What Does Dezincification Mean?
Dezincification is a process which selectively removes zinc from an alloy, leaving behind a porous, copper-rich structure that has little mechanical strength.
Dezincification can show itself in a variety of ways depending on the water composition and service conditions. It may present itself as dull red spots on the surface of brass. It can also manifest as seepage of water through the walls of fittings or leakage at valves. Extreme dezincification can cause actual breakage, with a dull coppery appearance to the fracture surface.
Copper alloys that contain less than 15% zinc and alpha brasses inhibited by arsenic or antimony are resistant to dezincification, when in service in water or soil environments.
Corrosionpedia Explains Dezincification
Dezincification is the leaching of zinc from copper alloys in an aqueous solution. It is an example of dealloying in which one of the constituents of an alloy is preferentially removed by corrosion. Dezincification is similar to graphitization in that one constituent of the alloy (zinc) is selectively removed, leaving the other (copper) behind.
Dezincified brass retains the original shape and dimensions of the metal component before corrosion, but the residue is porous and has very little strength.
Conditions favoring dezincification are:
- Contact with slightly acid or alkaline water
- Water with little aeration
- Low flow rates of the circulating liquid
- Relatively high tube-wall temperatures
- Permeable deposits or coatings over the tube surface
An in-service valve suffering from dezincification has a white powdery substance or mineral stains on its exterior surface. The valve may exhibit water weeping from the valve body or stem/bonnet seal. Two types of corrosive attack characterize dezincification in this case:
- Plug-type dezincification: This type of dezincification penetrates deeply into the sidewalls of valves and fittings.
- Uniform-layer dezincification: This type of dezincification uniformly reduces the wall thickness of the valve or fitting.
Elevated temperature and coupling to a more noble metal can increase the dezincification. If brass bosses are used on copper hot water cylinders, the combined effects of the high water temperature and coupling to a large area of copper can give rise to significant dezincification even in waters that normally would not cause problems.
Avoiding and mitigating dezincification failures in aggressive water is probably best accomplished by use of dezincification-resistant brass alloys.