Rustproof iron

Rustproof iron is a type of iron that is resistant to rust (i.e. iron oxide), hence the name. It was developed in the early 20th century by alloying normal iron with its fellow metal, chromium. However, rustproof iron eventually developed into its much more common namesake: stainless steel.

Before this industrial version of rustproof iron, other forms had been developed independently throughout history. The most notable is the Delhi iron pillar (of indeterminate age, but at least 1,600 years old), located in New Delhi, India.