Exposure tests of CuZn21Si3P in drinking water and dezincification test solution have revealed good dezincification resistance of the lead‐free brass CuZn21Si3P. To clarify the underlying mechanism, electrochemical measurements in tap water and surface characterisation of samples exposed to drinking water were performed. Open circuit potential measurements, electrochemical impedance spectroscopy and micro‐electrochemical polarisation curves, as well as light‐microscopy, X‐ray diffraction and Auger electron spectroscopy measurements reveal details on the corrosion behaviour and scale formation. Microstructural effects, electrochemical behaviour of the separate phases, and the role of the alloyed phosphorous in the dezincification process could be elucidated. Conclusively, a corrosion mechanism is proposed, that unites and explains the findings of long‐term exposure.