In the process of processing, transportation, storage, etc., the surface of the workpiece often contains oxide scale, rust mold residues, molding sand, welding slag, dust, oil and other dirt. In order to make the deep layer firmly attached to the surface of the workpiece, the surface of the workpiece must be cleaned before painting, otherwise, it will not only affect the adhesion and corrosion resistance of the coating and the base metal, but also make the base metal continue to corrode even if it is protected by the coating, so that the coating will peel off, and the mechanical properties and service life of the workpiece will be affected. Therefore, the surface treatment of the workpiece before painting is an important guarantee and measure to obtain a high-quality protective layer and prolong the service life of the product.
In order to provide a good surface of the workpiece, the surface of the workpiece is treated with the following points before painting:
1. No oil and moisture
2. No rust and oxide
3. No adhesive impurities
4. No residues such as acid and alkali
5. The surface of the workpiece has a certain roughness
Commonly used surface treatment methods are:
Manual processing: e.g. scrapers, wire brushes or grinding wheels. The rust and oxide scale on the surface of the workpiece can be removed by hand, but the manual processing has high labor intensity, low production efficiency, poor quality, and incomplete cleaning.
Chemical treatment: It is mainly to use acidic or alkaline solution to react with oxides and oil stains on the surface of the workpiece, so that it can be dissolved in an acidic or alkaline solution to achieve the purpose of removing rust oxide scale and oil stain on the surface of the workpiece. Chemical treatment is suitable for cleaning thin plates, but the disadvantage is that if the time is not properly controlled, even if corrosion inhibitors are added, the steel can be over-corroded. For more complex structural parts and porous parts, after pickling with acid solution, the residual acid immersed in the gap or hole is difficult to be completely removed, if it is not properly treated, it will become the hidden danger of corrosion of the workpiece in the future, and the chemical is volatile, the cost is high, the chemical discharge after treatment is difficult, and if it is not properly treated, it will cause serious pollution to the environment. With the improvement of people’s awareness of environmental protection, this treatment method is being replaced by mechanical treatment.
Mechanical treatment method: mainly including shot blasting method and shot peening method. Shot blasting cleaning is a method that uses centrifugal force to accelerate the projectile and eject it to the workpiece for rust removal and cleaning. However, the flexibility of shot blasting is poor, limited by the site, there is some blindness when cleaning the workpiece, and it is easy to produce dead corners that cannot be cleaned on the inner surface of the workpiece. The structure of the equipment is complex, there are many wearing parts, especially the blades and other parts wear out quickly, the maintenance man-hours are man-hours, the cost is high, and the one-time investment is large. Shot blasting is further divided into shot blasting and sandblasting. The surface treatment is carried out with shot blasting, the striking force is large, and the cleaning effect is obvious. However, the treatment of the thin plate workpiece by shot peening is easy to deform the workpiece, and the steel shot hits the surface of the workpiece (whether shot blasting or shot peening) to deform the metal substrate, because Fe3o4 and Fe2o3 have no plasticity, they are peeled off after breaking, and the oil film is deformed together with the substrate, so for the workpiece with oil stains, shot blasting and shot peening can not completely remove the oil stains. Among the existing workpiece surface treatment methods, sandblasting is the most effective cleaning effect. Sandblasting is suitable for the cleaning of the workpiece surface with high requirements. However, at present, China’s general sandblasting equipment is mostly composed of original and bulky sand conveying machinery such as hinge dragons, scrapers, and bucket elevators. The user needs to build a deep pit and make a waterproof layer to install machinery, the construction cost is high, the maintenance workload and maintenance cost are huge, and a large amount of silica dust can not be removed in the sandblasting process, which seriously affects the health of the operator and pollutes the environment.
Introductory knowledge of steel structure – related knowledge of surface treatment before painting
