Selective Leaching, also called de-alloying is a type of corrosion where one element of a metal or alloy is singled out for corrosion attack. It is always the less noble alloy-ing component which is selectively removed, leaving the more noble metal in place. In other words, the more anodic metal goes into ionic solution. Selective leaching can be considered a form of galvanic or bimetallic corrosion on a microscopic scale. Alloys with components far from each other in the electrochemical series are sus-ceptible.
For example, carbon (graphite) is electrically active and forms a galvanic couple with iron (Fe) in cast iron.
Key Properties of Selective Leaching Corrosion
- Removal of one element from an alloy by corrosion.
- Appears as porous surface.
- Also known as parting or de-alloying.
- Caused by a susceptible element, which is anodic, within the alloy which cor-rodes at different rates.
Common forms of Selective Leaching Corrosion are:
Dezincification – Zinc (Zn) and copper (Cu) go into solution. Only copper is re-de-posited since its electromotive force (EMF) is greater than zinc.
Graphitization – Grey cast iron is susceptible to selective leaching. Iron goes into solution leaving weaker carbon/graphite matrix behind. The cast iron loses its strength and can cause unexpected failure of the metal.
Where does it occur?
- Brass valves
- Cast iron water systems
- Copper alloy components
How to Control Selective Leaching Corrosion?
Selective Leaching Corrosion may be controlled in a variety of ways, including:
- Use of different alloys
- Addition of inhibitor elements
- Applying protective coatings
Source: Chesterton ARC Manuals
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