Question

how galvanised article is protected against rusting even if the zinc coating is broken

Answer 1

Zinc is more reactive than iron, and loses electrons more easily. The outer layer of zinc of any galvanized material, reacts with the atmospheric oxygen to form Zinc Oxide (ZnO), which is stronger than Zinc. Thus, even if the outer layer of zinc undergoes corrosion, the material is getting coated with a stronger substance (ZnO), and thus is better able to resist corrosion.