1.What is the purpose and cause of poor or failed passivation treatment?
Purpose: To further enhance the rust resistance of galvanized steel sheets, most galvanized coils undergo a "passivation" treatment before leaving the factory. This treatment applies an extremely thin, dense passivation film (usually containing or without chromium) to the zinc surface. This film effectively isolates the zinc from air and moisture.
Problem: Uneven passivation solution concentration, uneven spraying, inappropriate temperature, or incomplete drying can result in an incomplete or partially missing passivation film. Without or with weak protection, the zinc layer will preferentially react with moisture, resulting in yellow spots.
2.What are the causes and manifestations of residue on the coating surface?
Cause: During the post-galvanizing cooling process, if the water used is impure (containing impurities such as chloride ions and iron ions), these impurities will remain on the surface of the coating, becoming a starting point for moisture absorption and corrosion.
Symptoms: These yellow spots typically appear as fingerprints, water stains, or spots.
3.What is the reason why the zinc layer itself is impure?
If too much iron impurities or other metal elements are mixed into the galvanizing solution (zinc pot), the chemical properties of the coating itself will become unstable, making it more prone to galvanic corrosion and causing yellow spots to appear prematurely.
4.What are the inducing factors in the storage and transportation environment?
Humid Environment
This is the most direct cause. Zinc is stable in dry air, but in high-temperature, high-humidity environments, it reacts with moisture and oxygen to form white zinc oxide. This reacts further with carbon dioxide and other substances, ultimately forming yellow zinc hydroxide or basic zinc carbonate.
Condensation ("Inner Rust")
Symptom: This is a very typical situation. When galvanized coils are transported from a warm environment to a cold warehouse, or when there is a large temperature difference between day and night, "condensation" forms inside the coils.
Consequences: Because galvanized coils are tightly wound, this condensation is trapped between the layers and cannot evaporate. Long-term exposure to the zinc layer can lead to large, regularly spaced, watermark-like yellow spots or white rust.
Contact with Corrosive Substances
Sources of Pollution: Storage in environments containing acidic gases, salt (such as in coastal areas), chloride ions, industrial pollutants, etc. can significantly accelerate corrosion of the zinc layer.
5.What are the preventive measures?
Manufacturers: Require high-quality passivation and ensure pure cooling water.
Storage:
Store in a cool, dry, well-ventilated indoor warehouse.
Avoid storage with corrosive substances.
Use dry wood between coils to avoid direct contact with the ground.
If wrapping with film, ensure the internal air is dry or use specialized packaging material with a vapor phase rust inhibitor.