The Delhi Iron Pillar Does Rust. Its Own Rust Then Guards the Iron, and Has for 1,600 Years. IIT Kanpur Worked Out How

In 1997 the Archaeological Survey of India put an iron cage around the Delhi iron pillar. For centuries visitors had stood with their backs to it and clasped their hands behind it; the band at chest height was polished bright as a mirror. With the hands gone, a metallurgist from IIT Kanpur photographed that band at intervals and watched it do the one thing the pillar is famous for not doing. It rusted. Within a year the bright band was dark brown. By early 2000 it was black. So the question was never whether the pillar rusts. It is why the rust slows to almost nothing.