Thread Content
If the foundation of the coating (surface pretreatment) is not properly prepared, it will be difficult for the coating to achieve the required lifespan for protection, or catastrophic consequences may occur. Surface pretreatment can create conditions for high-performance coatings in two aspects: the first is mechanical, by providing surface roughness for the coating ； The second aspect is chemical, which ensures close contact between the molecules of the coating and the surface of the steel substrate. We can imagine what the effect of applying paint to a smooth surface or a dirty surface would be, and thus understand the importance of surface pretreatment in achieving a good coating quality. 1. Overcoming the adverse effect of smooth surfaces on coatings: It is a well-known fact that if a surface is very smooth, there will be no good adhesion between the coating and the surface; thus, the coating on the glass surface can be removed easily using a scraper or even fingernails. On the contrary, if the surface of the workpiece is as rough as sandpaper, then it is not easy to remove the coating. After shot blasting, the surface of steel becomes as rough as sandpaper, with numerous tiny peaks and valleys – this is what we refer to as surface roughness. The coating penetrates to the bottom of the wave troughs, with the wave crests acting like teeth that firmly grip the coating. Therefore, in the coating industry, some people describe surface roughness as “anchors” or “mechanical teeth”. 2. Eliminating the harmful effects of surface contaminants on the coating. Contaminants on the surface of workpieces are composed of many different substances; those on steel surfaces include dirt, dust, grease, oil, rust, oxide scale, and sometimes water vapor as well. If these contaminants are covered by a coating, the mechanical and chemical adhesion of the coating will be affected, causing the coating to fail. On the contrary, if these contaminants are removed before painting, a complete coating that bonds well to the steel can be formed on the surface of the workpiece, thereby ensuring effective adhesion of the coating. A coating with good adhesion acts as an effective barrier, preventing moisture from coming into contact with the steel. It should be clear that, during the corrosion of steel, water vapor acts as an electrolyte. 3. Harmful substances invisible to the naked eye: There are also some chemical hazards that cannot be seen with the naked eye. The most harmful chemical pollutants are certain soluble salts, such as chlorides and sulfides, etc. If these harmful substances are not completely removed, they will allow water vapor to penetrate the coating and reach its bottom, causing bubbling and separating the coating from the substrate, which accelerates the corrosion of the steel. During the surface preparation of rusted steel structures prior to painting, the areas where corrosion pits appear after shot blasting (especially the bottoms of these pits) may contain soluble salts. Dry shot blasting cannot remove these salts completely; therefore, it is advisable to use specialized on-site testing equipment before painting to check whether there are soluble salts on the surface of the workpiece and what their concentration is. If the concentration of soluble salts exceeds the allowable level, measures must be taken to remove them.