What Is the Rust on Brass Fittings?

Black Corrosion

• The most commonly seen corrosion on brass fittings is cupric oxide or copper (II) oxide, which forms a black patina on the surface of the fitting. Cupric oxide is the product formed when the copper in the brass reacts with oxygen in the atmosphere and water. Cupric oxide is the dominant form of oxidation when the oxygen reacts to the copper in the brass.

Red or Yellow Corrosion

• Cupric oxide is not the only potential form of corrosion produced by this oxidation reaction. Cuprous oxide or copper (I) oxide can also be formed. This compound is a red or yellow patina, both of which are less obvious on the surface of the fitting. Even if cuprous oxide does form, it degrades into cupric oxide, forming the more common black patina.

White Corrosion

• The white corrosion is formed when the zinc in the brass undergoes an oxidation reaction to form zinc oxide. It is more common when the zinc content of the brass is above 15 percent. The formation of zinc oxide is more destructive to the brass than either copper oxide product. When zinc oxide forms, the brass undergoes dezincification. The zinc in the brass is leached out of the fitting and deposited as a white powder on the surface. The brass becomes porous after the zinc is removed, allowing air and water to enter and corrode the inner surfaces of the fitting, rather than just corroding the surface.

Green or Blue Corrosion

• Green or blue corrosion is the most common corrosion to brass that does not result from an oxidation reaction. The green or blue patinas are formed when cupric oxide, which forms the black patina, reacts with sulfur dioxide or carbon dioxide in the atmosphere. The green or blue corrosion is more commonly seen on pure copper, such as the green patina of the Statue of Liberty, but it still does form on brass in the same manner.