Hot-dip galvanizing covers a steel part with zinc by immersing it in molten zinc. The coating stands between the steel and its environment, reducing direct exposure to moisture and oxygen.
A coating with two roles
Zinc also offers sacrificial protection. If a small area of steel is exposed by a scratch, nearby zinc corrodes preferentially and helps protect the steel until that zinc is consumed. Zinc corrosion products can form a protective patina on the coating. This mechanism differs from a paint film that relies chiefly on an intact barrier.
Where protection eventually ends
Protection is not unlimited: environment, coating thickness, cuts and time all matter. Large exposed areas or exhausted zinc can allow rust to begin. Galvanizing is also different from stainless steel, where chromium in the alloy forms a protective surface film.
Related: why stainless steel resists rust.