There are many types of corrosion. I will talk about the main one and take the metal as an example. Generally the corrosion of metal means the formation of the more chemically stable state of the metal, such as oxide, hydroxide, etc...As metal generally have free electrons. When in contact with acid, there will be some chemical reactions. Electrons will be donated to the ions in the acid, leaving the metal in the ionic state, which is essential for the metal to form the final stable state such as those mentioned above. That's why a lot of acids are corrosive when compared to the natural corrosion in air or water. Generally speaking, the acid will speed up the process, the rate of corrosion, as a result of the chemical reaction.