Iron Pillar of Delhi

This pillar is made of iron ( not sure if any other ingredient is also used ) and is 7 mtr tall. Though it's date of establishment is not sure, it' widely accepted as somewhere around in 900 BC. Amazing fact about the pillar is that, though for all these past centuries it was exposed to wars / sun / rain / pollution etc...it has not yet corroded. What technique would have our forefathers used ? It has not yet been discovered as to what all other ingredients are used in it's making. It's a marvelous work of ancient India. Just a small piece of work to astonish the modern world.