.dezincification

Dezincification selectively removes zinc from the brass alloy. It leaves behind a porous, copper-rich structure that has little mechanical strength. An in-service valve suffering from dezincification has a white powdery substance or mineral stains on its exterior surface. 

Reason for dezincification

Zinc is a highly reactive metal in galvanic series ranking. Zinc has a very weak atomic bond relative to other metals. Zinc galvanic polential is -.76V where for Copper it is +.34V. Simply, zinc atoms are easily given up to solutions with certain aggressive characteristics. During dezincification, the more active zinc is selectively removed from the brass, leaving behind a weak deposit of the porous, more noble copper-rich metal.

Factors which cause increased rates of dezincification are Copper-zinc alloys containing more than 15% zinc are susceptible to dezincification. high temperature, high chloride content of water, and low water speed.  Adding tin, Ni and Al in the alloy can reduce dezincification. Dezincification occurs in pump casing, impellers and in valves