A construction company wants to build a new steel bridge over a river in Ontario. They are considering using either painted steel or galvanized steel for the beams of the bridge. Which option do you think is better and why?
It is difficult to answer this question as the choice would depend on the type of impurities present in the water flowing in the river, their concentration, the type of steel being used, etc.
Galvanized steel is steel coated with a layer of zinc. Zinc protects steel from corrosion as it reacts with oxygen in the air to form zinc oxide ZnO,
2 Zn + O2 --> 2 ZnO
and later reacts with carbon dioxide to form zinc carbonate
ZnO + CO2 --> ZnCO3
Zinc carbonate is a strong material that covers the underlying steel structure and prevents further corrosion. Galvanization has been used over a long duration of time to protect steel structures from corrosion.
Modern paints are also very efficient in protecting metal structures from corrosion. They cover the metal in a very effective manner and have a high affinity that allows them to stay on the surface even in rough environmental conditions. Paints with additives can be used for protecting metals from different corrosive gases like oxygen, carbon dioxide, etc. and other corrosive salts dissolved in the water.
Modern steels are inherently resistive to corrosion due to the formulation of metals used in the alloy. If the steel being used for the bridge can resist corrosion on its own, the paint used would be for decorative purposes only. In this case there is no use of using galvanized steel which tends to lose its visual appeal with time as the layer of zinc carbonate is formed.