1.How should the substrate be pretreated?
Degreasing and Cleaning
Purpose: To remove residual grease, lubricating oil, dust, and particles from rolling or storage processes.
Method: Alkaline cleaning agent spraying or soaking, combined with brush rollers and electrolytic degreasing, followed by rinsing with clean water.
Function: To ensure that subsequent treatment solutions can fully contact the clean metal surface.
Chemical Conversion Treatment
This is the most critical pretreatment step, forming a dense non-metallic crystalline film on the clean metal surface.
Main Types:
Phosphating: Primarily used for cold-rolled sheets. Forms a phosphate crystalline film (such as zinc phosphate), which is porous and greatly enhances paint adhesion (providing mechanical anchoring) and offers some short-term rust prevention.
Chromating: Forms a chromium-containing passivation film. Excellent corrosion resistance, but due to environmental concerns (hexavalent chromium), it is gradually being replaced by chromium-free methods.
Chromium-Free Passivation: The current mainstream trend. By employing zirconium-based, titanium-based, silane, or organic-inorganic hybrid systems, an environmentally friendly passivation film can be formed, which can also provide good adhesion and corrosion resistance.
2.How should the coating be treated?
Hot-dip galvanizing
Process: Cold-rolled steel sheet is immersed in molten zinc to form a zinc-iron alloy layer and a pure zinc layer.
Function: Zinc acts as a sacrificial anode, providing electrochemical protection to the steel substrate. Even if the coating is scratched, zinc will corrode before iron. It is the most commonly used substrate in the construction and home appliance industries.
Hot-dip galvanized aluminum-magnesium
Upgraded coating: Aluminum, magnesium, and other elements are added to the zinc bath.
Advantages: Its corrosion products are denser, capable of automatically repairing scratches. Its corrosion resistance (especially edge protection) is several to ten times greater than ordinary galvanizing, and its lifespan is longer.
Electro-galvanizing
Process: A zinc layer is uniformly deposited on the surface of the steel sheet through electrolysis.
Characteristics: The coating is uniform, with a smooth and delicate surface, resulting in a good appearance, but the coating is usually thinner. It is commonly used in home appliances and interior parts where high surface quality is required.
3.What is the function of a coating system?
Primer
Function: Provides excellent adhesion to the pre-treated substrate or coating and a good base for the topcoat. Typically offers anti-corrosion properties (e.g., epoxy primer, polyester primer).
Topcoat
Function: Provides color, gloss, weather resistance, and other desired special properties.
Main Resin Types:
Polyester: High cost-performance ratio, good balance between weather resistance and processability, most widely used.
Silicone-modified polyester: Better weather resistance than ordinary polyester, used for building exteriors where durability is critical.
High-durability polyester/fluorocarbon: Excellent weather resistance, good gloss and color retention, used in high-end buildings.
Polyvinylidene fluoride (PVDF): Recognized as a top-tier weather-resistant coating, used in landmark buildings or harsh environments.
Plastic Sol: A thick PVC coating providing excellent scratch resistance, corrosion resistance, and flexibility; commonly used in household appliances, sound insulation panels, etc.
Back Coating
Applied to the back of the roll material, usually a single layer, providing basic protection, decoration, and insulation.
4.What are the effects of post-processing and special processing?
Embossing
After the coating has cured, embossing rollers are used to press three-dimensional textures such as wood grain and stone grain onto the surface, enhancing the decorative effect.
Lamination/Lamination
PVC, PET, or other plastic films are applied to the surface of the color-coated sheet, providing additional protection, special textures, or patterns (such as imitation marble or printed patterns).
Waxing or Protective Film Application
A peelable wax or plastic protective film is applied to the surface of the finished roll material to prevent scratches during transportation and processing.
5.How are the superior performances of color-coated steel coils achieved?
The superior performance of color-coated steel coils is not achieved by a single coating, but rather relies on a "system engineering" approach: Metal substrate → Plating (optional, such as hot-dip galvanizing) → Pretreatment (phosphating/passivation) → Primer → Topcoat → Post-treatment
Each layer performs a specific function:
The plating (such as zinc) provides long-lasting electrochemical corrosion protection.
The chemical conversion coating provides coating adhesion and short-term corrosion protection.
The primer enhances adhesion and provides barrier corrosion protection.
The topcoat provides final properties such as decoration, weather resistance, and chemical resistance.