In galvanizing, the protective film effect (often called the zinc patina) refers to the formation of a dense oxide/carbonate layer on the zinc surface.
This layer is impermeable to oxygen and moisture, which slows corrosion and prevents rust. This protection functions in the atmosphere, as well as in water, soil, and concrete.
However, if the surface is exposed to strong acids, alkalis, or salts, this protective film is destroyed. Once the barrier is compromised, the zinc begins to corrode rapidly, forming white rust.