The treatment of metal surfaces such as ferrous metals (iron, iron alloys and steel), copper, precious metals and aluminium alloys with strong acids for the purpose of removing inorganic contaminants, metal oxides and scale, is generally referred to as Pickling.
Pickling is one method of preparing a metal surface for processes such as electrodeposition, chemical coating, painting, etc. In a pickling process, a metal object containing, for example, a high percentage of iron, is immersed in an aqueous acid bath (usually inorganic acids), commonly called “Pickling Liquor.” On immersion, the metal oxide such as iron oxide and scale are removed by direct attack of the acid on points on the metal surface at which the iron oxide or scale attaches.
Deposits of oxide and scale are usually quite pervious to the constituents of the pickling liquor, and slough off rapidly because of the action of the acid. At the same time that the pickling liquor is attacking the metal surface directly beneath the unwanted coating, it is also attacking the already clean metal surfaces and dissolving off quantities of metal, often causing pitting or roughening, i.e., corrosion. Acid is used up needlessly in this type of chemical process. In order to prevent such corrosive attack on clean metal, inhibitors are generally added to the pickling liquor. These inhibitors slow down acid attack on clean surfaces. Thus, the rate of scale removal by pickling is substantially the same as without the inhibitor, while the loss of metal by acid corrosion is decreased.