Aluminum diffusion coating, also known as aluminizing coating, is a process that enhances the oxidation and corrosion resistance by infiltrating aluminum elements into the metal surface, and is widely used in occasions that require high temperature oxidation resistance and corrosion resistance. This article describes in detail the properties, preparation process, and practical application of aluminum diffusion coatings in industry.
Performance analysis
Resistance to high-temperature oxidation
The most significant property of the aluminium diffusion coating is its excellent oxidation resistance at high temperatures. The aluminum oxide film formed on the surface of the workpiece effectively isolates oxygen and thus protects the substrate. Studies have shown that when the thickness of the aluminized layer reaches more than 0.3mm, the coating can significantly improve the oxidation resistance of steel at high temperatures from 750°C to 1200°C. Especially in the continuous high temperature environment, the greater the thickness of the aluminized layer, the better the oxidation resistance.
Corrosion resistance to hydrogen sulfide environments
In high-temperature environments containing hydrogen sulfide, steel often faces serious corrosion problems. The aluminized coating exhibits excellent corrosion resistance in hydrogen sulfide media due to its passivation layer. The oxide film of aluminum is not only resistant to oxidation, but also acts as a barrier in corrosive gases.
Resistance to acids and corrosion in a variety of corrosive gas environments
The aluminizing coating not only enhances the corrosion resistance of the steel in acid solution, but also improves its corrosion resistance in sulfur-containing gases (such as V0, NaSO4, etc.).
Changes in mechanical properties
The aluminizing process has an effect on the grain structure of the steel at high temperatures. The study shows that the surface hardness of the aluminized workpiece can reach HV500, but its fatigue strength decreases with the increase of the thickness of the aluminized layer. This change is mainly due to the coarse grain size after high temperature diffusion, resulting in a slight decrease in plasticity and toughness. These properties can be improved by quenching and tempering, restoring a certain strength and toughness.
Formation and organizational structure
Method of formation of aluminized coating
The formation of an aluminized coating is mainly done by two methods: melt infiltration and chemical reaction infiltration. The former relies on the mutual miscibility of molten aluminum and the surface of the workpiece to form an aluminum-rich alloy layer, which usually adopts liquid aluminization, thermal spray aluminization or vacuum evaporation process. The latter relies on a chemical reaction to infiltrate the surface of the workpiece, with typical methods including solid aluminizing and gas aluminization.
Tissue structure of aluminized coating
The microstructure of the aluminizing coating varies depending on the aluminizing method. Generally, the structure of the aluminized layer from the matrix to the surface layer is a solid solution and a series of metal compound phases, such as FAI, FeAI, FeA1 phase, etc. These structures improve the compactness and corrosion resistance of the coating.
Preparation process
Thermal spray aluminization
The thermal spray aluminizing process includes steps such as surface pretreatment, aluminum spraying, paint protection, and diffusion annealing. Through high-temperature annealing, the metallurgical combination of the coating and the substrate can be realized, and the oxidation resistance of the material can be effectively improved. The thickness of the coating is recommended to be 0.3~0.5mm to ensure the anti-oxidation effect.
Solid aluminized
The solid aluminizing process achieves the penetration of aluminum into the matrix by heating and diffusing the workpiece together with a powdered aluminizing agent. The process requires the selection of appropriate temperature and time control to prevent coarse grains.
Gas aluminization and slurry aluminization
Gas aluminizing uses the reaction of aluminum chloride at high temperature to form an aluminum permeable layer, which is suitable for some specific occasions. Slurry aluminization is done by coating the surface of the workpiece with a slurry and then diffusing heat to form a coating.
Application of aluminium diffusion coatings
Anti-corrosion applications
Aluminum diffusion coatings can be used in equipment that resists hydrogen sulfide corrosive environments, such as heat exchangers, sulfuric acid converters, etc. These applications take advantage of the corrosion-resistant properties of the coating to protect the substrate from corrosive gases.
Resistance to high-temperature oxidation applications
Aluminized coatings exhibit excellent oxidation resistance in high-temperature environments and are widely used in heat treatment fixtures, high-temperature fasteners, and refinery equipment. The coating's resistance to high-temperature oxidation makes it a reliable protective coating material.
summary
The aluminium diffusion coating significantly improves the resistance to high-temperature oxidation, corrosion and mechanical properties of the steel through the aluminizing process. Its excellent antioxidant properties make it widely used in harsh environments such as high temperature and hydrogen sulfide, which prolongs the service life of the material. Different aluminizing processes (e.g. solid aluminization, liquid aluminization, etc.) can be selected according to the needs to further improve the quality and effect of the coating.
