1.What are the hazards of direct painting?
The rust layer destroys the adhesion of the coating
Rust (mainly iron oxide) is a loose and porous substance with poor bonding to the substrate (such as steel plate, galvanized plate) and cannot form an effective bond with the color coating (resin, pigment and other components). After direct coating, the coating will be delaminated, bubbled, and peeled off due to "unsound substrate", and will fail in a short period of time.
Rust will continue to spread
Rust contains moisture and electrolytes. Even if it is covered by the coating, electrochemical corrosion will continue to occur under the coating (forming "micro batteries"), causing the rust area of the substrate to expand. It may eventually spread outward from the damaged coating (such as scratches), causing the entire color-coated plate to lose its protective effect.
Appearance defects of the coating
The unevenness of the rusted area will lead to uneven coating thickness, pinholes, shrinkage holes, color difference and other problems, affecting the aesthetics of the product, especially in scenes with high requirements for appearance such as buildings and home appliances.
2.What are the different treatment methods for different degrees of rust?
Mild rust (surface rust): All visible rust and scale can be removed by mechanical grinding (such as sandpaper, wire brush), chemical pickling (such as hydrochloric acid, phosphoric acid solution) or sandblasting to expose the clean metal substrate.
Severe rust (rust depth > 0.1mm or pitting): It is necessary to evaluate whether the substrate thickness loss is within the allowable range. If the rust has caused the substrate to become thinner, perforated, or formed deep pits that cannot be removed, the substrate is no longer suitable as a color coating substrate and needs to be scrapped.
3.How to perform surface activation and passivation treatment?
After rust removal, the surface of the substrate needs to be activated (such as weak acid cleaning) to remove residual rust and impurities, and then passivated (such as chromate passivation, chromium-free passivation) to form a dense oxide film to enhance the adhesion between the substrate and subsequent coatings and inhibit secondary corrosion.
4.How to repair substrate defects?
If there are slight pits on the surface after rust removal, they can be filled by applying a primer (such as epoxy primer) to ensure that the substrate surface is flat and provide a uniform adhesion base for the color coating.
5.Limited treatment for localized minor rust?
For substrates with only a small amount of floating rust on the edge or part (and without damaging the substrate body), after strict grinding and passivation, they can be used in scenes with lower performance requirements (such as temporary enclosures and packaging materials), but two conditions must be met:
The rust area accounts for less than 5%, and there is no deep pitting;
After treatment, the primer thickness needs to be increased (such as dry film thickness ≥ 15μm) to enhance the protection ability.
However, the service life of such products is much lower than that of normal color-coated plates, and it is not recommended to be used for long-term outdoor or high-corrosion environments.