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The effect of trace addition of Zn on the stress corrosion cracking (SCC) resistance and microstructure of Al-4.2Cu-1.4Mg alloy was studied by means of the slow strain rate technique (SSRT), potentiodynamic polarization test, transmission electron microscopy (TEM) and electron back scattered diffraction (EBSD). The results show that the stress corrosion cracking resistance is improved at the beginning and then decreased with the increase of Zn content. The optimum amount of Zn addition is 0.29% to improve SCC resistance. EBSD results show that there are much more low-angle grain boundaries with low energy in the 0.29% Zn contained alloy, which is probably the main reason why Zn can significantly influence SCC resistance of the alloy