Mechanism and Formula of Phosphating Primer

Hua Dongxu1,Xu Qinzheng2 (1.Henan Shenma Nylon Chemical Co., Ltd., Pingdingshan, Henan 467013, China; 2.Solvay (China) Co., Ltd., Shanghai 201100, China)

Abstract : The mechanism and the development of the phosphating primer is outlined. The basic formulas for typical phosphating primers are described. In addition, the characteristics of the different formula and phosphating primer have been analyzed.

0 Introduction
Phosphating primer is The high-efficiency metal surface treatment agent developed by Bakelite in the United States in the 1940s, also known as washing primer. The thin film (10-15 μm) formed by the phosphating primer is firmly attached to the metal surface, which can phosphatize and passivate steel and non-ferrous metals. It is widely used to enhance the rust prevention of steel and non-ferrous metals, and is particularly suitable. Protection of equipment and structures in the ocean, hot and humid areas. For large components, it can replace phosphating treatment. But generally speaking, it can’t replace the primer, it should be coated with other primers or anti-rust paint to get better results. As a rust-proof coating for non-ferrous and ferrous metals, the paint can replace the phosphating treatment of steel. It can increase the adhesion between organic coatings and metal surfaces, prevent rust, and extend the service life of organic coatings. It is suitable for the coating of various ships, bridges, water tanks, pipes, docks, buoys, meters and other various metal components and equipment.

Since the 1980s, the domestic has introduced the international advanced level of the British Haden company’s phosphating primer technology. The current phosphating primer is XF06-2 phosphating primer developed by AVIC Beijing Aviation Materials Institute and Tianjin Paint Factory. The developed X06-1 ethylene phosphating primer, and XF06-2 has significantly better water resistance than X06-1 ethylene phosphating primer. Subsequent application process research also showed that XF06-2 was used as the phosphating primer, H06-1010h epoxy paint was used as the intermediate primer, and XF06-2 was dried at room temperature for 4 h or 60 ℃ for 3 h, and then coated with H06-10l2. h Epoxy primer, the performance of the final coating system meets the technical requirements, and has gradually become the current phosphating primer process.

1 Mechanism of phosphating primer
The development of phosphating primers began in the 1940s, and research into its mechanism of action is ongoing. Rosenbloom H believes that the paint first reacts with the metal to form an inorganic salt film, and then deposits an organic film on the inorganic salt film. Kruger J and Bloom M believe that the acidic phosphate formed by the reaction between phosphoric acid and hemiacetal reacts with the metal substrate to form a film, but Maller C denies the film formation mechanism. Now the general view is that the phosphating primer reacts with the substrate to produce a chemical bond to form phosphate and chromate. At the same time, phosphoric acid reacts with the tetrabasic zinc yellow to form chromic acid. The primary alcohols form aldehydes, and they themselves reduce to trivalent chromium salts and chromium phosphates. Cr3 + has a strong complexing ability, can complex with hydroxyl groups in polyvinyl butyral, and is deposited on the phosphate and chromate inorganic layers on the surface of the substrate. Alcohol-soluble polyvinyl butyral film is complexed with Cr3 + to become a yellow translucent paint film that is resistant to water and insoluble in alcohol.

In short, the main film-forming material of phosphating primer is composed of polyvinyl butyral resin, zinc chrome yellow and additives. Alcohols are used as solvents, and phosphoric acid is used as the treatment liquid, usually two-liquid type (single cans are also available, but the antirust effect is poor). Phosphoric acid reacts with tetrabasic zinc yellow, which can produce an inorganic phosphate coating film similar to general chemical phosphating treatment. At the same time, the metal surface is passivated under the action of chromate, and the chemical stability is significantly enhanced. The butyraldehyde resin is transformed into insoluble substances, which blocks the voids of the substrate, forms a tightly sealed layer, and completely protects the metal.
As a metal surface anticorrosive coating, phosphating primer can completely replace the phosphating process, and can replace the phosphating treatment of steel. It can increase the adhesion of organic coatings and metal surfaces, prevent rust, and prolong the service life of organic coatings. [1]. Simple construction, fast drying, good miscibility and broad application prospects. It is worth noting that phosphating primer cannot be used as a separate primer. Pre-coat a very thin film before applying the primer. In order to enhance the primer adhesion and rust prevention ability, it has the dual effects of phosphating and passivation to extend the service life of the surface coating. After applying the phosphating primer, it is still necessary to apply the primer. This article focuses on the formulation and performance characteristics of several phosphating primers.

2 Basic formula and performance of phosphating
The two-component phosphating primer is composed of component A (paint liquid) and component B (phosphating liquid) in a fixed proportion. When using, add component B phosphating solution to the paint solution with stirring (uneven mixing will form gel around the phosphating primer). It must be used up within 8 hours after mixing. Although the mixed paint liquid does not gel after 8 hours, it often loses its performance characteristics. (General type) The basic formula of the phosphating primer is shown in Table 1.

(Common type) Basic formula of phosphating primer
(Common type) Basic formula of phosphating primer

Four basic zinc yellow is the most common variety, mainly used in ferrous metals (steel, iron, cast iron), as well as those using lead chrome yellow, only limited to the surface of steel, can not be heated and dried (including the upper paint can not be dried). The anti-corrosion performance of dilute salt solutions is not as good as that of tetrabasic zinc yellow, but in fresh water and high humidity conditions, the effect is worse than that of tetrabasic zinc yellow. The basic formula of the two-component lead-chrome yellow phosphating primer is shown in Table 2.

Basic formula of two-component phosphating primer with lead chrome yellow

The basic formula of one-component phosphating primer is shown in Table 3.

One-component phosphating primer base formula

Phosphating primer is generally produced by ball milling. The better the pigment is dispersed, the better the primer will be. Although the phosphating primer cannot passivate the metal and form a phosphate cover film, but because the entire paint is thin and easily penetrated by gas or liquid, the ordinary primer and topcoat must be applied after the phosphating primer is applied, otherwise Metals are still prone to corrosion. The one- and two-component phosphating primers have their own characteristics. The two-component has a longer pot life and the one-component has the opposite effect. The phosphating effect is equivalent.

3 Features of phosphating primer
(1) Easy to use, as long as you have general spraying tools and places, you can spray phosphating primer;
(2) Good film formation and fast drying. Generally, the thickness of sprayed coating is 5 ~ 10μm, The primer can be sprayed after drying for 1 hour;
(3) It has excellent adhesion on the aluminum surface and provides a good coating layer surface for the next layer. It also has good adhesion on other metal surfaces;
(4) The phosphating primer layer has certain anti-corrosion performance, and it has temporary protection ability
when the primer is spray-painted ; (5) As the phosphating primer can be sprayed with general spray equipment Spraying, so the consumption of manpower, man-hours and energy during construction is very small;
(6) The old coating of the phosphating primer can be removed with a general paint remover, and the coating is cleaned without pollution;
(7) There is no residual chromate or water in the gap between the rivet head and the connection part, which avoids the factors that are prone to corrosion in use.

4 Conclusion
Phosphating film has changed from coarse-grained thick film to micro-crystalline thin film type. Normal and low-temperature phosphating has become the most active research area at present, and the accelerator has developed from a single accelerator to a multi-component accelerator. The surface adjustment has also become international. Proven treatment. Phosphating treatment is developing in the direction of low cost, low energy consumption, low toxicity, no pollution, uniform and dense phosphating film, thin film and good corrosion resistance. At the same time, a series of emerging technologies such as ultrasonic phosphating technology are maturing.

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