1.What is the main reason why galvanized coatings can prevent rust?
Physical barrier function: Isolates the steel substrate from the external environment.
Cathode protection function: Zinc is more reactive than iron. When the coating is damaged, zinc acts as the anode, preferentially corroding and thus protecting the steel, which acts as the cathode.
Scratches destroy this protection. Cold galvanizing coatings contain a high concentration of zinc powder (typically over 90% zinc in the dry film). After application, these zinc powder particles form a conductive film that electrically connects to the original galvanized layer and the exposed steel substrate, re-establishing the cathodic protection effect.
2.How to improve the quality of surface treatment?
Cleaning and Degreasing: Use a specialized metal cleaner or solvent (such as acetone) to thoroughly remove oil, dust, and moisture from the scratches and surrounding area.
Grinding:
Purpose: To remove loose zinc plating, existing oxides, or white rust from the scratches, exposing a fresh, bright metal surface on the steel substrate. This is crucial for ensuring adhesion of zinc-rich coatings.
Tools: For linear scratches, an angle grinder with a cup-shaped wire brush or sandpaper is best.
Scope: The grinding area should be slightly wider than the scratch itself, extending approximately 10-20 mm into the intact galvanized layer on both sides, creating a smooth transition zone.
Further Cleaning: Thoroughly remove grinding dust using compressed air or a clean cloth.
3.How to choose the right repair materials?
"Cold galvanizing paint" or "galvanizing repair agent" must be used, not ordinary rust-preventive paint.
Characteristics: These products are single-component or two-component, rich in zinc powder, and typically gray after opening the can.
Forms: Available in brush and spray can types.
Brush type: More suitable for wider, deeper scratches, allowing for a thicker coating in one application.
Spray can type: Suitable for fine, shallower scratches, easier to treat, and produces a thinner, more even coating.
4.What are the steps involved in painting?
Stir thoroughly: Zinc powder in zinc-rich coatings tends to settle, so it must be thoroughly stirred before use.
Application:
Use a brush or spray can to evenly apply the coating to the prepared area.
Ensure the coating completely covers the exposed steel and the polished galvanized layer.
For best results, at least two coats are usually required. Refer to the product instructions for the interval between coats.
Achieve sufficient thickness: The dry film thickness of the repaired coating should be as close as possible to the thickness of the original hot-dip galvanized layer. The final coating should have a uniform gray metallic sheen.
5.What are some important points to note?
Do not directly cover: Never paint directly onto untreated scratches. Otherwise, the paint will cover the loose oxide layer, resulting in extremely poor adhesion, and internal corrosion will continue to develop, causing the coating to blister and peel off quickly.
Performance differences: Repaired cold-dip galvanized coatings do not have the same bonding strength and durability as the original hot-dip galvanized layer, but it is the most effective and economical remedy without re-hot-dip galvanizing.
For load-bearing structures: If the galvanized coils are used on critical load-bearing structures such as the floating bridges you mentioned earlier, scratch repair should be more rigorous. It is recommended to follow the standard procedures described above, and if necessary, even apply a matching epoxy intermediate coat and polyurethane topcoat for additional protection.