Chromium diffusion coatings (chrome wetting coatings) are widely used in a wide range of high-performance applications due to their excellent corrosion, oxidation and abrasion resistance
Corrosion resistance
The chromium element in the chromium diffusion coating can significantly improve the corrosion resistance of the steel. When the chromium content reaches 12.5%, an iron-chromium alloy structure with good corrosion resistance is formed. Chromized steel performs well in a variety of corrosive environments, and can resist the erosion of humid atmosphere, water, strong alkali, superheated steam, nitric acid and other media. Its corrosion resistance is even comparable to that of high-nickel stainless steel, especially in acidic environments. Even so, the corrosion resistance of the chrome coating may be compromised if there are surface defects such as porosity or inclusions.
Antioxidant properties
The chromium infiltration coating has outstanding performance in oxidation resistance, especially in the temperature range of 700~800 °C, and its high temperature oxidation resistance is better than that of many other alloy steels. However, when the temperature exceeds 900°C, the oxidation rate accelerates due to the secondary diffusion of chromium, and the oxidation resistance of the coating begins to decline. As a result, chromium coating is suitable for medium to high temperature operating environments, but has limited oxidation resistance to very high temperatures.
Hardness and abrasion resistance
The chrome coating has a very high hardness and can effectively improve the wear resistance of steel. When the chromium infiltration temperature is 850~1000°C, the coating thickness is 10~20μm, and the hardness can reach HV1400~2000. Depending on the carbon content of the steel, the hardness of the chrome coating will vary, and usually the chrome layer of medium carbon steel will be harder and more wear-resistant.
Solid powder chromium
Solid powder chromium is a traditional and commonly used method of chromium wetting. In this process, the workpiece is embedded in a powder containing chromium powder, chemical catalysts (such as halides) and fillers, heated to high temperatures, and the chromium element is diffused to the surface of the steel to form a chromium coating. The process is easy to operate, but the problem of dust pollution is more serious.
Molten salt chromium
Molten salt chromium infiltration is carried out in a high-temperature molten salt bath, which uses the activity of the salt to promote the diffusion of chromium elements to the surface of the workpiece by immersing the workpiece in a molten salt solution. The commonly used molten salt is borax, with the addition of chromium powder or ferrochrome powder and reducing agents such as aluminum powder. The advantages of the molten salt chromium infiltration method are that the raw material is economical, the equipment is simple, and it is suitable for large-scale production.
Gas chromium
Gas chromium infiltration is the process of generating chromium halide vapor at high temperatures and allowing these vapors to react with the surface of the workpiece to form a chromium layer. Although the gas chromium infiltration speed is fast and the quality of the seepage layer is better, there are problems of gas explosion, toxicity and corrosiveness, which requires stricter process control.
Chromium coating is widely used in engineering materials that require abrasion, corrosion and high temperature resistance. For example, molten salt chromium is often used to make high-strength, wear-resistant parts such as sharpening rods, steel files, high-pressure valves, and burner nozzles. The chrome coating significantly increases the service life of the workpiece, especially under extreme conditions.
Chromium coatings are an indispensable part of a wide range of high-performance engineering materials due to their excellent corrosion, oxidation and abrasion resistance. With the continuous improvement of the process, the preparation methods of chromium coating are becoming more and more diverse, and the application fields are also expanding. In the future, as the demand for new materials increases, chrome coatings will play an even more important role in the industrial sector.