The applied zinc layer chemically protects the underlying vulnerable steel in two distinct ways. First, it forms a tough physical barrier that completely blocks atmospheric oxygen and liquid water. These two elements are strictly required for unprotected iron metal to rust and physically decay.
Second, the metallic zinc coating acts chemically as a highly effective sacrificial anode material. Zinc is significantly more chemically reactive than iron on the standard metal activity series. Therefore, the zinc metal will lose its valence electrons much more easily than the iron.
If the galvanized outer coating is deeply scratched, the newly exposed steel remains completely protected. The nearby reactive zinc corrodes preferentially and quickly oxidizes into soluble Zn2+ ions. The underlying iron metal remains completely reduced and entirely safe from destructive chemical rust.
This fascinating electrochemical process is widely known in chemistry as sacrificial cathodic protection. Over time, the outer zinc layer reacts with atmospheric oxygen, moisture, and ambient carbon dioxide. This chemical reaction forms a durable patina of basic zinc carbonate known as hydrozincite.
The exact chemical formula for this tightly clinging mineral layer is Zn5(CO3)2(OH)6. This solid carbonate layer strongly resists any further chemical weathering or physical decay. Galvanization is incredibly unique because the zinc coating chemically sacrifices itself to save the steel.