Pickling may also be referred to as acid pickling. Acid cleaning removes oils and light oxides from metal surfaces by soaking, spraying, or manual brushing or wiping. The process is carried out at ambient or elevated temperatures. Common cleaning fluids are acid solutions commbined with water-miscible solvents, wetting and emulsifying agents. Cleaning acids include hydrochloric (HCl), nitric (HNO3), phosphoric (H3PO4), and sulfuric (H2SO4), the selection depending on the base metal and purpose of cleaning. For example, phosphoric acid produces a light phosphate film on the metallic surface, which can be a useful preparation for painting.
The distinction between acid cleaning and acid pickling is a matter of degree. Acid pickling involves a more severe treatment to remove thicker oxides, rusts, and scales; it generally results in some etching of the metallic surface, which serves to improve organic paint adhesion.
Pickling - Scale and rust normally are removed from the steel surface by pickling in a dilute solution of hot sulfuric acid or ambient temperature hydrochloric acid.