Effect of coating thickness on corrosion resistance of color coated sheet
As mentioned above, the corrosion resistance of the substrate is one of the key factors affecting the service life of the color coating plate, and the corrosion resistance of the substrate depends directly on the durability of the galvanized layer, and the quantitative evaluation formula for the durability of the galvanized layer is usually used at present.
N=t2 / TV x 0.9 (1)
In the formula, n is the service life, the T2 is the thickness of the galvanized layer, and the M is TV; the annual corrosion rate of the zinc coating is g / (m.A).
The annual corrosion rate of zinc coating, which is the loss of zinc layer per unit area after exposure to the first year, can be seen in the data of ISO12944 (2), as shown in Table 2. Australia's atmospheric exposure data basically verified the corresponding ISO data representing the environment, as shown in Table 3.
It is estimated that the zinc coating (280g / m m) of the 140g / M can be used for at least 30 years in a low or very low corrosive environment. But the data of the American galvanizing Association on the life of galvanized products is conservative. Even in a rural climate with low corrosion, the weight of the zinc layer for 30 years of durability is at least not less than 400g / M (double sided).
On the basis of galvanizing, color coated products are coated on the basis of surface cleaning and chemical conversion film, and then coated with primer (focusing on adhesion and corrosion protection) and the two layers of dense organic coating (focusing on weather resistance and decoration) as protective barrier, which effectively prevents the immersion of water molecules and corrosion medium, and has the nature light destruction against ultraviolet rays. The ability to solve. The thickness of the coating should be reached to the specified thickness, so as to get a tight screen coating, reduce the permeability, oxygen permeability and prevent the corrosion of the coating. As for the film, the film thickness is the key factor affecting the corrosion.
The corrosion test of neutral salt fog shows that the corrosion resistance of the coating is often not up to 500h when the thickness of the coating is less than 10 m. In the interval of 10~20 mu m, the thickness of the coating has a significant effect on reducing the corrosion level of the coating, and the corrosion level of the coating is increased with the thickness of 20~26 mu m. It is not obvious; moreover, the difficulty of controlling the coating process with the thickness increases, and the coating above 26 m is easy to produce edge thickening and other bad phenomena. It can be seen from the comprehensive evaluation of cost and performance that 20 mu is a minimum value to achieve a good corrosion resistance to atmospheric environment.