What are the effects of dirt on the coating of steel structure surface？

Surface pretreatment is to remove visible and invisible dirt on the material surface. So how do they affect the life of the coating?

The influence of dirt on coating life mainly comes from two aspects, one is visible dirt, the other is invisible. The former is mainly the dirt, dust, grease, oil, rust, oxide skin and sometimes water vapor on the steel surface. If the dirt is under the coating, the mechanical and chemical adhesion of the coating will be affected, and the life of the coating will be shortened. Only when these things are completely removed can a complete coating with good bonding with steel be formed on the workpiece surface, and the adhesion of the coating will be effectively guaranteed. This is like forming an effective barrier on the surface of the workpiece to prevent moisture from contacting the workpiece. Because in the process of steel corrosion, water vapor is an electrolyte, which can accelerate the corrosion process.

As for the chemical harmful substances invisible, they are some soluble salts, such as chlorides and sulfides. They mainly use water vapor to penetrate the coating and reach the bottom of the coating to form bubbles to separate the coating from the substrate, which will accelerate the corrosion of steel. Therefore, the highest requirements for conductivity and chloride content are usually included in the requirements for chemical indicators of sand blasting abrasives. For example, for garnet sand, the chloride content of SEPPE garnet is lower than 0.0001%, and the conductivity can be as high as 12ms/m. In this way, the use of garnet sand blasting can not only form a certain roughness on the surface, but also minimize the amount of invisible objects, greatly extending the life of the coating.