Pickling

From Wikipedia:


 * Pickling is a metal surface treatment used to remove impurities, such as stains, inorganic contaminants, and rust or scale from ferrous metals, copper, precious metals and aluminum alloys. A solution called pickle liquor, which usually contains acid, is used to remove the surface impurities. It is commonly used to descale or clean steel in various steelmaking processes.

Pickle Liquor
F. Axtell Kramer wrote in Live Steam & Outdoor Railroading, May/June 2021:


 * For many years, and in many articles, diluting concentrated liquid sulfuric acid to make "pickle liquor" for cleaning copper and brass before and after silver brazing has been the standard procedure.


 * This is a hazardous procedure, and unnecessary. For safety, use solid sodium bisulfate instead. I can verify that it works as well as dilute sulfuric acid solutions. With use, the sulfuric acid pickle soon becomes a solution of copper bisulfate anyway.  Sodium or copper - it makes no difference.  It is the bisulfate part, an acide itself, that does the work.