Selective leaching (also known as dealloying, paring, selective corrosion, or demetalification) is a type of corrosion common in some solid solution alloys. When selective leaching occurs, one component in the alloy is removed preferentially from the material. Generally, the less noble metal is removed through a microscopic galvanic corrosion mechanism. The most vulnerable alloys are ones with a large distance between their constituent elements on the galvanic series (copper and zinc in brass, for example).
As mentioned, the selective leaching of zinc from brass alloys is the most common occurrence of this type of corrosion. This particularly occurs in alloys with larger than a 15% concentration of zinc in the presence of oxygen and moisture (for example, brass taps in chlorinated water). However, interestingly enough, copper and zinc are believed to both dissolve out of solution simultaneously, but copper precipitates back into the material. The material that is left behind is a copper-rich sponge with weakened mechanical properties that has changed from yellow to red in color.
Although zinc is the most commonly affected element in selective leaching, iron is also commonly leached from grey cast iron in a process known as graphitic corrosion. This process leaves behind the graphite grains which severely reduce the mechanical properties but can also inhibit further leaching from the material. Like most types of corrosion, selective leaching is typically to be avoided.
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