electrode consumption of each ton of steel by 20-30, and reduce the power consumption of steelmaking by about 5. Moreover, due to the low resistance of the anti-oxidation coating, the current density passing through the electrode can be increased and the electrode diameter n can be reduced. However, the manufacturing cost of graphite electrode with anti-oxidation coating is increased by 8-10, and the electrode holder of steel-making furnace needs to be technically reformed due to the use of anti-oxidation coating electrode in electric furnace steel plant, which has a large amount of engineering and investment, and brings some troubles to steelmaking operation. Therefore, with the popularization and application of high-power graphite electrode, oxidation-resistant coated graphite electrode is not produced in China. (b) boride coating this method is to coat the surface of graphite products with a layer of carbonizable liquid containing boride, and its oxidation resistance can be greatly reduced. The oxidation loss of uncoated graphite products is 100, while that of graphite products with boride coating is only 1. This method is mainly used to manufacture aircraft brake pads. (c) The coating material consists of 20-70 aluminum rich andalusite (3a io82sio 2), 5-14% silicon, 5-70 silicon carbide or boron carbide. The coating is sprayed on the graphite products. After drying and heat treatment, the coating is well bonded to the surface of graphite products and can withstand severe thermal expansion. This method is mainly suitable for crucible, mold for casting molten refractory, blast furnace block, etc.
Vapor deposition method:
Pyrolytic carbon and pyrolytic graphite have good high temperature resistance and corrosion resistance, so the oxidation resistance of graphite materials can be improved by depositing a certain thickness of pyrocarbon or pyrolytic graphite on the surface of graphite materials by chemical vapor deposition It is easy to peel off in practical application. If the needle like SiC crystal is embedded in the pyrocarbon deposition process, and the axial direction of the crystal is perpendicular to the base plane of the pyrocarbon, the anisotropy of the pyrocarbon can be reduced, the c-axis strength of the pyrocarbon can be increased, and other properties, such as expansibility, can be improved. In addition, the needle like SiC embedded vertically destroys the pyrocarbon layer Thus, the spalling of pyrolytic layer is reduced. The oxidation resistance and corrosion resistance of the product are enhanced due to the formation of so during oxidation and the formation of a protective layer by melting. However, due to its high cost and being only suitable for small-sized products, the technology is mainly used in aerospace materials and other fields.
Impregnation method is to impregnate graphite materials with oxidation resistant materials to reduce the porosity of graphite products, so as to reduce the contact surface between carbon and oxygen and improve the oxidation resistance of graphite materials. In order to achieve satisfactory results, different impregnation methods (a) should be used to soak phosphoric acid solution according to the specific conditions of impregnant_ The oxidation resistance of graphite products is 760 ℃ or higher. The specific method is to soak the graphite products in phosphoric acid solution containing phosphate for about 10 minutes. Then, soak the