Definition - What does Parting mean?

Parting is a type of selective leaching or corrosion in which one constituent of a solid solution alloy is preferentially removed, leaving behind an altered (weakened) residual structure.

Parting can be used to produce powdered materials with extremely high surface area, such as Raney nickel. Parting also occurs in cast iron, where iron dissolves from the iron-carbon matrix, leaving the iron brittle and weak (also known as graphitization).

Parting is also known as dealloying, demetalification, selective corrosion, selective leaching or parting corrosion.

Corrosionpedia explains Parting

Parting is a corrosion process in which the more active metal is selectively removed from an alloy, leaving behind a porous weak deposit of the more noble metal. Specific categories of parting often carry the name of the dissolved element. For example, the preferential leaching of zinc from brass is called dezincification. If aluminum is removed, the process is called dealuminification, and so forth. In the case of gray iron, dealloying is called graphitic corrosion.

In parting, the less noble metal is removed from the alloy by a microscopic-scale galvanic corrosion mechanism. The most susceptible alloys are ones containing metals with a large distance between each other in the galvanic series, such as copper and zinc in brass. The elements most susceptible to selective removal include:

  • Zinc
  • Aluminum
  • Iron
  • Cobalt
  • Chromium

In the parting process, typically one of two mechanisms occurs: alloy dissolution and replating of the cathodic element, or selective dissolution of an anodic alloy constituent. In either case, the metal is left spongy and porous and loses much of its strength, hardness and ductility.

In the dezincification of brass, selective removal of zinc leaves a relatively porous and weak layer of copper and copper oxide. Corrosion of a similar nature continues beneath the primary corrosion layer, resulting in gradual replacement of sound brass by weak, porous copper. Unless arrested, parting eventually penetrates the metal, weakening it structurally and allowing liquids or gases to leak through the porous mass in the remaining structure.

Countermeasures involve:

  • Using alloys not susceptible to grain boundary depletion
  • Using a suitable heat treatment
  • Altering the environment (e.g. lowering oxygen content)
  • Using cathodic protection

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