Glossary

Acid pickling is a metal surface treatment process that uses acidic solutions to remove impurities, such as rust, scale, oxides, and other contaminants, from the surface of metals—most commonly steel, stainless steel, aluminum, and copper alloys. The goal of pickling is to produce a clean, reactive metal surface that’s suitable for further processing, such as coating, plating, welding, or forming.

During acid pickling, the metal is immersed in or sprayed with a chemical bath containing acids such as hydrochloric acid (HCl), sulfuric acid (H₂SO₄), nitric acid (HNO₃), or hydrofluoric acid (HF), depending on the metal type and contaminants being removed. The acid reacts with the oxides and corrosion products on the surface, dissolving them and leaving behind bare metal.

In the case of stainless steel, a combination of nitric and hydrofluoric acids is often used to remove heat tint and oxides that form during welding or heat treatment, restoring the alloy’s natural corrosion resistance by promoting the formation of a new, uniform passive oxide layer afterward.

After pickling, the metal is thoroughly rinsed with water to remove any residual acid, and often neutralized with an alkaline solution to stop further chemical reaction.