Galvanized coatings protect steel in three ways:
Firstly, the coating weathers at a very slow rate giving a predictable and long life.
Secondly, the coating corrodes preferentially to provide cathodic (sacrificial) protection to any small areas of steel exposed through drilling, accidental damage or cutting; scratches are sealed by weathering products from the zinc.
Thirdly, if the damaged area is larger, the sacrificial protection prevents the sideways creep of rust which can also undermine paint coatings.
Visit the Wedge Group Galvanizing website for more information on Hot Dip Galvanizing - Three-way Protection