Chemical oxidation of steel
Chemical oxidation of steel involves immersing a steel workpiece in a specific chemical solution. Through a chemical reaction, a layer of oxide film (commonly known as “bluing” or “blackening”) forms on the surface of the workpiece, primarily composed of iron oxide. This film typically appears bluish-black or black, with a thickness of 0.5-2 microns. Although thin, it effectively prevents the steel surface from coming into contact with corrosive media such as air and water, providing a degree of protection. Furthermore, the oxide film absorbs lubricants, improving the wear resistance of steel parts. It is commonly used in the surface treatment of mechanical parts, tools, weapons, and more. For example, chemical oxidation treatment of machine tool guide rails not only prevents rust but also reduces friction and wear between the guide rails and the slide, extending their service life.
The principle of chemical oxidation of steel is to use an oxidizing agent to react with iron under certain conditions, forming a dense oxide film. Commonly used oxidizing solutions are alkaline solutions, primarily composed of sodium hydroxide, sodium nitrite, and sodium nitrate. Sodium hydroxide provides the alkaline environment, while sodium nitrite and sodium nitrate serve as oxidants. At high temperatures (typically 130-145 °C), the iron on the steel surface reacts with the oxidizing agent in the solution to form a ferroferric oxide ( Fe₃O₄ ) oxide film. The reaction process can be divided into three stages: First, iron is oxidized to ferrous ions in the alkaline solution; then, the ferrous ions are further oxidized to ferric ions; finally, the ferrous and ferric ions combine with hydroxide ions to form ferroferric oxide precipitates, which adhere to the steel surface, gradually forming a continuous oxide film. The oxide film is firmly bonded to the substrate and exhibits good adhesion.
The chemical oxidation process for steel primarily involves pretreatment, oxidation, and post-treatment. Pretreatment aims to remove oil, rust, and scale from the steel surface, ensuring uniform formation of the oxide film. Pretreatment steps include degreasing, pickling, and water washing. Degreasing can be performed by soaking in an alkaline solution or wiping with an organic solvent to remove surface grease. Pickling typically uses hydrochloric acid or sulfuric acid solutions to remove rust and scale. Water washing removes residual degreasing agent and acid to prevent them from interfering with the oxidation reaction. Oxidation involves immersing the pretreated workpiece in a high-temperature alkaline oxidizing solution for 30-60 minutes, depending on the required oxide film thickness. The solution should be stirred regularly during the treatment process to ensure a uniform reaction. Post-treatment includes water washing, neutralization, drying, and oil immersion. Water washing removes residual oxidizing solution from the surface. Neutralization uses a weak acid solution (such as acetic acid) to neutralize residual alkaline substances. Drying is performed at 80-100°C to prevent the oxide film from absorbing water. Oil immersion (such as engine oil or rust-proof oil) fills the pores of the oxide film and improves corrosion resistance.
Chemical oxidation technology for steel boasts advantages such as simplicity, low cost, and high production efficiency. Compared with electroplating and painting processes, it requires minimal equipment investment, requiring only simple equipment such as oxidation tanks and heating devices. Processing time is short, making it suitable for mass production. The oxide film produces a uniform color and offers a certain decorative effect. However, this technology also has limitations: the oxide film is thin and has limited corrosion resistance, requiring additional protective measures (such as oil immersion or painting) in humid or highly corrosive environments. The oxide film also has low hardness and poor wear resistance, making it unsuitable for highly loaded friction parts. The alkaline oxidation solution is hot, consumes significant energy, and operates in a harsh operating environment, requiring protective measures.
With the development of industrial technology, steel chemical oxidation technology has continued to make progress in process optimization and environmental improvement. By adjusting the formula of the oxidation solution, such as adding catalysts or surfactants, the density and corrosion resistance of the oxide film can be improved; using a low-temperature oxidation process (such as below 100°C) can reduce energy consumption and operational difficulty. In terms of environmental protection, we develop low-toxic and low-pollution oxidation solutions to reduce the use of harmful substances such as sodium nitrite. At the same time, we strengthen wastewater treatment to achieve standard discharge of harmful substances. In addition, combining chemical oxidation with other surface treatment technologies, such as phosphating after oxidation or applying organic coatings, can significantly improve the comprehensive properties of steel surfaces. In the future, chemical oxidation technology for steel will continue to play an important role in the field of low-cost protection, while developing in a more environmentally friendly and efficient direction.