The corrosion behavior of as-cast magnesium alloys ͑AM50, AZ91, and AZ91Si͒ was investigated in a 0.1 M sodium sulfate solution at the corrosion potential (E corr ) using electrochemical impedance spectroscopy. Transmission electron microscopy was used to analyze the corrosion product layer, and phase shifting interferometric microscopy was carried out to characterize the reactivity of intermetallic particles. Due to its microstructure, the AM50 alloy presented uniform corrosion during immersion, whereas corrosion of the AZ91 alloys began in the grain body and progressively spread to the eutectic areas. For the AZ91 alloys, the dissolution of the ␣-eutectic phase led to a strong aluminum enrichment of the corrosion product layer and, when a threshold was reached in the level of Al 2 O 3 in the magnesium oxide ͑or hydroxide͒ layer, a change of phenomenology occurred in the impedance diagrams. In addition, electrochemical results revealed that an increase of silicon concentration for the AZ91 alloys decreased the corrosion resistance. This was attributed to an increase of the number of Mg 2 Si particles, accelerating the dissolution of eutectic areas.In the automotive industry, magnesium-aluminum-zinc and magnesium-aluminum-manganese alloys are popular, in particular, the AZ91 and AM50 alloys. The corrosion resistance of magnesiumaluminum alloys has been partly explained in terms of increased passivity by incorporation of aluminum, which stabilizes the magnesium oxide layer even though the layer is thinner when the amount of Al is increased. 1,2 Nevertheless, in the aerospace industry, the use of magnesium alloys ͑Mg-Al with rare-earth elements͒ is limited due to their poor mechanical properties and low corrosion resistance. 3 The primary reason for the low corrosion resistance of magnesium alloys is the internal galvanic attack caused by alloying or impurity elements. Therefore, inclusions of metals with a low hydrogen overvoltage constitute efficient cathodes for magnesium and cause severe galvanic corrosion. 4,5 Fe, Cu, and Ni form particles which are highly cathodic in comparison with the Mg matrix. Ni and Cu are usually at low levels in alloys from primary production. Fe is the most troublesome element in the alloys. 5 The addition of manganese to Mg-Al alloys induces the formation of Al-Mn-Fe particles, and the corrosion resistance of the alloys can be improved by decreasing the galvanic potential difference between the intermetallic particles and the surrounding matrix. More precisely, Lunder et al. 6 showed that the corrosion resistance of the alloys was dependent on the Fe/Mn ratio in the materials and thus in the particles. In fact, the kinetics of the cathodic reaction were strongly influenced by the amount of Mn in the particles.According to Daloz et al.,7 the presence of zinc strongly influences the potential of both matrix and precipitate and thus also has beneficial effects on the corrosion resistance of Mg-Al alloys.For high temperature applications, Mg-Al-Si alloys are used. Silicon hardly dissolve...

a b s t r a c tThe influence of artificial ageing on the corrosion behaviour of the recently developed 2050 Al-Cu-Li aluminium alloy in chloride-containing solutions was studied. Corrosion tests showed that artificial ageing changed the corrosion morphology of the alloy from intergranular to intragranular and decreased the corrosion potential of the alloy. Transmission electron microscopy observations combined with small-angle X-ray scattering measurements were used to establish a link between the distribution of T 1 precipitates and the corrosion behaviour.⇑ Corresponding author. Tel.: +33 (0)5 34 32 34 07; fax: +33 (0)5 34 32 34 98.