The effects of Mg and Ag contents on the corrosion resistance of Al-Cu-Mg-Ag alloy were investigated by intergranular corrosion (IGC) testing. The corrosion mechanism of the alloys with different chemical composition was discussed by transmission electron microscopy (TEM) and electrochemical analyses. The results indicate with increasing Mg and Ag content, the age hardening rate of the alloy increases. The size of the strengthening phases Ω decreases with increasing Mg content whereas phase volume fraction increases. More solution atoms are consumed to form the precipitation on the grain boundaries of the alloy with higher Mg content, which increases the potential difference between precipitation free zone (PFZ) and the matrix and subsequently, the IGC resistance of Al-Cu-Mg-Ag alloy decreases. Increasing the Ag content, the main strengthening phases transform from great amounts of θ' with less Ω to plentiful Ω with less θ'. The variation of Ag content hardly affects the potential of PFZ, and the corrosion resistance of the alloy depends mainly on the width of PFZ. With increasing Ag content, the precipitations on the grain boundaries are refined; the width of PFZ decreases and correspondingly, the corrosion passageway becomes narrow. The IGC resistance of Al-Cu-Mg-Ag alloy is enhanced with increasing Ag content.
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