Brass is an alloy consisting of copper (Cu) and Zinc (Zn) and exists in two general categories:
Red Brass: The two main constituents are Zn at less than 15wt % Cu at greater than 80wt %. This alloy has a red tint because all the Zn is dissolved in the Cu. This material rarely shows dezincification, but little difficult to machine.
Yellow Brass: The two main constituents are Zn at more than 25wt % Cu at less than 60wt %. This alloy has a yellow tint because the Zn forms its own separate regions in the alloy. This material is cheaper and easier to machine than red brass. A good visual picture is to imagine this brass to be a chocolate chip cookie. The chips would represent the Zn rich phase. The rest of the cookie is the copper rich phase. Machining of yellow brass is improved by adding lead (Pb). During machining, the lead acts as a lubricant and causes the chips created by the cutting to break off easier.
Dezincification of Brass
Dezincification is the corrosion of an alloy (i.e. metal) where the Zn in the alloy reacts preferentially in a corrosive environment. Basically it is a process in which zinc is lost and copper is left behind. Red brass (i.e. <15 wt% Zn) rarely suffers this type of corrosion because the Zn is still in the Cu matrix. Yellow brass with >25wt% Zn is susceptible to dezincification. This is because there are Zn rich regions in the alloy. The less noble metal, Zn, is removed from the alloy by galvanic corrosion on a microscopic-scale. The most susceptible alloys are the ones containing regions of metals with high difference in the galvanic series, e.g. copper (much more noble) and zinc (more reactive) in brass.
A schematic of galvanic corrosion is given in Figure #1. There are two effects of dezincification on a brass alloy. The Zn rich regions are eliminated and a white zinc oxide (called a meringue) is formed on the fitting interior (see Figure #2). This meringue can plug fittings and restrict water flow (See Figure #3). In addition, the brass becomes porous (See Figure 4). The porosity weakens the brass and also may cause leaking.
Metal suppliers are developing yellow brass alloys to decrease the occurrence of dezincification. There are many factors that contribute to dezincification; these include water chemistry, temperature, flow and pH. For this reason, it is hard to predict how well an alloy will perform in any situation. For this reason, dezincification resistant alloys have been developed; however, their performance is not guaranteed. These fittings are marked DZR; this means dezincification resistant (see Figure #5).