Corrosion is a process by which some objects made of iron got covered with an orange or reddish-brown colored layer at some point in time.
The formation of this layer is the result of a chemical process known as rusting, which is a form of corrosion.
Water, air, and acid are three of the most common corroding chemicals.
Whenever the acid reacted with the iron in the nail, it has started to eat it away. The nail has rusted or corroded.
A metal corrodes whenever a chemical attacks its surface. Metals first lose their shiny surface and when the layer corroded is removed, they lose their strength.