Short answers, pulled from the story.
Pickling is a metal surface treatment that uses chemical removal of surface impurities to erase scale left by hot working processes. This process involves dipping workpieces into pickle liquor containing strong acids like hydrochloric acid or sulfuric acid before further manufacturing steps.
Hydrochloric acid has become the primary choice for steelmaking because it works much faster while minimizing the loss of base metal during the process. Modern mills run production lines at speeds reaching 800 feet per minute, which requires rapid reaction cycles that sulfuric acid cannot match.
Carbon steels with alloy content under 6 percent undergo treatment in hydrochloric or sulfuric acid baths. Steels containing over 6 percent alloy require a two-step process using acids like phosphoric or nitric acid, while rust-resistant chromium-nickel steels receive a bath mixture of hydrofluoric and nitric acid.
Acid reactions can cause hydrogen atoms to react with the metal surface and create internal brittleness that leads to cracking in certain high-carbon steel alloys. This structural weakness makes some alloys unsuitable for standard acid pickling procedures unless engineers carefully balance speed against safety.
The Environmental Protection Agency classifies spent pickle liquor as hazardous waste under federal regulations due to its contents of acidic rinse waters, iron chlorides, metallic salts, and waste acid. Since the 1960s, hydrochloric pickling sludge often enters regeneration systems that recover hydrochloric acid before remaining waste requires neutralization and landfill disposal.