Aluminium alloys possess a unique combination of strength and low density which historically have made these alloys ideal for many structural applications However addition of alloying elements in appreciable concentrations in particular Cu and Mg make Al-alloys inherently susceptible to localised corrosion In this work we adopt a bottom-up approach in an attempt to quantify the critical microstructural feature size (viz precipitate size) that is capable of triggering a cascade of pitting events and eventual degradation of corrosion resistance This is accomplished by exploiting the well characterised hardening response in a model alloy Al-1 1Cu-1 7Mg (at %) for which pitting resistance of the alloy was tracked with aging time and hence microstructural evolution Corrosion performance and microstructural characterisation were carried out using a combination of electrochemical testing coupled with high-resolution scanning transmission electron microscopy (HRSTEM) and atom probe tomography (APT) Results indicate at least for this particular alloy that second phase features below a critical width of approximately 3 nm can be tolerated from a corrosion perspective This study has potentially wide consequences in the understanding of aluminium alloy corrosion initiation and the development of highly corrosion resistant aluminium alloys (C) 2010 Elsevier Ltd All rights reserved