Oxidation resistance of graphite materials

It is well known that in the manufacturing process of graphite products, the pyrolysis and condensation of organic carbonaceous materials make the artificial graphite material porous, and the general porosity is about 20% to 30%. Most of these are open pores, which lead to an increase in the diffusion rate and diffusion depth of the oxidizing gas, and the deterioration of the oxidation resistance. Generally, oxidation begins at about 400°C in the air. One of the most important measures to prevent the oxidation of graphite materials is to reduce the contact area between the graphite material and oxygen, make it essentially covered with an antioxidant material on the pores or active centers of the graphite material so that the surfaces are not directly exposed to the air. At present, the anti-oxidation methods of graphite materials can be roughly divided into three types: one is surface coating; the other is vapor deposition; and third, impregnation. Among them, the impregnation method has the advantages of simple equipment, easy operation, remarkable effect, and good economical efficiency, and is increasingly receiving attention.
1 surface coating method
The coating method is to spray a layer of oxidation-resistant coating on the surface of the graphite material. Generally, the coating is different according to the coating substance, the coating process is different, and the properties of the manufactured product are also different.
(a) Spray metal, carbide, silicide, etc.
Sprayed 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 the graphite electrode. The commonly used coating process is aluminum and refractory “alternating spraying and burning method” on the electrode surface to obtain an anti-oxidation coating graphite electrode. The anti-oxidation coating must meet the following requirements: 1) It can withstand high temperature without melting, the coating decomposition temperature is above 1850°C; 2) it is well combined with the electrode surface, and Has a similar thermal expansion coefficient; 3) The conductivity is higher than the graphite electrode substrate; 4) It should have a certain mechanical strength; 5) No adverse effects on the metallurgical operation and molten steel quality. Graphite electrodes with an anti-oxidation coating can reduce the electrode consumption per ton of steel by 20% to 30% compared with electrodes of the same quality, and can reduce the power consumption of steel making by about 5%, and because the resistance of the oxidation resistant coating is small, can increase the current density through the electrode, reducing the electrode diameter.
However, due to the increase in the production cost of the graphite electrode of the anti-oxidation coating by 8% to 10%, and the use of the anti-oxidation coating electrode in the electric furnace steel plant requires technical transformation of the electrode holder of the steelmaking furnace, and the amount of the transformation project and the investment amount are relatively large. , And bring some trouble to the steelmaking operation.
(b) Boron compound
This method is a surface coating of graphite products containing a boride-containing carbonizable liquid, its oxidability can be greatly reduced, the test at 820 °C in still air shows that the uncoated graphite products’ oxide loss is 100%, and contains The boride-coated graphite products have a loss of oxidation of only 1%. This method is mainly used in the manufacture of aircraft brakes and so on.
(c) Coated mullite, etc.
The coating material contains 20 to 70% of mullite (3Al2O32SiO2), 5 to 14% of silicon, 5 to 70% of silicon carbide or boron carbide, and the coating is sprayed on the graphite product and then dried and heat-treated. That is, the surface of the graphite product is well bonded and can withstand severe thermal expansion. This method is mainly applied to the casting molds, blast furnace refractory blocks, and blast furnace blocks.
2 Vapor deposition
Pyrolytic charcoal and pyrolytic graphite have good high temperature resistance and corrosion resistance, so the deposition of a certain thickness of pyrolytic carbon or pyrolytic graphite on the surface of graphite material by chemical vapor deposition method can improve the oxidation resistance of graphite materials, but The most serious drawback of pure pyrolytic carbon coatings is their large anisotropy and they are prone to spalling in practical applications. If during the deposition of pyrolytic carbon, the needle-shaped silicon carbide crystals are deposited on the deposited carbon, and the axial direction of the crystal is perpendicular to the base surface of the pyrolytic carbon, the anisotropy of the pyrolytic carbon can be reduced, and can increase the pyrolysis carbon’s C axial’s strength, improve other properties such as expansibility. In addition, the vertically embedded acicular silicon carbide destroys the layered structure of the pyrolytic carbon, thereby reducing the spalling of the pyrocarbon. Since silicon forms SO2 when oxidized, it melts to form a protective layer. The product’s oxidation resistance and corrosion resistance are enhanced. However, due to its high cost and only for small-size products, the technology is currently mainly used in aerospace materials and other fields.
3 dipping method
The impregnation method is to impregnate the graphite material with an oxide-resistant material to reduce the porosity of the graphite product so as to achieve the purpose of reducing the contact surface between carbon and oxygen and improve the oxidation resistance of the graphite material. In order to achieve satisfactory results, different impregnation methods should be adopted for the specific conditions of the impregnating agent.
(a) Phosphoric acid solution
This process is simple and easy. The oxidation resistance of the obtained graphite products is 760°C or more. The specific method is to immerse graphite products in phosphate-containing phosphoric acid solution for about 10 minutes. The soaked article is then heated to at least 500°C for about 5 to 10 minutes. Oxidation tests at 680°C and 800°C showed that the oxidation resistance of the soaked graphite products was significantly stronger than that of the unsoaked graphite products.
(b) Diborosilicate glass
The borosilicate glass is placed in a stainless steel crucible in an immersion apparatus, vacuum heated and melted, put in graphite product, and the ammonia gas is input for a certain period of time so that the molten glass is immersed in the graphite product. Impregnated graphite products have greatly improved performance compared to pre-impregnated graphite products. Among them, the compressive strength has doubled, the flexural strength has more than doubled, the porosity has decreased significantly, and the resistivity has hardly changed. The oxidation resistance is enhanced and the oxidation loss is significantly reduced.
(c) Dipping antioxidants
The impregnating agent is an antioxidant, and the antioxidant is a mixture of one or two or more alkaline earth metal inorganic salts and one or two kinds of phosphoric acid or phosphate.and a small amount of Si, Ca and other metal salts are added to the oxidation preventing mixturecan make conductive additives and arc stabilizers. With this can produce electrode products with good arc resistance and conductivity. The graphite products impregnated with antioxidants have been tested at 700°C for 6 hours and show no oxidation on the surface of the graphite products.
(d) Immersion of metal salts
A saturated solution of a refractory metal salt is used as an impregnant. Before the impregnated graphite product is heated at a high temperature, the graphite product is soaked with an ammonium hydroxide or ammonium hydrogen sulfate aqueous solution for 10 minutes to convert the refractory metal salt deposited in the pore into an oxide. . Then, the graphite article is heated at a high temperature in a vacuum or a protective gas to convert the metal oxide into a carbide. This method can be applied to the tip of a graphite electrode, especially under smelting conditions that control carbon concentration.
The method of anti-oxidation treatment of graphite materials includes mixing method and dip coating method in addition to the above three methods. The mixing method is mixed with refractory metal or metal powder during the production of graphite products. The dip coating method combines the advantages of the impregnation method and the coating method, greatly reducing the contact surface area of ​​the product with oxygen. Of course, this treatment method depends on the use of the product and the treating agent used.
Artificial graphite electrode is usually treated by impregnation method. The cost of impregnation solution and the cost of impregnation treatment and related treatment are not very high. Therefore, it is the most effective method to improve the quality of electrode and enhance the oxidation resistance of graphite electrode in smelting.
Author：Ling Wang, Bin Xu