From the testing laboratory: Damage analysis of a leaking fitting
5. March 2020
Illustrative image of a bath tap.
1) Background
A fitting from a hotel failed because of a leak that caused water damage. The material used was a cast brass alloy of type CuZn39Pb1Al-C (CC754S) in accordance with DIN EN 1982.
The fitting failed after only ten months of use in a hotel room. Water escaped through cracks on the underside of the fitting body (see Fig. 1).
2) Damage analysis
Macroscopic examination shows a few clearly visible cracks on the outside of the fitting body (Fig. 2). Greenish-white corrosion products have accumulated at some of these locations. By contrast, the water-contacting inner surface shows numerous very fine cracks and several larger ones (Fig. 3).
Hardness testing of the fitting body gave a value of 120 HB. This appears high compared with forged brass components used in drinking-water applications. However, there are no standardised maximum hardness requirements for cast components.
Metallographic analysis in the cracked region shows that the cracks run in a straight line through the component wall from the inside out, with only a few branches (Figs. 4–6).
The microstructure is a typical α/β structure dominated by the β-phase, which forms a continuous phase. The crack runs almost exclusively through this darker β-phase. Selective corrosion of the β-phase is visible at the crack initiation site and at other locations on the inner wall. Cracks initiate from these areas (Fig. 7).
3) Conclusions of the fitting damage analysis
The damage mechanism responsible for the leak was stress corrosion cracking initiated by local dezincification on the water-contacting inner wall of the fitting.
This damage mechanism typically requires several factors to act together:
a corrosive medium (in this case, drinking water), a susceptible material microstructure, and tensile stresses arising either from operating loads or residual stresses in the component.
In this case, two factors were particularly important: the unfavourable microstructure with a continuous matrix of susceptible β-phase, and tensile residual stresses generated during casting. Together, they led to stress corrosion cracking (SCC).
