CORROSION FATIGUE OF AN ALUMINUM-ZINC-MAGNESIUM ALLOY AND AN ALUMINUM-MAGNESIUM - LITHIUM ALLOY

Corrosion fatigue and related electrochemical experiments have been conducted on a high purity Al-Zn-Mg ternary alloy and a high purity Al-Mg-Li ternary alloy. The electrochemical experiments were performed in solutions of various concentrations of sodium chloride and sodium sulfate as functions of pH, temperature and degree of aeration. Thin sheets of the alloys were examined in strain controlled fatigue with controlled environmental conditions. Samples were heat treated to obtain the maximum tensile strength, electropolished, and stored in vacuum to remove absorbed hydrogen. Fatigue tests were performed in dry nitrogen gas as a reference environment, and compared with tests in humid nitrogen, distilled water, 0.5 molar sodium sulfate and 0.5 molar sodium chloride.; The corrosion test results indicate that the corrosion potentials of the Al-Mg-Li alloy are considerably more active than those of the Al-Zn-Mg alloy and that the pitting potentials are slightly more active. Therefore, while the lithium alloy pits more readily at the same potential in a given environment, the overpotential or energy required to induce pitting in the lithium containing alloy is greater than that required for Al-Zn-Mg alloys.; The fatigue resistance of the Al-Mg-Li alloy in dry nitrogen is virtually identical to that of the Al-Zn-Mg alloy. However, in aggressive environments, the fatigue lives and fatigue limits were decreased in the Al-Zn-Mg alloy while only the fatigue lives were affected for the Al-Mg-Li alloy. Pre-exposure of the Al-Zn-Mg alloy to humid air followed by testing in dry nitrogen resulted in the same fatigue lives as testing in humid nitrogen. The Al-Mg-Li alloy was unaffected by pre-exposure to humid air. The preexposure embrittlement of the Al-Zn-Mg alloy could be reversed by storage in vacuum.; These results are consistent with the hypothesis that corrosion fatigue of these alloys in environments containing water or water vapor results from hydrogen assisted fracture. The difference in the behavior of the alloys is attributed to either reduced hydrogen absorption or to reduced susceptibility to hydrogen assisted fracture.