Metal surface phosphating is a commonly used pretreatment technology, mainly used for phosphating on metal surfaces, and phosphating can also be applied to non-ferrous metals (such as aluminum and zinc). Metal surface phosphating is a commonly used pretreatment technology, mainly used for phosphating on metal surfaces, and phosphating can also be applied to non-ferrous metals (such as aluminum and zinc). Chengdu Xingke Chemical Coating Technology Co., Ltd. introduces the metal surface phosphating process:
The purpose of phosphating is to provide protection to the base metal, to prevent the metal from being corroded to a certain extent; to be used for primer before painting, to improve the adhesion and corrosion resistance of the paint layer; to reduce friction and lubrication in the metal cold working process .
Metal surface phosphating is a simple, reliable, low-cost, and easy-to-use process for greatly improving the corrosion resistance of metal surfaces, and is therefore widely used in actual production.
The modern phosphating process is generally:
Degreasing â†’ washing â†’ descaling â†’ surface adjustment â†’ phosphating â†’ washing â†’ drying.
The fifth process, phosphating, refers to a chemical treatment method in which a metal surface is contacted with an acidic solution containing dihydrogen phosphate to form a chemical reaction to form a surface of a stable insoluble inorganic compound film on the surface of the metal. The film formed is referred to as a phosphate film. Its film formation mechanism is: (taking zinc as an example)
1. Dissolution process of metal When the metal is immersed in the phosphating solution, it first reacts with phosphoric acid in the phosphating solution to form a generation of iron phosphate, and a large amount of hydrogen is precipitated. Its chemical reaction is;
The above formula shows that at the beginning of phosphating, only the metal dissolves, and no film is formed.
2. Acceleration of the accelerator The hydrogen released by the upper reaction is adsorbed on the surface of the metal workpiece, thereby preventing the formation of the phosphate film. Therefore, an oxidizing accelerator is added to remove hydrogen. Its chemical reaction formula is: 3Zn(H2PO4)2+Fe+2NaNO2=Zn3(PO4)2+2FePO4+N2â†‘+2NaH2PO4+4H2O
3. Hydrolysis reaction and tertiary dissociation of phosphoric acid The basic component of the phosphating bath is one or more heavy metal acid phosphates, which have the molecular formula Me(H2PO4)2. These acid phosphates are soluble in water at a certain concentration. Hydrolysis and overtones occur at pH and produce free phosphoric acid.
As the hydrogen ion concentration on the surface of the metal workpiece drops sharply, the dissociation equilibrium of the phosphate groups shifts to the right and eventually becomes phosphate.
4. Formation of a phosphating film When the trivalent phosphate dissociated from the metal surface and the metal ion (the surface of the workpiece) in the phosphating bath are saturated, that is, the crystal is deposited on the surface of the metal workpiece, and the crystal grains continue to grow until A continuous, water-insoluble, firm, phosphating film is formed on the surface of the metal workpiece.
A part of the ferrous ions dissolved in the metal workpiece is consumed as a component of the phosphating film, and the ferrous ions remaining in the phosphating bath are oxidized to ferric ions. Http://news.chinawj.com.cn Editor: (Hardware Business Network Information Center) http://news.chinawj.com.cn
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