1.How to remove surface oil stains?
During rolling and storage, substrates come into contact with lubricating oil and rust-proofing oil, requiring degreasing. Common methods include:
Chemical degreasing (most common): Use an alkaline solution (such as sodium hydroxide or sodium carbonate) or a specialized degreasing agent for spraying or soaking to dissolve the oil through emulsification, then rinse thoroughly with water.
Physical degreasing: For substrates with minimal oil contamination, hot air drying and brushing can be used for removal, suitable for environmentally sensitive applications.
After degreasing, test for water film continuity: Pour water onto the substrate surface. If the water film adheres evenly (without any dry spots), the oil has been completely removed.
2.How to remove rust/scale and treat surface rust?
Cold-rolled steel may develop minor rust due to improper storage. Oxidation on the surface of hot-dip galvanized steel can form "white rust," which requires rust removal:
Acidic Rust Removal: Spray with a dilute hydrochloric acid or sulfuric acid solution to dissolve the rust (reacting to form soluble salts), then rinse with water.
Mechanical Rust Removal: For thick scale, use a wire brush or high-pressure water jet to remove it, avoiding any residual acid.
Note: Rust removal on galvanized steel requires a controlled time (to avoid corrosion of the zinc layer). A "weak acid rust remover" is typically used.
3.How to oxidize/phosphate to form an "adhesion bridge"?
After cleaning, the substrate surface is still relatively smooth, resulting in insufficient coating adhesion. Therefore, a passivation or phosphating process is necessary to form a thin film (0.5-5μm thick) on the surface. This enhances both coating adhesion and corrosion resistance.
Phosphating: The most traditional method involves reacting a solution of zinc phosphate or iron phosphate with the substrate to form a porous phosphate crystalline film (such as a zinc phosphate film). The coating "embeds" into the pores, significantly improving adhesion. However, the phosphating solution contains phosphorus, making environmentally friendly processing expensive.
Passivation: An environmentally friendly alternative to phosphating, commonly used are chromate passivation (which forms a chromate film with strong corrosion resistance, but contains hexavalent chromium, which is not environmentally friendly) and chromium-free passivation (such as silane passivation and titanate passivation, which form a thin film through chemical adsorption, are environmentally friendly and compatible with most coatings).
Currently, mainstream color coating lines mostly use "chromium-free passivation," which meets environmental requirements while also meeting the adhesion requirements of subsequent coatings.
4.What are the key factors affecting coating quality?
Substrate pretreatment quality: Incomplete degreasing can lead to localized coating flaking; uneven passivation can reduce corrosion resistance (e.g., localized rusting during salt spray testing).
Coating viscosity and roller accuracy: Viscosity fluctuations can lead to uneven coating thickness (e.g., color variations within a batch of panels, which are actually thickness variations leading to varying light reflections); roller surface wear (e.g., scratches) can cause streaking defects in the coating.
Curing temperature and time: Insufficient curing (e.g., low temperatures) can result in low coating hardness (e.g., easily scratched by fingernails); excessive curing (e.g., prolonged curing) can lead to brittle coating (e.g., prone to cracking when bent).
Coating type matching: The primer and topcoat must be compatible (e.g., epoxy primers are compatible with all topcoats, while polyester primers require pre-tested adhesion when paired with fluorocarbon topcoats). Otherwise, intercoat flaking can occur.
5.What is the principle of roller coating?
The coating principle is to transfer the coating to the substrate surface using a coating roller. The equipment consists of a coating roller, an idler roller, and a scraper. The substrate is transported by the idler roller. The coating roller, after being dipped in the coating, contacts the substrate surface and transfers the coating. The scraper scrapes off excess coating from the coating roller (controlling coating thickness).
Key parameters:
Roller speed to substrate speed ratio: Forward coating (the coating roller rotates in the same direction as the substrate) is suitable for thin coatings (such as primers, with a thickness of 5-10μm), while reverse coating (the coating roller rotates in the opposite direction to the substrate) is suitable for thicker coatings (such as topcoats, with a thickness of 10-30μm).
Coating viscosity: Viscosity must be stable (usually controlled within 20-50 seconds, measured by a 4-cup coating). Too high a viscosity can easily cause streaking, while too low a viscosity can cause sagging (partially thick coating, resulting in sagging).