What is the Mehrauli Pillar in the complex of Qutub Minar Primarily famous for?

Mehrauli Pillar is famous for Excellent quality steel because its high resistance to corrosion and has been called a "testimony to the high level of skill achieved by the ancient Indian iron smiths in the extraction and processing of iron". The corrosion resistance results from an even layer of crystalline iron hydrogen phosphate hydrate forming on the high-phosphorus-content iron, which serves to protect it from the effects of the Delhi climate.