1.What is the principle that causes "white rust" corrosion and indirectly causes shedding?
Mechanism: When the surface of a galvanized coil is exposed to humid air, especially when the relative humidity exceeds 60%, and condensation forms on the surface due to temperature fluctuations, an electrolyte forms.
Zinc reacts with water, oxygen, and carbon dioxide to form basic zinc carbonate (Zn₅(CO₃)₂(OH)₆), commonly known as "white rust."
2.What causes shedding?
Early Stage: White rust is a loose, porous, and non-protective corrosion product. It coats the zinc surface.
Severe Stage: If exposed to high humidity, the white rust will continue to thicken and its adhesion will be poor. When flakes of white corrosion product powder and flake off from the substrate, we see a phenomenon similar to "sloughing." This is actually the flaking of corrosion products, not the peeling of the intact zinc layer.
3.What are the processes that trigger "wet storage corrosion" under certain conditions?
Scenario: During storage or shipping, narrow gaps form between tightly wound galvanized coils.
Process:
Moisture intrusion: High humidity or direct exposure to rain.
Oxygen concentration gradient: Deep within the coil's gaps, oxygen is difficult to penetrate, while oxygen is abundant outside.
Oxygen concentration gradient cell: This forms a macroscopic battery. The oxygen-deficient area (inside the coil) becomes the anode, while the oxygen-rich area (at the edge of the coil) becomes the cathode.
Accelerated corrosion: Zinc in the anode area corrodes faster, producing large amounts of white corrosion products. These products accumulate in a confined space, generating significant pressure.
Result: The coils eventually become extensively adhered. When forcibly pulled apart, the zinc layer, along with its corrosion products, tears off, exposing the steel substrate. This is a highly destructive peeling process.
4.How will the adhesion foundation be affected in the long term?
Even if the zinc coating isn't severely rusted, prolonged exposure to high humidity can allow moisture to penetrate the zinc-iron interface through microscopic pores or defects in the zinc layer.
This weakens the bond between the zinc layer and the steel substrate, creating an intermetallic compound layer (such as the Fe₂Al₅ inhibition layer). This, in the long term, reduces adhesion and makes it more susceptible to peeling during subsequent processing (such as bending and stamping).
5.What are the synergistic effects with other factors?
Humidity rarely causes problems alone; it often works in conjunction with other factors to exacerbate the problem:
Humidity + High Temperature: High temperatures accelerate all chemical reactions, accelerating corrosion.
Humidity + Pollutants:
Chloride Ions: In coastal areas or near chemical plants, salt in the air dissolves in water, forming a highly corrosive electrolyte that significantly accelerates zinc corrosion.
Sulfides: In industrial areas, acidic gases such as sulfur dioxide dissolve in water, forming acids that directly corrode the zinc layer.