1.What are the common pollutants on the surface of galvanized steel sheets?
Physical pollutants: dust, oil (rolling oil, anti-rust oil), zinc powder particles, fingerprints (containing salt and grease).
Chemical pollutants: passivation film on the surface of the zinc coating (such as chromate passivation layer, which may affect the adhesion of the coating), oxide layer (natural oxide film of zinc, which is thin but needs to be activated), and residual acid and alkali substances.
Mechanical defects: burrs, scratches, zinc nodules (affecting coating flatness).
2.What is the surface treatment process?
Pretreatment: remove visible contaminants
Activation treatment: improve the adhesion of the zinc coating and coating
Post-treatment: ensure the surface is clean and dry
3.How do ensure the surface is clean and dry?
Washing and drying: All chemical treatments must be rinsed with deionized water or pure water at least 2-3 times to ensure that there is no chemical residue on the surface.
Drying method: Natural drying, hot air drying or blowing with clean compressed air to ensure that the surface is completely dry.
Surface condition inspection:
Visual inspection: The surface is free of oil, water stains, dust, and feels smooth and non-sticky.
Adhesion prediction: Paint within 30 minutes after drying to avoid re-oxidation of the surface.
4.How to deal with special situations?
Refurbishment of old galvanized sheets: First remove the aging coating and rust on the surface, then use 10% dilute sulfuric acid to lightly etch for 10 seconds to remove the old oxide layer, then wash with water and process according to the new sheet process.
Galvanized sheet with chromate passivation layer: If the passivation layer is intact and free of oil stains, it can be directly painted after light grinding; if the passivation layer is damaged or contains hexavalent chromium, it needs to be removed with a weak acid solution.
Fingerprint removal: wipe with a special fingerprint cleaner, or wipe repeatedly with a cloth soaked in alcohol, avoid direct washing with water.
5.What are the precautions?
Avoid excessive treatment: Do not soak in strong acid or alkali for a long time.
Control the intensity when grinding to remove only surface pollutants and oxide layer without damaging the base zinc layer.
Aging after treatment: Painting should be completed within 4 hours after surface treatment (shortened to 2 hours in a humid environment) to avoid secondary contamination or oxidation.
Tools and environment control: Handling tools (rags, sandpaper, containers) must be dedicated to avoid cross contamination (e.g. rags that have been stained with oil cannot be used to clean degreased surfaces).
The handling environment should be kept dry and ventilated, with a relative humidity of ≤70% and a temperature of 15-35℃ (low temperature will prolong the drying time, and high temperature may cause oxidation of the zinc layer).
Compatibility with coatings: The surface of galvanized sheet is a zinc layer (low polarity). It is recommended to choose a coating that is compatible with the zinc layer (such as epoxy primer, polyurethane paint) and avoid using coatings that are corrosive to zinc (such as some alkyd paints).