Short answers, pulled from the story.
Pickling is a metal surface treatment that uses a chemical solution called pickle liquor, typically containing acid, to remove impurities, stains, rust, scale, and inorganic contaminants from metals such as steel, copper, precious metals, and aluminium alloys. It is most commonly used to descale or clean steel during steelmaking processes.
Hydrochloric acid is the primary acid used in modern steelmaking because it pickles faster and reduces base metal loss, though sulfuric acid was previously more common. Steels with alloy content above 6% require phosphoric, nitric, or hydrofluoric acid. Copper alloys are usually pickled in dilute sulfuric acid, while brass uses concentrated sulfuric and nitric acid with sodium chloride and soot.
The EPA classifies spent pickle liquor as a hazardous waste because it contains acidic rinse waters, iron chlorides, and metallic salts. Steel pickling sludge can be neutralized with lime to raise its pH, after which the EPA no longer considers it hazardous and it may be disposed of in a landfill.
Hydrogen embrittlement occurs when hydrogen released by the pickling acid reacts with the metal surface, making it brittle and causing cracks to form. It is a particular risk for some alloys and high-carbon steels.
Eco pickled surface, or EPS, is a mechanical alternative to acid pickling that does not rely on chemical reactions. It is considered environmentally friendly and imparts a high degree of rust resistance to carbon steel, eliminating the need for the oil coating that acid-pickled carbon steel requires.
In jewelry making, pickling removes the copper oxide layer that forms on copper and sterling silver during soldering and annealing. A diluted sulfuric acid bath is the traditional method, though citric acid is an alternative.