The first line of defense, the physical shielding effect of the coating. The coating causes the substrate to be isolated from the external environment, cut off the channel of the original battery, and prevent the corrosion of the electrochemistry, thus protecting the substrate and resisting the environmental corrosion. This protective effect is determined by the ability and endurance of the organic coating to isolate the environment, and the two capabilities of the organic coating are closely related to the external factors, such as the composition, layer, thickness, uniformity, machinability, and other external factors such as the environment and human maintenance. The damage of coating physical shielding is mainly due to the penetration of coating. The coatings and coatings produced by existing coatings and technology tend to have tiny pinholes, or the penetration or leakage of H2O, O2 and other ions can be caused by the aging and cracking of the coating. When the coating is permeated between the coating and the substrate metal layer, the local galvanic reaction will occur. Based on the above considerations, for a certain type of substrate, when a certain type of coating can not meet the requirements of adhesion and weather resistance at the same time, a multi coating method can be used, that is, a coating with good adhesion is used as a primer or a layer of binder, and then coated with good weatherability and decorative paint as a finish. Properly increasing the number of layers and total thickness can reduce the number and penetration of pinholes. Baking based coatings have better weather resistance than self drying coatings.
Second, third line of defense, chemical conversion coating, physical and chemical protection of coating. Small cracks on the chemical conversion film help to infiltrate the liquid organic coatings or adhesives, enhance the bonding between the coating and the conversion coating through physical entangling, and then increase the bonding strength to the substrate. In addition, the coating can be improved by chemical conversion of the hydrated oxide on the surface of the membrane and the chemical effect of the coating. Focus on. The chemical conversion film is also a physical shield relative to the substrate. Moreover, as the standard electrode potential of the chemical conversion film, such as passivation film or chromium compound, is more positive than that of the Fe element, the chemical corrosion process of the substrate can be delayed as a sacrificial anode when the coating permeation occurs. It can be seen that chemical conversion coating is an important part of ensuring the quality of color coated steel sheet. The coating is a dense layer of metal or alloy film covering the surface of the substrate. The surface of the coating can participate in passivation or oxidation with phosphate or environmental medium, and play the role of chemical conversion film. In addition, the coated metal also has a special sacrificial cathodic protection effect on trimming.