dezincification

Dezincification is the selective corrosive removal of zinc from brass and other zinc-containing copper alloys, leaving behind a weak, porous copper-rich residue. The process occurs when the alloy is exposed to certain corrosive environments, particularly those containing dissolved oxygen, chlorides, or organic acids. The zinc preferentially dissolves into the surrounding fluid while copper remains behind, creating a layer that looks intact on the surface but has lost its structural integrity and may crumble under stress.

The phenomenon is most notorious in brass plumbing fittings, where it appears as "red rot": a reddish or pink discoloration accompanied by pitting, pinhole leaks, and eventual failure of the fitting. This happens in systems carrying slightly acidic water or where ammonia-based cleaners accumulate. Brass pipes that were standard in mid-20th century domestic water systems have caused significant plumbing failures for this reason. The affected metal becomes brittle and loses corrosion resistance, making it dangerous for pressure applications.

Mechanism and variables

Dezincification occurs through galvanic corrosion within the alloy itself. Zinc is more electronegative than copper, so it acts as the sacrificial anode in the presence of moisture and oxygen. The rate of attack depends on alloy composition, water chemistry, temperature, and flow velocity. Brass with zinc content above 15 percent is more susceptible. Stagnant water or low-velocity systems are particularly vulnerable because corrosion products cannot be swept away and may accumulate locally, intensifying attack. Water pH below 7 and presence of chlorides or sulfates significantly accelerate the process.

Resistance to dezincification can be improved through alloying additions. Tin additions of around 0.7 to 1.5 percent, producing admiralty brass, substantially reduce susceptibility by stabilizing the microstructure. Arsenic or antimony additions at trace levels also inhibit the mechanism, though these are less common now due to health and environmental concerns. Protective coatings and inhibitor-based water treatment provide alternative control methods in systems already installed.

The term dezincification appears in standards such as ASTM B117 and B154, where brass is graded by its resistance to this failure mode. Understanding this failure mechanism is essential for material selection in water systems, marine environments, and any application where brass contacts slightly acidic or chloride-bearing fluids for extended periods.