Although galvanized steel sheet has strong rust resistance, it can still rust in certain situations. The main reasons include the following:

1. Damage or wear of the zinc coating
- Mechanical damage: During transportation, processing, or installation, the zinc coating may be damaged by collisions, scrapes, cutting, or welding, exposing the underlying steel and causing rust.
- Localized corrosion: If the coating contains pores, bubbles, or uneven thickness (such as a thin electroplated zinc coating), corrosive media may penetrate the weak areas and attack the base metal.

2. Environmental factors
(A) Extremely corrosive environments:
- High humidity and high salinity (such as coastal areas): Chloride ions accelerate corrosion of the zinc coating.
- Acid rain or industrial pollution: Acidic substances (such as SO₂ and H₂S) can damage the zinc oxide protective film (ZnO/Zn(OH)₂).
- Long-term exposure to chemicals, Such as strong acids, strong bases, or certain organic solvents. 
(B) Water accumulation or prolonged moisture: If water accumulates on the surface of the galvanized sheet for a long time (such as at joints), the zinc layer may develop "white rust" (basic zinc carbonate) in an oxygen-deficient environment, reducing its protective properties.

3. Natural zinc layer wear
- Time factor: The zinc layer continuously protects the steel through sacrificial anode action, but it gradually oxidizes and wears away over time. In harsh environments, the zinc layer wears away faster, eventually exposing the base iron and causing rust.
- Insufficient coating thickness: If the zinc coating does not meet the standard (e.g., less than 80g/m²), the protective lifespan will be significantly shortened.

4. Electrochemical corrosion (dissimilar metal contact)
- Direct contact between galvanized steel and metals with a higher potential (such as copper or stainless steel) can cause galvanic corrosion, accelerating zinc layer wear, and even causing localized rust.

5. Improper welding or cutting
(A) Welding area: High temperatures can burn away the zinc layer. Without anti-corrosion treatment (such as a recoat of zinc-rich paint), welds are prone to rust. 
(B) Cutting edge: The galvanized layer at the cut may not be completely covered and needs to be sealed to prevent rust.
Preventive measures
(1) Select the appropriate coating: Select the galvanizing process (hot-dip galvanizing is thicker and more durable than electroplating) and coating thickness according to the environment.
(2) Avoid mechanical damage: Protect the coating during processing and installation, and repair damaged parts in time (such as spraying zinc-based paint).
(3) Environmental control:
- Avoid long-term contact with corrosive media.
- Keep the surface clean and dry to prevent water accumulation.
(4) Isolate dissimilar metals: Use insulating materials (such as rubber pads) to isolate different metals to avoid galvanic corrosion.
(5) Post-maintenance: Regularly inspect and apply protective coating (such as epoxy zinc-rich paint) to aged coatings.

Summary: The rust resistance of galvanized steel sheets depends on the coating quality, use environment, and maintenance measures. If rust occurs, it is necessary to analyze whether it is caused by coating damage, environmental corrosion, or other factors, and take targeted measures to repair and protect it.