Look at a clean iron or steel nail and the surface looks unaffected because the oxide layer is so thin. When the oxide coating is thicker, it may appear as a discolouring, or tarnishing, of the surface. However, unlike some other metals such as aluminium and copper, iron's oxide coating is not able to keep water and oxygen out. On the contrary, it is a porous coating, which is why it rusts. Steel nails need water and oxygen from the air to rust quickly.
Water alone is a poor corroding agent.
The chemistry of smelting The processes that occur inside the blast furnace are quite complex, and different reactions happen at the top and bottom of the furnace. The objective is to ensure that the right chemical reactions occur in each part of the furnace so that controlled and continuous processing is achieved. Iron oxide sinks down the furnace where it reacts with carbon monoxide gas coming up from the bottom of the furnace. The two then react to produce carbon dioxide gas and liquid iron. Liquids descend through the mixture in the furnace.