Final answer:
Zinc is commonly used to coat galvanized steel, providing both a protective barrier against corrosion and acting as a sacrificial metal for cathodic protection.
Step-by-step explanation:
The metal commonly used as a thin coating on galvanized steel to protect the underlying iron against corrosion is zinc. Galvanization is a method of protecting iron or similar metals from corrosion by coating with a layer of zinc, which is more easily oxidized. This layer serves as prophylactic protection, preventing oxygen from contacting the underlying iron, and as a form of cathodic protection where zinc will oxidize before the iron, should the coating be breached. This process is essential in preventing rust and maintaining the integrity of the steel over time.