1.What is the main reason for the whitening of the galvanized steel surface?
Condensation and humid environment problems: Galvanized parts are stacked tightly during transportation or storage, and air is not circulated, causing condensation to remain on the surface for a long time.
Surface residual chemicals ◦ Passivation layer damage: Passivation treatment after galvanizing (such as chromate passivation) can prevent corrosion. If the passivation layer is damaged or the residual acid, alkali, and salt (such as chloride and sulfate) on the surface are not thoroughly cleaned, it will accelerate the corrosion and whitening of zinc.
Electrochemical corrosion (dissimilar metal contact): When galvanized steel contacts metals with higher potentials such as copper and lead, a galvanic effect is formed in a humid environment. Zinc is accelerated to corrode as an anode and generates white corrosion products.
Zinc layer quality defects
Uneven/too thin zinc layer: Insufficient local zinc coverage, electrochemical corrosion is induced after the matrix iron is exposed.
Abnormal zinc alloy phase: Too much brittle ζ phase (FeZn₁₃) in the coating, and the corrosion resistance is reduced.
2.What is the mechanism by which condensed water and humid environment cause white rust?
Zinc reacts with water and oxygen in a humid environment to form zinc hydroxide, which further reacts with carbon dioxide to form basic zinc carbonate (a white porous powdery substance).
Typical scenarios: in shipping containers, high humidity warehouses, and open-air storage without covering during the rainy season.
3.What are the common contaminants remaining on surfaces?
Common pollutants:
Sweat, fingerprints (containing salt)
Sulfide/chloride ions in rainwater (coastal areas)
Industrial dust (containing acidic components).
4.What are the solutions and preventive measures?
Storage and transportation
Avoid close stacking: Use wooden spacers to separate galvanized parts and ensure ventilation.
Dry environment: The humidity in the warehouse is controlled below 60%, and a desiccant or dehumidifier is used.
Moisture-proof cover: When storing outdoors, cover with waterproof cloth to avoid rain infiltration.
Surface cleaning and protection
Remove pollutants in time: rinse the surface sweat and salt with clean water, and wipe dry with a soft cloth.
Reapply protective layer: ◦ Light white rust: wipe with a hard brush dipped in 5% acetic acid solution (or diluted white vinegar), rinse with clean water and dry thoroughly.
Stubborn white rust: Use zinc material special passivation solution to re-passivate and form a protective film.
Rust prevention with oil: Apply light anti-rust oil (such as vaseline) for short-term storage.
Avoid contact between dissimilar metals. Use insulating materials (rubber pads, plastic sleeves) to isolate different metals.
5.What is the difference between white rust and normal zinc oxidation?
Normal oxidation: Zinc forms a dense gray-white oxide film (ZnO) after being exposed to air, which has a protective effect and is not easy to fall off.
White rust (abnormal corrosion): White loose powder, which may expose the base after wiping and needs to be handled in time.