Dezincification of brass – or selective corrosion

What is selective corrosion (dezincification of brass)?

Zinc and copper can be dispersed under certain conditions in copper alloys with a zinc content of more than 20%. After the dissolution of the brass, the copper is separated again in porous form while the zinc is discharged in dissolved form. The causes of this oxidation process of “dezincification” are various:

• water quality, e. g. chloride content
• an existing biofilm
• stagnant water in line systems
• very hard water
• ph value below 7

mostly in combination with high temperature >110 °C.

The copper red discoloration of the affected areas is characteristic. Such a dezincification of standard brass (CuZn39Pb3/CuZn40Pb2) is a very rare phenomenon, but generally leads to the hazard of a premature component failure and high costs, due to the embrittlement caused by the dezincification.

The difference with stress corrosion
Copper-zinc alloys with a zinc content of more than 15% also tend to stress corrosions. However, in this case two further conditions must occur simultaneously, in addition to the metallurgic precondition:

1. The material is under a considerable internal and/or external tensile stress.
2. During a simultaneous impact of a corrosive media (ammoniac, amine, ammonium salts, sulphur oxide)

The stress corrosion can be avoided by precautionary heat treatments to adjust the hardness below HB 115 and by the observation of torque limits during the assembly of the brass parts.

Our recommendation
In order to avoid both problems for temperatures >100 °C, we basically recommend steel or stainless-steel couplings from our program to prevent component failure by selective corrosion or stress cracking.
The alternative use of dezincification-resistant brass can also be economic for higher quantities.

A possible failure of our continuously hot pre-treated brass couplings due to stress corrosion can be reliably, quickly and easily prevented by observing the permissible tightening torques.
For corrosion protection we generally recommend additional suitable coatings or, depending on the application, steel or stainless steel couplings.

We would be pleased to advise you.