Bicycle chains are made from an alloy of iron. Bicycle chains rust without protection. Paint is not used to protect bicycle chains from rusting.
Suggest how bicycle chains can be protected from rusting.

Rusting (the corrosion of iron) requires the presence of both oxygen and water. Protection methods work by providing a barrier to prevent one or both of these from reaching the iron surface. For a bicycle chain, which has many moving parts, a solid barrier like paint is unsuitable as it would be inflexible and quickly wear away. A liquid barrier is needed. Applying oil or grease coats the metal surface. This layer is waterproof and prevents oxygen and water from coming into contact with the iron, thus preventing rust. It also has the added benefit of lubricating the chain, reducing friction and wear between the moving parts.