Surface treatment of carbon steel and low alloy steel by phosphating
Abstract: Phosphating is a surface treatment technology for carbon steel and low alloy steel, which is widely used in fields such as automobile, motorcycle, bridge and building. It has the advantages of good anti-corrosion and wear-resistance, little pollution to environment, short process time, high efficiency, low cost and so on. The purpose of this paper is to introduce the process of phosphating surface treatment, analyze the mechanism of phosphating process and analyze the current development status of phosphating surface treatment.
1 Introduction
Nowadays, with the rapid development of economy and life, the requirements of steel products are increasingly reduced. At the same time, the environment of the use of the products is becoming more and more severe, and the requirements for anti-corrosion and wear-resistance are increasing. Therefore, surface treatment technology has become the key to improve the performance of steel products. Phosphating surface treatment is an effective surface treatment technology for carbon steel and low alloy steel. It has the following advantages: strong adhesion and corrosion resistance, low cost, fast process, no pollution and easy operation.
2 Process of Phosphating
Phosphating is a chemical process for surface treatment of carbon steel and low alloy steel. It is generally divided into three steps: pre-treatment, phosphating and post-treatment.
2.1 Pretreatment
Pre-treatment is the primary condition for phosphating. Generally speaking, it includes pickling and degreasing, de-scaling and rust removal. The purpose of pre-treatment is to remove impurities, oxide scale, grease and oil stains on the surface of steel parts, and to reveal the active sites of the surface of steel parts which are conducive to the reaction of phosphating.
2.2 Phosphating
Phosphating is the main reaction process of phosphating. In this process, zinc phosphate, manganese phosphate or iron phosphate solutions are used as immersion baths to dissolve phosphates and form insoluble phosphates on the surface of stee. The insoluble phosphates form a protective film on the surface of steel to improve the anti-corrosion and wear-resistance.
2.3 Post treatment
Post-treatment is to further improve the performance of the protective film formed by phosphating. It includes passivation treatment and lubrication treatment. The passivation treatment can fill up the micro-holes on the surface of the phosphating film and improve its alkali resistance. The lubrication treatment can further enhance the adhesion strength, corrosion resistance and wear resistance of the phosphating film.
3 Mechanism of phosphating
The mechanism of phosphating is mainly composed of three parts: hydrogen bond formation, electrostatic attraction and ion exchange.
3.1 Hydrogen bonding
Hydrogen bonding is a kind of physical bond formed by the sharing of a pair of electrons between two atoms or molecules. In the reaction of phosphating, a hydrogen bond is formed between the metal surface and the phosphate ions, which not only enhances the adhesion of the protective film, but also improves its corrosion resistance.
3.2 Electrostatic attraction
Due to the difference of electrostatic potential in the phosphating reaction, an electrostatic attraction force is generated between the phosphating and the metal surface. The electrostatic attraction force plays an important role in the formation of the protective film, which not only improves the adhesion of the film, but also enhances the wear resistance of the film.
3.3 Ion exchange
The ion exchange is a kind of surface reaction based on the exchange of ions between the metallic surface and the phosphating solution. The exchange of ions can not only change the electrochemical potential of the surface of the metallic material, but also promote the formation of insoluble phosphates on the surface of the material, so as to establish a stronger protection film.
4 Current status of phosphating
Phosphating surface treatment has been widely applied in automobile, motorcycle, bridge and building industry. At present, there are two popular phosphating processes in the industry, namely zinc phosphate and iron phosphate.
The first is zinc phosphate process. This process has the advantages of low cost and high corrosion resistance of the film, and is suitable for general steel parts. The second is iron phosphate process. This process has a longer process time and higher cost compared with zinc phosphate process, but has higher corrosion resistance and wear resistance, so it is suitable for steel parts with higher requirements.
At present, phosphating has gradually developed from a single process to a multi-process combination process, i.e. to combine pre-treatment, phosphating and post-treatment processes together. This helps to further improve the performance of the phosphating film. In the future, in order to meet the increasing requirements of automotive industry, phosphating process will further develop and perfect in the aspects of continuous operation, automatic control and environment protection.
5 Conclusion
Phosphating is an important method for surface treatment of carbon steel and low alloy steel. Its advantages of low cost, fast process and no pollution are widely recognized. The magnesium phosphate process has become increasingly popular in the industry due to its superior performance. The development of phosphating process in recent years has made great progress in terms of process development, automation and environmental protection. In the future, we should continue to optimize the process parameters, improve the automation of production and reduce the environmental pollution to meet the requirements of application market.
