Corrosion resistant medium strength Al-Cu-Mg alloys

Commonly used Al-Cu-Mg alloys possess properties desirable for many structural applications. A heterogeneous microstructure, consisting of various Cu and Mg-containing intermetallic and constituent particles, is responsible for observed strengths. Unfortunately, this microstructure is also electrochemically heterogeneous, which makes these alloys inherently susceptible to localised corrosion. Hardening for Al alloys with low Cu and Mg content, in particular Al-1.1Cu-1.7Mg (at%), has been shown to occur in two stages, with approximately 60% of the peak hardness achieved within a few minutes of aging at 150°C. This initial hardening has been attributed to either the formation of a fine dispersion of nanometre lengthscale GPB zones or Cu and Mg atom clusters. However, it is unclear whether these strengthening heterogeneities are large enough to act as distinct electrochemical entities and ultimately increase the susceptibility of the alloy to localized corrosion. Our previous work has shown nanoscale precipitates can lead to localised corrosion. However if the finer features responsible for initial hardening are not detrimental to the corrosion susceptibility of the alloy, it may be possible to create highly corrosion resistant medium strength alloys; through alloying with low levels of Cu and Mg and using an appropriate heat treatment schedule. In this study, the corrosion resistance of an Al-1.1Cu-1.7Mg (at%) alloy as a function of ageing time at 150°C and 200°C is evaluated. Anodic polarisation and potentiostatic transient experiments were carried out in deaerated 0.01 M NaCl on samples which were aged for various times ranging from the as-quenched state to 648 h.