How can metal corrosion be prevented by protective coatings and galvanising?

• A. Protective coatings (paint, lacquer) insulate metal from air and water; galvanising coats metal with zinc to corrode instead.
• B. Protective coatings cause faster corrosion; galvanising accelerates rust.
• C. Only galvanising; coatings have no effect on corrosion.
• D. Corrosion cannot be prevented.

Answer: (A)

The idea being tested is how protective coatings and galvanising prevent corrosion by creating barriers and by using a more reactive metal to protect the iron.

Protective coatings such as paint or lacquer form a physical layer that blocks air and water from reaching the metal surface. With air and moisture kept away, the chemical reaction that turns iron into rust can’t start, so the metal stays protected. It’s important that this coating remains intact, because any cracks or chips can expose the metal and allow corrosion to begin again.

Galvanising works mainly through sacrificial protection. Coating the metal with zinc puts a more reactive metal on the outside. If moisture is present, zinc tends to corrode first, drawing away the corrosive attack that would otherwise hit the iron. This can happen even if the coating is scratched, because the zinc near the scratch will continue to corrode, protecting the underlying metal. The zinc also helps form a protective layer of its own corrosion products, which adds to the protection.

So, coatings prevent corrosion by acting as a barrier, while galvanising provides sacrificial protection by sacrificing the coating metal to protect the iron underneath.