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  Antirust Mechanism of Phosphate Antirust Pigment

1. Zinc phosphate
Zinc phosphate is an important antirust pigment. As one of the substitutes for lead and chrome antirust pigments in the paint industry, it has been used since 1965 and is widely used in oily and water-based primers as chemical antirust pigments. To improve and enhance its antirust performance, it is currently the largest and most widely used non-toxic antirust pigment.
The standard zinc phosphate antirust pigment is zinc orthophosphate white powder, mainly dihydrate type (Zn3(PO4)2•2H2O) and mixed type of dihydrate and tetrahydrate (Zn3(PO4)2•4H2O). Its antirust mechanism is very complicated. The main anti-rust mechanism of zinc orthophosphate pigment:
The dissolved ferrous salt and iron salt are hydrolyzed to release protons, and then gradually react with zinc orthophosphate to generate water-soluble first-generation ferrous phosphate and second-generation ferrous phosphate in turn, and finally generate insoluble third-generation ferrous phosphate. Passivation occurs on the surface of the metal substrate, forming a phosphating film with complex composition, dense and strong adhesion such as Me (metal)-Zn-P2O5.
a) In addition to phosphating, they can also react with the carboxyl and hydroxyl groups in the base material and some ions in the corrosive environment to form complexes, and this complex can react with corrosion products. Forms a tightly adherent protective film on the surface of the material.
b) The anti-rust effect of zinc orthophosphate is mainly due to the presence of crystal water in it, which slowly hydrolyzes to form zinc hydroxide and second-generation zinc phosphate ions, which form a protective film composed of complexes with adhesive corrosion inhibition To achieve anode protection.
c) Zinc orthophosphate slows down the diffusion rate of ammonium ions through the coating film, thereby playing a role in corrosion inhibition.

2. Modified zinc phosphate ZPA-2
Zinc phosphate releases phosphate and zinc ions during the slow hydrolysis process, and the combination of phosphate ions and iron salts forms a dense and strong complex protective film with strong adhesion, which protects the metal surface. Ordinary zinc orthophosphate The effect is remarkable in a slightly acidic environment, but in neutral or alkaline water-based coatings, due to poor hydrolysis, the release of phosphate ions is slow, and at the same time, the activity of zinc orthophosphate is not enough, and there is a phenomenon of delay in effectiveness, resulting in neutral or alkaline water-based coatings. The effect in the environment is not ideal, and the problem of flash rust cannot be well overcome, so the modification of zinc orthophosphate is the current development direction of zinc phosphate development. The modified zinc phosphate produced by our company is based on the above theory, and the surface modification treatment is carried out on zinc orthophosphate, so that various components can cooperate with each other in the early, middle and late anti-rust process, and overcome the problem of anti-rust delay in the early stage of zinc phosphate .
3. Aluminum phosphate series
The main variety of aluminum phosphate antirust pigments is aluminum dihydrogen tripolyphosphate.
a) The main component is aluminum dihydrogen tripolyphosphate (AlH2P3O10•2H2O), which is a white non-volatile powder, orthorhombic, with a relative density of 2.31, slightly soluble in alcohol, and partially hydrolyzed in the air for a long time. The acid is relatively stable, with good storage stability and weather resistance;
b) It is weakly acidic, with a pKa of 1.5-1.6. Compared with other solid acids, its acidity (by mass) is extremely high, and it contains many active groups per unit mass, so it can produce effective antirust effect with a small amount ;
c) Aluminum tripolyphosphate can release tripolyphosphate ions (P3O105-) with strong complexing ability, and has a high complexing ability for divalent and ferric ions, so that MxFey (PO4) can be formed on the surface of the iron substrate Dense passivation film with z structure.
There are 2 hydrogen atoms in AlH2P3O10•2H2O which are weakly acidic and can combine with metals to form a tough passivation film. The reaction steps are as follows:
However, due to acidity, water solubility and other reasons, aluminum tripolyphosphate is not suitable for direct use as an anti-rust pigment, and must be modified to make it have a suitable pH value, solubility, dispersibility, etc., in order to become a practical and usable pigment. Active anti-rust pigments.

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