The term “pickling” refers to the chemical removal operation of encrustations present on the metal surfaces of equipment and / or pipes. The pickling is carried out on metal surfaces to remove the oxides that cover the metal, bringing it back to the bare. Most of the pickling activities involve pipes and / or equipment made of iron or iron alloys, but they can also involve metal pipes of other material, for example alloy steels (stainless steel). The oxidized layer formed on a metal surface by external agents constitutes a protection for the metal itself; its removal makes the metal susceptible to new attacks with consequent reformation of the protective oxide layer. The oxide removal operation is usually completed with a passivation step in order to create a protective layer on the bare metal surface which slows down the reoxidation of the metal.