Application and Process Flow of Anti-rust Phosphating Process
Introduction
Phosphating is widely used in the finishing of metal parts because of its sufficient coating thickness, good anti-rust and anti-corrosion property and low cost. Phosphating film has relatively high chemical stability and mechanical properties, and it can not only absorb paints, but also improve the performance of the coating. It is widely used in automotive parts, agricultural machinery and engineering machinery, etc.
Process Flow
The protective corrosion process for phosphating film can be divided into degreasing, desmutting, prewashing and activating, pickling, phosphating, rinsing and passivation, etc. It is mainly used for anti-rust and anti-corrosion treatment of metal surface forming a thin, uniform and stable zinc-iron or zinc-iron-manganese phosphate composite film.
1. Degreasing
It is to remove the grease and oil on the surface of the parts, preventing the process of forming the film. Generally, alkaline detergent, coal tar emulsion or degreasing gel is used for degreasing.
2. Desmutting
In this step, it is used to remove the impurities remaining on the workpiece such as residual soap film and stubborn dirty. There are generally two kinds of desmutting processes: chemical desmutting and mechanical desmutting. Chemical desmutting refers to the use of a reducing agent or chelating agent to chemically react with the sulfide, while mechanical desmutting means to remove the smut on the surface of the parts by chemical-mechanical means.
3. Pre-washing
This process is to make the surface of the parts enter the activation agent uniformly and reliably, preventing the activation reaction from attenuation due to the presence of grease, oil and other organic substances.
4. Activation
The activation step is used to chemical reaction with the iron on the surface of the parts to form a iron oxide film to provide condition for the subsequent phosphating process. Generally, the activator adopts piperazine-containing activator and acetic acid activator.
5. Pickling
This process is to further remove the oxide skin, rust, scale and other impurities on the surface of the parts to make the parts enter the phosphating process step and ensure the adhesion of the phosphating film.
6. Phosphating
This is the main part of the protective corrosion process for phosphating film. Generally, the single layer phosphate film is formed by the chemical reaction of phosphoric acid-zinc salt at room temperature, and two-layer phosphate film is formed by adding tin and manganese salt at room temperature.
7. Rinsing
This process is to clean the parts to remove the residual acid and salt ions in the phosphating, ensuring that contaminants will not enter the passivation process to affect the performance of the protective film.
8. Passivation
This step is to protect the phosphating film and improve its salt fog resistance, acid and alkali resistance properties. Generally, chromic acid passivation and nitric acid passivation are adopted. Chromate passivation belongs to conversion film passivation, and it is a chemical reaction to form a film layer. Nitric acid passivation belongs to film-forming passivation, and it is a physical adsorption of a film layer.
Conclusion
Though anti-rust phosphating process is simple and the cost is low, it should be recognized. The formation of phosphating film with strong anti-rust and anti-corrosion effects depends on the stability and uniformity of the process parameters of each step. Therefore, attention should be paid to controlling the processing time, temperature, phosphating concentration and parts flow rate to ensure the forming of the phosphating film with excellent properties.
