Dezincification of brass is a form of selective corrosion that happens when zinc is leached out of the alloy leaving a weakened porous copper fitting. This commonly happens in chlorinated water or in water that has high levels of oxygen and carbon dioxide. Signs of dezincification are white powdery zinc oxide coating the surface of the fitting, or yellow brass that turns red. Selective corrosion can be a problem when it weakens a fitting, leaving it vulnerable to failure and eventual leaks.
Brass is an alloy, or mixture, of copper and zinc. Since brass is a malleable metal that is easy to cast and has many desirable properties, it is often used in plumbing. When a brass alloy is made up of more than 15 percent zinc, there is an increased risk of selective corrosion because zinc is a highly reactive metal with a weak atomic bond. As the percentage of copper in the alloy increases, the risk for dezincification decreases.
There are two types of dezincification that can occur that are often more common in different parts of the world, depending on local conditions. The plug type is localized and deep. Neutral or alkaline water that is high in salt and warmer than room temperature may cause zinc to leach. Over time, penetration can cause holes and leaks or failures in threaded parts.
Uniform dezincification happens over a broader area than the plug type, thinning the wall uniformly. Slow water flow or stagnant water, especially when the water is high in oxygen and carbon dioxide, may make a fitting more susceptible to uniform corrosion. Slightly acid water that is low in salt and about room temperature may also facilitate leaching of zinc.
There are several methods to increase resistance to corrosion and zinc leaching. Fittings with less than 15 percent zinc are less susceptible to leaching and corrosion, provided there aren’t elements known to react with copper present in the water and aeration is minimal. Manufacturers may also add trace amounts of other elements to the alloy to ward off corrosion. Arsenic, antimony, phosphorus, and tin may be added to the alloy, with the addition of antimony being the most effective.
The use of dezincification resistant brass is becoming more common. Resistant brass is marked CR (corrosion resistant) in the United States. In the United Kingdom, it is marked DZR (dezincification resistant), and in Australia it is marked DR (dezincification resistant).