Pickling

Definition - What does Pickling mean?

Pickling is a treatment of metallic surfaces in order to remove:

  • Impurities
  • Stains
  • Inorganic contaminants
  • Rust
  • Scale

Pickling is used as a surface treatment of ferrous metal and copper/aluminum alloys. It is the process of dissolving oxide and scale from a metal's surface, and is commonly used to descale or clean steel in various steelmaking processes.

Pickling is also known as acid descaling.

Corrosionpedia explains Pickling

Pickling involves the immersion of the metal in an acid solution, usually hydrochloric or sulphuric acid, in order to remove mill scale and rust from the surface. After immersion in these acids the metal requires a thorough hot water rinse. It is preferable that the treatment is followed by application of a priming coat. Hydrochloric acid is more expensive than sulfuric acid, but it pickles much faster while minimizing base metal loss.

Carbon steels, with an alloy content of 6% or less are often pickled in hydrochloric or sulfuric acid. Steels with an alloy content greater than 6% must be pickled in two steps and other acids are used, such as phosphoric, nitric and hydrofluoric acid. Rust- and acid-resistant chromium-nickel steels are pickled in a bath of hydrochloric and nitric acid. Most copper alloys are pickled in dilute sulfuric acid, but brass is pickled in concentrated sulfuric and nitric acid mixed with sodium chloride and soot.

Disadvantages of pickling include:

  • It is difficult to handle because of its corrosiveness.
  • It is not applicable to all steels.
  • Hydrogen from the acid reacts with the surface and makes it brittle, causing cracks.
  • Acid concentrations and solution temperatures must be carefully monitored to assure desired pickling rates.

Pickling sludge is composed of:

  • Acidic rinse waters
  • Iron chlorides
  • Metallic salts
  • Waste acid
  • Spent pickle liquor

Pickle sludge from steel processing is usually neutralized with lime and disposed of in a landfill. The byproducts of nitric acid pickling are marketable to other industries, such as fertilizer processors.

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