Why is surface treatment important before coating a metal surface?

Thread Content

Steel surfaces often have rust, scale, oil stains, and the like. Scale is a scale-like layer that forms on steel during hot rolling at high temperatures. Its coefficient of expansion is lower than that of steel, and it tends to crack under thermal cycling. The lattice constant of iron is 2.86 angstroms, while that of iron oxide is 8.3 angstroms; as a result, the two do not adhere well to each other and will peel off over time. Moreover, it is brittle and imperfect, allowing water vapor to penetrate through its cracks. The potential of the oxide scale is higher than that of the steel, and a corrosion cell is formed in humid conditions, with the oxide scale acting as the cathode and the steel as the anode. Moreover, at this time the area of the oxide scale is large while the area of steel in the gaps is small, resulting in a large cathode and a small anode, which causes concentrated corrosion pits. Under the loose oxide scale, the oxygen concentration is low while the oxygen concentration outside is high, which in turn creates an oxygen concentration difference cell; therefore, the oxide scale must be removed before painting. From a polymer technology perspective, coatings not only form thin polymer films but also serve as excellent adhesives. Plain plastic film cannot prevent metal corrosion; it must be firmly bonded to the steel surface. Generally, paint adheres to metal surfaces due to the polar attractive forces between molecular secondary bonds; the magnitude of these forces is inversely proportional to the sixth power of the distance between molecules – the smaller the distance, the greater the force ； However, the attraction range must be within 5 angstroms to be effective, that is, within 3 times the atomic diameter of oxygen. Therefore, if there is oil on the steel surface, even a monolayer of oil with a thickness of over 5 angstroms will cause the coating to lose its adhesion. However, when the steel surface becomes oily, the free energy of its outer layer drops to a certain level, sometimes even lower than that of some coatings. This not only significantly reduces the wetting adhesion but can also cause shrinkage cavities. In such cases, oil-based paints (with low surface free energy) can still suffice, whereas highly polar coatings such as epoxy and polyurethane coatings fail to adhere. Therefore, metal surface treatment requires both rust removal and oil removal. The rust on steel surfaces is loose oxides such as γ-FeOOH, α-FeOOH, and Fe2O3; in industrial areas with SO2 present, the rust layer contains ferrous sulfate ； The rust layer in coastal areas contains sodium chloride, and these impurities accelerate corrosion; for example, ferrous sulfate hydrolyzes to produce sulfuric acid, which corrodes iron and then generates more ferrous sulfate, creating a continuous cycle. Therefore, rust must be removed, and ferrous sulfate is deeply buried at the bottom of the corrosion pits (referred to as ferrous sulfate nests), making it difficult to remove completely; sometimes it needs to be washed away with clean water before sandblasting.