| Selective Leaching |
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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 Brass es with less than 85% content of Copper (dezincification) in presence of oxygen and moisture, eg. from brass taps in Chlorine -containg 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. 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 . |
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