Selective leaching

Selective leaching, also called dealloying, demetalification, parting and selective corrosion, is a corrosion type in some solid solution alloys, when in suitable conditions a component of the alloys is preferentially leached from the material. The less noble metal is removed from the alloy by microscopic-scale galvanic corrosion mechanism. The most susceptible alloys are the ones containing metals with high distance between each other in the galvanic series, eg. copper and zinc in brass.

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The elements most typically undergoing selective removal are zinc, aluminium, iron, cobalt, chromium, and others.

The most common example is selective leaching of zinc from some brasses with less than 85% content of copper (dezincification) in presence of oxygen and moisture, eg. from brass taps in chlorine-containing water. It is believed that both copper and zinc dissolve simultaneously and copper precipitates back from the solution. The material remaining is a copper-rich sponge with poor mechanical properties, and color changed from yellow to red. To combat this, arsenic or tin can be added to brass, or gunmetal can be used instead. Plumbing fittings that are resistant to dezincification are appropriately marked, with the letters "CR" (Corrosion Resistant) in the UK, and the letters "DR" (Dezincification Resistant) in Australia.

Dealuminification is a corresponding process for aluminum alloys. Similar effects for different metals are decarburization (removal of carbon from the surface of alloy), decobaltification, denickelification, etc.

Graphitic corrosion is selective leaching of iron from grey cast iron, where iron gets removed and graphite grains remain intact.

Countermeasures involve using alloys not susceptible to grain boundary depletion, using a suitable heat treatment, altering the environment (eg. lowering oxygen content), and/or use cathodic protection.

Selective leaching is sometimes used intentionally to prepare materials with high surface, eg. Raney nickel.