Among many surface treatment processes, one process can make the surface of the product black, rainbow or gray near the metal color. This process is phosphating process.

What is phosphating

Phosphating is a kind of chemical reaction, which forms a layer of film on the surface.

Phosphating process is mainly used on metal surface, which aims to provide a protective film for metal surface, which can insulate metal from air and prevent corrosion. It can also be used for bottoming before painting of some products. With this coating, it can improve adhesion and corrosion resistance of paint layer, improve decoration, make metal surface look more beautiful, and can also be cold in some metals It plays a role of lubrication in the process of processing.

After phosphating, the workpiece can not be oxidized and rusted for a long time. Therefore, phosphating is widely used and is also a common metal surface treatment technology. It is used more and more in automobile, ship, machinery manufacturing and other industries.

However, phosphating treatment also has the disadvantages of many solution sediment, rough surface, high phosphating temperature, long time and high cost.

The history of phosphating

In fact, phosphating process has been developing for a long time. It should be one of the earliest invention times in modern metal surface treatment, and its development has passed through different periods.

In 1869, it was found that phosphating film could be used on metal surface, which could effectively protect metal from corrosion for a long time, and applied for patent at that time, which laid a foundation for the technology and development of phosphating treatment.

From the beginning of the 20th century, phosphating treatment began to be used in industrial products, which also promoted the development and progress of phosphating technology. From then on, phosphating treatment has been developed rapidly and entered the practical application period.

In order to meet various demands, phosphating process is also improving in modern times, mainly towards the direction of low temperature, low slag, environmental protection and non-toxic.

Classification and application of phosphating

Usually, a kind of surface treatment will show a color, but phosphating treatment can show different colors by using different phosphating agents according to the actual needs, which means that we often see that phosphating treatment has gray, color or black.

Iron phosphating

After phosphating, the surface will show rainbow color and blue color, so it is also called color phosphorus. The phosphating solution mainly uses molybdate as raw material, and forms rainbow color phosphating film on the surface of steel material, and is mainly used for coating the bottom layer to achieve the anti-corrosion ability of the workpiece and improve the binding force of the surface coating.

Zinc phosphating

The color is gray, so it is called grey film phosphating. The phosphating solution mainly consists of phosphoric acid, sodium fluoride and emulsifier, which will form gray phosphating film on the surface of the workpiece. It is also mainly used for the coating bottom layer, which is combined with the following spraying, painting or electrophoresis.

The gray phosphating film itself has certain corrosion resistance, so it can also be used as a corrosion-resistant coating on the surface of the workpiece, mainly used for some galvanized plate, cold rolled plate and surface phosphating of aluminum plate.

Manganese phosphating

The color is black or black gray, so it is also called black phosphating. It is mainly used with phosphating solution containing manganese ions, which will form a black phosphating film on the surface of the workpiece. It is also the best phosphating treatment with the best rust resistance performance. It can be used as a long-term rust prevention of parts and parts, and it is also the most widely used rust proof phosphating.

Manganese phosphating can have a smaller friction coefficient than other phosphating, so for those parts which often rub, black phosphating can be used to effectively reduce friction, such as some automobile parts and some fasteners.
 
Phosphating treatment

The main part of phosphating process is phosphating solution. The phosphating solution used by different phosphating will be different. After putting metal workpiece into phosphating solution, chemical reaction will occur between them, forming an iron phosphate element and hydrogen and oxygen.

The iron carbonate will crystallize slowly and deposit on the metal surface. When the crystallization is more and more, it will form a solid phosphating film on the metal surface.

After the workpiece is put into phosphating solution, it needs to shake for several minutes, mainly to exhaust the air and air bubbles trapped in the groove of the workpiece. The phosphating time is generally determined according to different work piece structure, material and temperature, usually about 30 minutes.