Etching can be performed with either a liquid phase or a gas phase in contact with the solid. I do a lot of work on etching silicon in aqueous solutions to form porous silicon. You can read more about ther here.
Laser induced processes with a reactive gas phase can also be used to modify the structure of solids and you can read more about that here.
You can learn more about surface structure modification and corrosion from the Surface Science and Corrosion Group in Erlangen.