The metal surface pretreatment and powder coating compatibility

Abstract: metal surface without oil, rust, and the formation of the conversion coating (phosphating film) can make the powder coating macromolecule polar groups and "close", not only enhance 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 promote the corrosion function of powder coating.

1, the introduction

Metal surface pretreatment is to increase the adhesion strength of the substrate and the coating. Therefore, pretreatment must be thoroughly remove oil and rust on the surface of the, and form a dense layer of conversion coating, this coating on the physical and chemical properties and service life was increased, the superiority of powder coating can be fully reflected. The following we will discuss different pretreatment (phosphate) process and powder coating the sex differences, as well as the impact on the quality of the coating.

2, test method,

Test material for the electric galvanized sheet of baosteel production, test plate size 150 * 750 * 0.8 mm. Before treatment agent used for guangdong province jiangmen chemical products, the integrity of the powder coating is Du Banghua companies provide used HouChun resistant polyester powder.

2.1 process is selected

To give full play to the anticorrosion properties of the powder coating, metal surface pre-treatment phosphating process is the key. To this end, we according to the actual circumstances of the actual production line, choose the following process:

Hot water washing to skim - water - watch, phosphating and water to pure water to wash, moisture drying to electrostatic spraying, powder curing.

2.2 conversion coating selected and determined

We know that the conversion film according to the chemical conversion film on the surface of the metal type can be divided into the following three kinds: iron oxide - phosphate film, it is amorphous film. This kind of film hole is more, the corrosion resistance is a bit poor, after processing needed to chromate treatment, coating binding force is good, film thickness is about 0.3 g/m2.

Department of zinc phosphate film (zinc, zinc, calcium, zinc, manganese, zinc, nickel, zinc, manganese, nickel, etc.), it is a crystalline type membrane. This kind of membrane is porous, with good coating performance, coating binding force is good, film thickness 1.0 ~ 2.0 g/m2.

Composite metal oxide film, it is amorphous film. This film is mainly composed of zinc oxide, which contain iron, cobalt and nickel. Although composite metal oxide has better coating performance, but its corrosion resistance is poorer, after processing chromate treatment should be carried out again. Complete compound metal oxide film usually containing cobalt and nickel is about 5.4 ~ 18.0 mg/m2; Chromium is about 6.5 ~ 16.0 mg/m2.

For weatherability pure polyester powder coating, the use of zinc phosphate film is more appropriate. So we choose three different zinc series phosphating solution related process test, namely zinc, zinc - manganese and zinc - manganese - nickel.

2.3 the coating adhesion test

Through the above three kinds of different pretreatment (phosphating process), the coating after related test board of the test of the project, including the coating adhesion, impact resistance, and the appearance of the coating quality, etc.

Using the process before the first (ordinary zinc phosphide) test board, electrostatic powder coating on the surface (hereinafter referred to as the "coating"), according to the method of the lattice (spacing 1 mm) measured surface coating adhesion is poor, the results reached level 3 above (greater than 35% off).

Using the process before the second (zinc - manganese phosphate) test board, after coating the surface according to the method of the lattice (spacing 1 mm) testing surface coating adhesion is good, the results to achieve grade 1 (5% off).

Using the process before the third (zinc - manganese - nickel phosphide) test board, according to the lattice method after surface coating (spacing 1 mm) testing surface coating adhesion, the results reached a level 0 (no loss).

The test results show that the zinc - manganese - nickel phosphate can well compatibility, weatherability pure polyester powder coating and the coating properties, corrosion resistance, impact resistance, etc.) are better than the other two phosphate system.

3, test results and analysis

3.1 phase analysis of different system phosphating film

Three kinds of phosphating film phosphating system structure of electron microscope analysis diagram as shown in figure 1.

The metal surface pretreatment and powder coating compatibility

From figure 1, you can get, the main metal ion components in phosphating film crystallization zinc, manganese, nickel, etc.

Galvanized surface of ordinary zinc phosphating film mainly zinc phosphate crystals, crystal in "activation center" pine shaped surface radiation, high porosity, as shown in figure 1 a.

Formed in the surface of galvanized zinc manganese series phosphating film is mainly manganese phosphate and zinc phosphate crystallization, crystallization polyhedral blocks shaped, disordered due to crystallization direction, the phosphating film thicker, gap is larger, as shown in figure 1 b.

Nickel zinc manganese in galvanized surface formation of phosphating film mainly zinc phosphate crystal of nickel, manganese phosphate and phosphoric acid, crystallization acicular or leaves shaped, crystallization direction most orderly, and a small amount of disorder, the phosphating film crystallization meticulous, as shown in figure 1.

3.2 with the sex of powder coatings

The crystallization of ordinary zinc series phosphating porosity is high, easy to absorb moisture from the air or the environment, lower the coating and the substrate. So this kind of phosphating film and poor powder coating the sex.

The crystallization of zinc manganese series phosphating film thicker, also can affect the adhesion strength of the coating and substrate. Especially after a period of time after the coating adhesion will appear local variation phenomenon. Therefore, this kind of phosphating film with powder coating the sex of instability.

The crystallization of nickel zinc manganese series phosphating medium porosity, membrane crystallization meticulous, powder coating curing coating molecules can seep into the phosphating film, increase the adhesion strength of the coating and substrate. Therefore, this kind of phosphating film has good compatibility with powder coating.

4, endnotes

In coating powder coating on the surface of galvanized sheet, to ensure that the anticorrosion properties of the coating, metal surface pretreatment and powder of sex is extremely important. Phosphating film system, crystal size, porosity and other factors all have different degrees of influence on powder coating.