Among the many surface treatment processes, there is a process that can make the surface of the product turn black, rainbow color or gray, which is close to the metal color, this process is the phosphating process.
What is phosphating
Phosphating is a chemical reaction that forms a film on the surface (phosphating film).
Phosphating process is mainly used in the metal surface, the purpose is also to provide a protective film for the metal surface, so that the metal is isolated from the air to prevent it from being corroded; It will also be used for the primer of some products before painting, with this layer of phosphating film can improve the adhesion and corrosion resistance of the paint layer, improve the decoration to make the metal surface look more beautiful, and can also play a lubricating role in the cold processing of some metals.
After phosphating, the workpiece will not be oxidized and rusted for a long time, so the application of phosphating is very wide, and it is also a commonly used metal surface treatment process, which is more and more used in automobiles, ships, machinery manufacturing and other industries.
However, the phosphating process also has the disadvantages of much solution sediment, rough surface, high phosphating temperature, long time and high cost.
The development history of phosphating
In fact, the phosphating process has been developed for a long time, it should be a modern metal surface treatment, invented earlier, its development has also gone through different periods.
In the United Kingdom in 1869, someone found that phosphating film can be used on the surface of the metal, can effectively protect the metal from corrosion for a long time, and at that time also applied for a patent, which also laid the foundation for the technology and development of phosphating.
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 process, since then phosphating treatment has been rapid development and entered the practical application period.
In modern times, in order to meet various needs, the phosphating process is also constantly improving, mainly towards the direction of low temperature, low slag, environmental protection and non-toxic development.
Classification and application of phosphating
Under normal circumstances, a surface treatment is to show a color, but phosphating treatment can be based on actual needs, through the use of different phosphating agents will show different colors, which is that we often see phosphating treatment has gray, color or black.
Iron series phosphating
After phosphating, the surface will show rainbow colors and blue, so it is also known as color phosphorus, phosphating liquid mainly takes molybdate as raw material, will form a rainbow color phosphating film on the surface of steel materials, 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 gray film phosphating, the main use of phosphating liquid is composed of phosphoric acid, sodium fluoride and emulsifier, etc., will form a gray phosphating film on the surface of the workpiece, which is mainly for the coating bottom, combined with the spraying, painting or electrophoresis and other processes.
Gray phosphating film itself also has a certain corrosion resistance, so it can also be used alone as an anti-corrosion coating on the surface of the workpiece, mainly used in some galvanized plates, cold rolled plates, and aluminum surface phosphating treatment.
Manganese phosphating
The color is black or black gray, so it is also known as black phosphating, mainly the use of phosphating solution containing manganese ions, will form a black phosphating film on the surface of the workpiece, it is also a kind of phosphating treatment with the best anti-rust performance, can be used as a long-term anti-rust parts, and is also the most widely used kind of anti-rust phosphating.
Manganese phosphating will have a smaller coefficient of friction than other phosphating, so for those parts that often friction can be used black phosphating, which can effectively reduce friction, such as some auto parts and some fasteners and so on.
Phosphating process
The commonly used phosphating process is to use chemical elements to react and form a film on the surface of the metal to play a role in corrosion prevention and bottling, and the modern phosphating process mainly has these processes.
Degreasing and degreasing
Metal materials in the storage and processing process will be exposed to a variety of oil or sweat, and these oil will be mixed with a variety of dust impurities, attached to the metal surface, will not only affect the formation of phosphating, but also reduce the phosphating film adhesion, corrosion resistance and beauty, phosphating quality will be greatly affected.
This requires cleaning the workpiece before phosphating, the commonly used method is chemical method, put the workpiece into the cleaning solution with a strong acid heating to 80 ° C for about 5 minutes, through the saponification reaction of the strong acid on the oil, the oil is removed from the workpiece.
Remove rust and peel
Steel in the process of processing will form a layer of oxide on its surface, and in the process of storage and transportation in the water or corrosive liquid will rust, these oxide and rust if it is not cleaned up, will accelerate the corrosion of the workpiece.
Therefore, rust removal and oxidation removal are also the main processes before the workpiece is phosphating, which will relate to the quality of phosphating.
There are mechanical methods and chemical methods to remove rust and oxidation, usually the chemical method will be used, the workpiece is put into the acid washing solution containing industrial hydrochloric acid, and the rust and oxidation can be completed by standing for about 30 minutes.
phosphating
The most important thing in the phosphating process is the phosphating liquid, the phosphating liquid used in different phosphating will not be the same, after the metal workpiece is put into the phosphating liquid, there will be a chemical reaction between each other to produce an element of iron phosphate, and it will also produce hydrogen and oxygen.
This iron carbonate element will slowly crystallize and deposit on the metal surface, when more and more of this crystallization, it will be connected to the metal surface, forming a layer of insoluble water firm phosphating film.
After the workpiece is put into the phosphating liquid, it needs to continue shaking for a few minutes, mainly in order to discharge the air and bubbles trapped in the groove of the workpiece, the phosphating time will generally be determined according to the different workpiece structure, material and temperature, usually about 30 minutes.
Other process
After each process, the workpiece needs to be washed, which is mainly to clean the chemical composition remaining on the surface of the workpiece, and can successfully complete the next process.
After the workpiece is phosphating and washed, it is necessary to dry the water on the surface, so that the entire phosphating process is completed and it can be re-processed or used.
Coating is a simple and practical method of modern metal anti-corrosion treatment, phosphating treatment is the key process of its pre-treatment, in addition to phosphating treatment process, the entire surface treatment coating industry has played a great role in promoting.
However, the phosphating process is also facing many problems, and it is necessary to improve the treatment process to develop in the direction of energy saving and environmental protection, non-toxic and improve efficiency.