1.What is surface activation treatment for cold-rolled coils? What is its core purpose?
Surface activation treatment refers to the cleaning and modification of the surface of cold-rolled coils through chemical or physical methods, transforming it from a relatively inert or contaminated state to a high-energy, highly active state.
Its core purpose is to improve the adhesion between the surface of the cold-rolled coil and subsequent coatings, oil films, or adhesives. Simply put, it's about making "paint adhere better," "rust-preventive oil cover more evenly," or "preventing it from peeling off during subsequent bonding."
2.Cold-rolled coils are usually clean before leaving the factory, so why is it necessary to carry out a special "activation" treatment?
Residual rolling oil: Despite degreasing and cleaning, a very thin oil film may still remain at the microscopic level.
Surface oxide layer: During the annealing process, a very thin, loose oxide layer (such as iron oxide) forms on the surface of cold-rolled coils.
Surface inertness: Pure steel surfaces quickly absorb dust and moisture from the air, forming a weak boundary layer that reduces surface energy.
Without activation treatment, these microscopic contaminants and low surface energy state act as an "isolation layer," causing subsequent paint coatings to peel off quickly or preventing the applied rust-preventive oil from spreading evenly, thus losing its protective function.
3.How exactly is the activation treatment achieved? What changes will it cause on the surface?
Chemical conversion treatments (e.g., phosphating, passivation):
Changes: Through chemical reactions, a dense, insoluble conversion film (e.g., phosphate crystal film) is formed on the steel surface. This film itself serves as a good substrate and increases the surface's micro-roughness, significantly improving coating adhesion and providing some corrosion resistance.
Physical/electrochemical treatments (e.g., electrolytic cleaning, plasma treatment):
Changes: Through the bubble bursting effect or high-energy particle bombardment generated by electrolysis, surface residues are thoroughly removed, and the surface's microstructure and chemical composition may be altered (e.g., increasing oxygen-containing functional groups), thereby significantly increasing surface energy.
4.What specific impact does the activation treatment have on the quality of the final product?
Coating Quality: Unactivated or poorly activated surfaces are prone to orange peel, pinholes, poor adhesion, and even large-area peeling after a period of time. A well-activated surface ensures a uniform, glossy, and durable coating.
Rust Prevention: Activated surfaces ensure that the rust-preventive oil spreads evenly into a continuous film, preventing pitting or rust spots caused by film breakage.
Adhesion Performance: For cold-rolled coils requiring subsequent spot welding or composite panel production, activated surfaces significantly improve the wettability and bonding strength of adhesives, preventing interface detachment.
5.How to determine or verify whether the surface activation treatment of a batch of cold-rolled coils is qualified?
Surface wettability test (dyne pen test): This is the quickest on-site method. A test pen with a specific surface energy (e.g., 38-42 dynes/cm) is used to draw lines on the surface. If the ink spreads evenly into a continuous line, the surface energy is adequate and activation is good; if the ink shrinks into droplets or discontinuous lines, the surface energy is low and activation is insufficient.
Adhesion test: After coating or oiling, a cross-cut test, tape peel test, or bending test is performed to see if the coating or oil film easily peels off.
Conversion film quality inspection: For chemically treated surfaces, the film weight (weight of the conversion film per unit area) and crystal density are tested to ensure the conversion film is uniform and intact.
Corrosion resistance test: Accelerated corrosion methods such as salt spray tests are used to evaluate the substrate's ability to resist early corrosion after activation treatment (especially chemical conversion treatment).