Answer to Question #149783 in Chemistry for Nilamar Butra

Question

“Galvanized iron” is steel sheet that has been coated with zinc; “tin” cans are made of steel sheet coated with tin. Discuss the functions of these coatings and the electrochemistry of the corrosion reactions that occur if an electrolyte contacts the scratched surface of a galvanized iron sheet or a tin can.

Answer 1

A galvanized steel sheet is corrosion-free, not merely because zinc shields the iron as paint would, but because zinc electrochemically prevents iron from being reduced.

Scratch a galvanized pail and the pail will not corrode; the zinc will be oxidized instead. In principle the iron object does not even have to be covered completely for protection to occur.

The zinc itself is relatively well protected because when some of it is oxidized, it absorbs CO₂ from the air and forms a tightly adhering zinc oxide-carbonate coating.

A "tin can" is a different story. Tin has a higher reduction potential than iron, and a greater tendency to remain reduced as the metal.

A tin can is tin-plated iron, and if the surface is scratched, the iron will oxidize preferentially instead of the tin.

Nothing electrochemical is gained by plating the can with tin; it is only a super-tight protective coating like paint. When the coating is breached, corrosion is rapid.