Weird A-Z. I is for Iron Pillar of Delhi

Weird A-Z. I is for Iron Pillar of Delhi

In the centre of the Qutb complex of monuments in Delhi, this dates to the fourth century CE, and bears an inscription that it was erected as a flagstaff in honour of the Hindu god Vishnu, and in the memory of the Gupta King Chandragupta II (375-413). 

It is composed of 98 percent wrought iron and has stood for 1600 years without rusting or decomposing. Around 24 feet (7.3m) tall, it weighs approximately 6.5 tons, and was manufactured by forged welding. Not until 2002 did scientists solve the secret of the non-corrosion of the iron column despite Delhi's harsh weather conditions. 

They discovered that a thin layer of 'misawite', a compound of iron, oxygen and hydrogen, had protected the cast iron pillar from rust. The protective film began to form within three years of the erection of the pillar and has been growing extremely slowly since then.

After 1600 years, the film has grown to just one-twentieth of a millimetre thick. The protective film was formed catalytically by the presence of high levels of phosphorus in the iron, as much as 1 percent compared to less than 0.05 percent typical in today's iron, i.e. around 20 times as much.

The high phosphorous content is a result of a unique iron making process which reduced iron ore into steel in one step by mixing it with charcoal. Modern blast furnaces use limestone in place of charcoal, yielding molten slag and pig iron, which is later converted into steel. 

In the modern process most phosphorous is carried away by the slag. The pillar demonstrates remarkable metallurgical knowledge, and a model developed for predicting growth of the protective film may also be useful for modelling long-term corrosion behaviours of containers for nuclear storage applications.