Metal pretreatment and its functions

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This post was last edited by PU3618988 on 2015-10-16 at 11:39. Pretreatment refers to the use of physical or chemical methods to remove contaminants such as grease and rust from the metal surface before painting; it also increases the roughness of the substrate, thereby enhancing the mechanical interlock between the coating and the base metal, and improving the coating’s adhesion to the material as well as its corrosion and rust resistance. The excellent adhesion of the coating to the substrate is a prerequisite for the coating to maintain its corrosion-resistant properties. Research has shown that the quality of surface treatment has an even greater impact on the lifespan of coatings compared to all other factors affecting that lifespan; therefore, the quality of surface pretreatment directly determines the lifespan and properties of the coatings. Applying different methods of surface pretreatment to metals can effectively improve the service life of the coatings. The functions of surface pretreatment mainly include the following three points: (1) Refining the metal surface. Studies have shown that frictioning the metal surface not only causes plastic deformation of the surface but also raises its temperature, thereby increasing the rate at which N and O from the atmospheric environment diffuse into the interior of the metal. This results in a gradual change in hardness from the metal surface toward the core of the metal, forming a modified layer that enhances the surface properties of the metal. Furthermore, the coating contracts to some extent during curing and cooling, which may cause the coating at the edges of the substrate to peel off. Therefore, performing some finishing on the metal surface before spraying can ensure that the fragile areas of the coating do not peel off due to contraction. (2) Purification: The pre-treatment process removes the oxide layer, grease, and other contaminants from the surface, bringing it to the cleanliness level required for spraying. For removing embroidery, methods such as acid washing, alkali washing, electrolysis, and grinding are generally used, while for degreasing, methods such as cleaning with organic solvents, boiling in alkaline solutions, cleaning with emulsions, and electrolysis are commonly employed. (3) Roughening: Roughening refers to the use of certain methods to create a uniform rough structure on the surface of the purified substrate. This uneven surface texture not only increases the bonding area, thereby providing more points of attachment with the coating, but also enhances the strength of the bond between the coating and the substrate. On the other hand, roughening the surface can activate the metal substrate, improving the spreading properties of the coating on the metal surface, and thus enhancing the bonding strength between the coating and the substrate surface.