Effect of Mg Content on Stress Corrosion Cracking of Al-X Mg Alloys

The stress corrosion cracking (SCC) behavior of Al-Mg alloys with different Mg contents (Al-X Mg, X=2.5, 4.5, 6.3) were researched by transmission electron microscopy (TEM), electrochemical tests and constant extension rate tensile experiments combined with scanning electron microscopy (SEM). The corrosive environment for the SCC tests was 3.5 wt.% NaCl solution. Electrochemical results indicated that corrosion potential decreased with an increase in Mg content, while corrosion current density (i_corr) increased significantly. The Al-2.5Mg alloy exhibited the lowest i_corr among the three investigated compositions, indicating that the Al-2.5Mg alloy exhibited the best corrosion resistance, which was corresponding to the EIS results. TEM observation results showed that the number of precipitates rises with an increase in Mg content. In the SCC tests, collinear bilateral pulls along double-sided specimens have been applied in constant extension rate tensile experiments. The Al-2.5Mg alloy exhibited the lowest susceptibility to SCC; the slopes of the da/dt-K curves during stage II increase with an increase in Mg content in the Al-Mg alloys and da/dt of Al-6.3Mg is the fastest at a given stress intensity factor, implying that stress corrosion is more and more serious with the increase of Mg content in the Al-Mg alloys. SEM micrographs has shown that the fracture is chiefly intergranular and transgranular and the stress corrosion crack surface is mainly intergranular corrosion (IGC).