As is well known, during the manufacturing process of graphite products, the thermal decomposition and condensation of organic carbon based raw materials form porous artificial graphite materials, with a general porosity of around 20% to 30%. Most of them are open pores, which increases the diffusion speed and depth of oxidizing gases, and deteriorates their antioxidant properties. Generally, oxidation begins at around 400 ℃ in air. One of the main measures to prevent the oxidation of graphite materials is to reduce the contact area between graphite materials and oxygen. Essentially, it is to cover the pores or active centers of graphite materials with antioxidant substances, so that their surfaces are not directly exposed to air. At present, the oxidation resistance methods of graphite materials can be roughly divided into three types: first, surface coating; The second is vapor deposition; The third is impregnation. The impregnation method has the advantages of simple equipment, easy operation, significant effects, and good economy, and is increasingly being valued.

1 Surface coating method

The coating method is to spray a layer of oxidation resistant coating on the surface of graphite material. Generally, the coating varies depending on the coating material, the coating process is different, and the characteristics of the resulting product are also different.

(a) Spraying metals, carbides, silicides, etc

The materials used for spraying include metals, carbides, silicides, and a mixture of the above materials. A layer of aluminum based metal ceramics is sprayed or fused on the surface of the graphite electrode. The commonly used coating process is the "alternating spraying and melting method" of aluminum and refractory materials on the electrode surface, which can obtain an anti oxidation coating graphite electrode. The anti oxidation coating must meet the following requirements: 1) It can withstand high temperatures without melting, and the coating decomposition temperature is above 1850 ℃; 2) Good adhesion with the electrode surface and similar coefficient of thermal expansion; 3) The conductivity is higher than that of graphite electrode substrate; 4) Should have a certain mechanical strength; 5) There is no adverse effect on metallurgical operations and steel quality. Compared with electrodes of the same quality, graphite electrodes with antioxidant coatings can reduce electrode consumption per ton of steel by 20% to 30%, and reduce steelmaking electricity consumption by about 5%. Moreover, due to the low resistance of the antioxidant coating, the current density passing through the electrode can be increased, and the electrode diameter can be reduced.

However, due to an 8% to 10% increase in the manufacturing cost of graphite electrodes with antioxidant coatings, and the need for technical modifications to the electrode holders of the steelmaking furnace when using antioxidant coated electrodes in electric furnace steel plants, the amount of modification work and investment are large, and it also brings certain troubles to steelmaking operations. Therefore, with the promotion and application of high-power and ultra-high power graphite electrodes, China currently basically does not produce antioxidant coated graphite electrodes.

(b) Boride coating

This method involves coating the surface of graphite products with a layer of carbonizable liquid containing borides, which can greatly reduce their oxidation resistance. Experiments in static air at 820 ℃ show that the oxidation loss of uncoated graphite products is 100%, while the oxidation loss of graphite products with boride coatings is only 1%. This method is mainly used for manufacturing aircraft brake pads, etc.

(c) Coated with aluminum rich andalusite, etc

The coating material contains 20-70% aluminum rich andalusite (3Al2O32SiO2), 5-14% silicon, and 5-70% silicon carbide or boron carbide. The coating is sprayed onto graphite products, and through dry heat treatment, the coating adheres well to the surface of the graphite products and can withstand severe thermal expansion. This method is mainly applicable to crucibles, molds for casting molten refractory materials, blast furnace blocks, etc.

2. Vapor deposition method

Pyrolysis carbon and pyrolytic graphite have good high-temperature resistance and corrosion resistance. Therefore, depositing a certain thickness of pyrolytic carbon or pyrolytic graphite on the surface of graphite materials through chemical vapor deposition can improve the oxidation resistance of graphite materials. However, the most serious drawback of pure pyrolytic carbon coatings is their high anisotropy, which makes them easy to peel off in practical applications. If needle shaped silicon carbide crystals are embedded on the deposited pyrolysis carbon during the deposition process, and the axis of the crystals is perpendicular to the basal plane of the pyrolysis carbon, the anisotropy of the pyrolysis carbon can be reduced, the strength of the C-axis of the pyrolysis carbon can be increased, and other properties such as expansion can be improved. In addition, the vertically embedded needle shaped silicon carbide disrupts the layered structure of the pyrolysis carbon, thereby reducing the layer peeling of the pyrolysis carbon. Due to the formation of SO2 during oxidation, silicon melts to form a protective layer, thereby enhancing the oxidation and corrosion resistance of the product. However, due to its high cost and limited applicability to small-sized products, this technology is currently mainly applied in fields such as aerospace materials.