Metal surfaces can contain impurities that may affect whether the product is used for further processing like plating with metal or painting. In various steelmaking processes for example, the stainless steel is dipped into a vat of pickle liquor (an acid solution; typically sulfuric, hydrochloric, phosphoric, nitric or hydrofluoric acids) to descale, clean and remove impurities and to modify the surface for further treatment. The pickling bath consists of a ratio between acid and metal ions to create an even metal surface.
Aluminum (and a few other metals) on the other hand, is often etched in solutions of caustic soda (NaOH) for a matte finish prior to being anodized (anodizing is an electrochemical process that converts the metal surface into a decorative, durable and corrosion-resistant finish). Aircraft manufacturers for example, use caustic soda to chemically etch aircraft parts to a specific thickness in order to save weight.
Due to the time-consuming and labor-intensive maintenance of the pickling bath, companies are looking for ways to run the pickling process continuously by removing impurities from the pickling liquor. Membrane technology has allowed these companies to run continuous pickling baths and reduce maintenance time.