Question

c) Rusting of iron can be prevented by painting. Then why do
we galvanize bridges?​

Answer 1

Paint or plating(like chrome plating) prevents the H2O molecules from reaching the surface of the metal so it can't get the oxygen. As a consequence, the iron can't rust since it can't get to the water molecules when it is painted.

Galvanizing protects from rust in a number of ways: It forms a barrier that prevents corrosive substances from reaching the underlying steel or iron. The zinc serves as a sacrificial anode so that even if the coating is scratched, the exposed steel will still be protected by the remaining zinc.

Both works like double shield..