Definition - What does Pickling mean?
Pickling is a metal cleaning process that uses very strong acids to clean the metal of certain types of surface conditions. These conditions include mill scale, oxides, impurities and stains. The solution of acid used when pickling is called the pickling liquor. During the process, the metal is submersed in the pickle liquor for a set amount of time and then removed.
Corrosionpedia explains Pickling
Pickling is excellent at removing mill scale because of the way the acids in the pickle liquor react with the mill scale layer. Mill scale has surface discontinuities that allow the pickle liquor to penetrate the mill scale surface. Once beneath the mill scale, the pickle liquor dissolves and removes the scale layer adhering to the base metal.
Care should be taken to minimize the time spent in the pickle liquor as this could result in deterioration of the base metal.
Pickling can be performed with a variety of acids. In the modern pickling process, the most common is hydrochloric acid, also known as muriatic acid. Sulfuric acid can also be used, although it is less widely used because it can rapidly damage and degrade the base metal. Vinegar (acetic acid) is another acid sometimes used.