Development and application of zinc phosphate
      Traditional anti-rust pigments limit their use due to their toxicity, so non-toxic and pollution-free anti-rust pigments have been developed, including phosphate series, borate series, molybdate series, metal powders, metal oxides, etc. . Among them, the research on the production and application of phosphate plays an important role. The British scholar Coslctt first invented the industrial method of using phosphate to prevent rust on steel in 1906. Since then, this method of phosphate treating metal surfaces has been gradually improved. In 1959, the United Kingdom first began to develop zinc phosphate, and now it has been widely used all over the world.

1. The composition of zinc phosphate The theoretical chemical formula of zinc phosphate is Zn3(PO4)2•2H2O, the molecular weight is 404.08, and the theoretical chemical composition is Zn 48.54%, P 15.33%, O 25.63%, H 0.50%. Zinc phosphate usually contains 4 molecular crystals Water has three crystal states of α, β and γ. It belongs to the needle-like and plate-like crystals of the orthorhombic system. When heated to 120°C, dihydrate can be obtained, and all crystal water will be lost at about 250°C. Zinc phosphate is almost insoluble in water, but soluble in dilute mineral acid, acetic acid and ammonia water. General characteristics of zinc phosphate antirust pigments, the main component is Zn3(PO4)2•nH2O (where n = 0, 2, 4). The relative density of zinc phosphate is 3.0 ~ 3.9, the refractive index is 1.2 ~ 1.6, the pH value is 6.5 ~ 8.0, the loss on ignition of zinc phosphate dihydrate is less than 10%, and the loss on ignition of zinc phosphate tetrahydrate is less than 16%. The oil absorption of zinc phosphate is 15 ~ 50 g/100 g, the water soluble part is generally less than 1%, and the particle size is 0.5 ~ 4 μm; while the component introduced in the British standard BS 5193 is Zn3(PO4)2•2H2O, burning The zinc content after burning is 50.5-52%, the loss on ignition at 800°C is 8.5-10.0%, the zinc content is 45.5-47.6% based on the collected material, and there are residues, etc. The zinc phosphate produced in my country is confirmed to be zinc phosphate with two molecules of crystal water by differential thermal analysis, thermogravimetric analysis, and X-ray diffraction test. The zinc content and oil absorption are the same as those of the British BS 5193.

  2. Anti-rust mechanism of zinc phosphate There are various opinions about the anti-rust mechanism of zinc phosphate. In 1963, Meyer proposed that zinc phosphate forms a complex with carboxylic acid in the system, and believed that zinc phosphate slowly dissociated into phosphate ions inside the coating, and the condensed phosphate ions reacted with the metal surface to form complex adhesive Me-Zn- The P2O5 compound covers the film, passivates the metal, or forms a complex complex between the metal surface and the paint. In addition, zinc ions also form insoluble complexes, which play the role of cathodic protection. Especially in the initial corrosion process, due to the presence of crystal water, it reacts with -OH and -COOH in the coating, which may produce a gel. This reaction is carried out by the incorporation of zinc phosphate in the multi-layer primer. When the -OH, -COOH groups in the primer react with the phosphate, inner layer adhesion will be obtained. Harrison et al. proposed in 1965 that the corrosion of steel due to the pollution of ammonium salts is very severe. Zinc phosphate can inhibit ammonium ions, slowly "phosphate" the metal surface through the penetration and diffusion of the paint film, and keep it in a passive state. The presence of phosphate will greatly reduce the harm of ammonium ions and control the corrosion of metals. Zinc ions may form cathodic protection on the steel surface, preventing further corrosion of the metal. Soluble phosphate reacts with ferrous ions generated in the cathodic region of the metal surface to form a cathodic inhibitor with an insoluble precipitate coating. However, zinc phosphate is practically insoluble, so its role has been questioned. Regarding this issue, Clay et al. carried out a constant current polarization test of iron on the zinc phosphate extract, which confirmed its anti-corrosion effect. In 1973, it was proposed that zinc phosphate can form a heteropolyacid complex with iron rust after hydrolysis. Forms a tough paint film to stabilize rust.

  3. Use of zinc phosphate Zinc phosphate is the most important variety of phosphate antirust pigments. Foreign countries believe that the selection of zinc phosphate from phosphate is a major development and promotion, and even regard it as one of the milestones of industrial development. It can be formulated with various antirust paints. Zinc phosphate has a small refractive index and high transparency, so it is easier to color in paint, and it can be combined with other pigments to prepare various colors. Zinc phosphate is formulated with a variety of base materials, including chlorinated rubber, hardened epoxy, medium oil alkyd and urethane resins. Used in water-based, compared with other standard anti-corrosion paints. Experimental description: In most cases, the antirust effect of zinc phosphate is comparable to red lead and calcium lead acid, and in some respects it is better than red lead. It is worth noting that the increase of zinc phosphate PVC will reduce the antirust performance, and the total PVC in zinc phosphate paint is generally 30~35% or even higher. Since zinc phosphate is an inactive pigment, the effective concentration in phosphate is higher. Its effectiveness can be enhanced by micronization. In addition, other anti-rust pigments or phosphates with higher alkalinity can also be combined to form a dense passivation film on the iron surface. Zinc phosphate is a colorless and non-toxic antirust pigment with good stability. It has good water resistance and corrosion resistance, and has dual functions of protection and decoration on the same coating. Zinc phosphate is neutral and has affinity with any paint, so it can maintain a good consistency. Since the zinc phosphate primer is non-toxic, it has been included in the British Safety Code 3443. At the same time, the practical application of flame cutting and welding of pre-painted structural steel has also been approved by the health department, thus simplifying the in-situ welding of pre-painted steel components. Zinc phosphate is widely used in alkyd, modified alkyd and urethane paints. It has a particularly fast drying effect. It can be topcoated in about 2 hours without reducing its corrosion resistance and has good adhesion. . Zinc phosphate has anti-chemical and flash effects, and can form scaly crystals with good adhesion with metals. In addition, zinc phosphate can also be used to make dental printing film materials.