What is the strongest kind of acid?
While sulfuric acid is a very strong acid, there are some acids that are even more powerful. In fact, if an acid is more acidic than sulfuric acid, it is called a superacid.
Most of the acids you might encounter are molecules that react with water by breaking up and donating a proton to the water. A proton is a hydrogen nucleus, and the water molecule that accepts it becomes a hydronium ion, H3O+.
Hydrogen chloride (HCl), for example, breaks apart in water to form a negative chloride ion Cl-, and donates its hydrogen nucleus to water to form the hydronium ion H3O+.
A strong acid is an acid that loses all of its hydrogen nuclei (protons) in water. Weak acids, like the acetic acid in vinegar, don’t lose all of their protons in water, so some of the molecules stay intact.
The carborane superacid H(CHB11Cl11) is a million times stronger than sulfuric acid. It donates a proton very easily.
But while the ease with which an acid donates a proton is how we define the strength of an acid, that is not what makes the acid corrosive, which may have been the main point of the question. What makes an acid corrosive is what is left behind after the proton leaves.
In the case of hydrofluoric acid, HF, what is left is the very reactive fluoride ion, and hydrofluoric acid reacts with almost anything. It even reacts with glass, so it can’t be stored in glass bottles.
In the case of carborane, what is left behind is a very stable ion that does not react with much. So although it is the strongest of the superacids, it can be stored in a glass jar.
But the strong superacid fluoroantimonic acid (HsbF6) is both an extremely strong superacid, and quite corrosive.
Another famously corrosive acid is a mixture of one part nitric acid to three parts hydrochloric acid. It is called aqua regia, because it can dissolve the noble metals gold and platinum.