Metal surface without oil, rust, and the formation of the conversion coating (phosphating film) can make the powder coating macromolecule polar groups and "close", enhanced the adhesion strength of the powder coating and substrate, but also the phosphating film anticorrosive function with powder coating of the anticorrosion performance together (is not a simple superposition), which will greatly enhance the antiseptic function of powder coating.

1, the introduction

Metal surface pretreatment is to increase the adhesion between substrate and coating. Therefore, the pretreatment must remove the oil and rust on the surface, and form a layer of dense conversion film, so that the physical and chemical properties and service life of the coating can be enhanced, the performance of powder coating can be reflected. Next we will discuss the compatibility differences between different pretreatment (phosphating) processes and powder coatings, as well as the effect on the overall coating quality.

2. Test method

The test material is galvanized sheet produced by Baosteel, the size of the test sheet is 150×750×0.8mm. The pre-treatment agent used is the product of Guangdong Jiangmen Chengxin Chemical Plant, and the powder coating used is the weatherproof pure polyester powder provided by Dubang Huajia Company.

2.1 Process selection

To give full play to the anti-corrosion properties of powder coating, the phosphating process of metal surface pretreatment is the key. Therefore, according to the actual production line, we choose the following process:

Hot water washing → degreasing → washing → table adjustment → phosphating → washing → pure water washing → moisture drying → electrostatic spraying → powder curing.

2.2 Screening and determination of transformation membrane

As we know, the conversion film on the metal surface can be divided into the following three types according to the type of chemical conversion film: iron oxide - phosphate film, it is amorphous film. This kind of film has many holes and poor corrosion resistance. After treatment, it needs to be treated with chromate. The coating has good adhesion and the film thickness is about 0.3g/m2.

Phosphate zinc film (zinc system, zinc - calcium system, zinc - manganese system, zinc - nickel system, zinc - manganese - nickel system, etc.), it is a crystalline film. The film is porous, has good coating performance, coating adhesion is good, film thickness 1.0 ~ 2.0g/m2.

Composite metal oxide film, which is an amorphous film. The film consists mainly of zinc oxide, which contains iron, cobalt and nickel. Although composite metal oxidation has better coating performance, but its corrosion resistance is poor, after treatment, chromate treatment is needed. The complete composite metal oxide film usually contains cobalt or nickel about 5.4 ~ 18.0mg/m2; Chromium about 6.5 ~ 16.0mg/m2.

Phosphate zinc film is suitable for weather resistant pure polyester powder coating. Therefore, we choose three different zinc phosphating fluids to carry out relevant process tests, namely zinc, zinc - manganese system and zinc - manganese - nickel system.

2.3 Coating adhesion test

After the above three different pretreatment (phosphating process), the coated test board was tested for relevant items, including coating adhesion, impact resistance and coating appearance quality.

Using a kind of pretreatment technology (ordinary zinc phosphatizing) of the test board, the surface of electrostatic powder spraying (hereinafter referred to as "spraying"), according to the hundred grid method (spacing 1mm) measured surface coating adhesion is poor, the result is more than 3 grade (more than 35% off).

Using the second pretreatment process (zn-Mn phosphating), the surface of the test board after plastic spraying according to the hundred grid method (spacing 1mm), the surface coating adhesion is good, the result is 1 grade (5% off).

Using the third pretreatment process (zn-Mn-Ni phosphating), the surface coating adhesion is good according to the hundred grid method (spacing 1mm), and the result is 0 (no fall off).

The test results show that the zn-Mn-Ni phosphating system is compatible with the weather-resistant pure polyester powder coating, and its coating properties (corrosion resistance, impact resistance, etc.) are better than the other two phosphating systems.

3. Test results and analysis

3.1 Crystal phase analysis of phosphating films in different systems

Electron microscopic analysis of phosphating film structure of three phosphating systems is shown in Figure 1.

Compatibility of metal surface pretreatment and powder coating

As can be seen from Figure 1, the main metal ion components of phosphating film crystallization are zinc, manganese, nickel, etc.

The ordinary zinc-series phosphating film formed on the galvanized surface is mainly zinc phosphate crystal, which is in the "activation center" and radiates in a pine dendritic plane with high porosity, as shown in Figure 1A.

The zinc-manganese phosphating film formed on the galvanized surface is mainly the crystallization of zinc phosphate and manganese phosphate, and the crystallization is in the shape of multi-faceted blocks. Due to the disordered crystallization direction, the phosphating film is thick and the gap is large, as shown in Figure 1B.

The zinc-Manganese-nickel phosphating film formed on the galvanized surface is mainly the crystallization of zinc phosphate, manganese phosphate and nickel phosphate. The crystallization is needle-like or leaf-like. Most of the crystallization directions are orderly, and a few are disorderly, resulting in the fine crystallization of the phosphating film, as shown in Figure 1.

3.2 Compatibility with powder coatings

Ordinary zinc phosphating crystal porosity is high, easy to absorb air or environmental moisture, so that the coating and substrate adhesion is reduced. Therefore, the compatibility of phosphating film and powder coating is poor.

Zinc-manganese phosphating crystal film is thicker, which also affects the adhesion between coating and matrix. After a period of time, the adhesion of the coating will appear local deterioration phenomenon. Therefore, the compatibility of phosphating film and powder coating is not stable.

Zn-mn-ni phosphating crystal porosity is moderate, the film crystallization is meticulous, the coating molecules can penetrate into the phosphating film when the powder coating is curing, increase the adhesion between the coating and the matrix. Therefore, this kind of phosphating film and powder coating compatibility is better.

4, endnotes

The compatibility between metal surface pretreatment and powder is very important when powder coating is applied on galvanized sheet. Phosphating film system, crystal size, porosity and other factors have different degrees of influence on the powder coating.