Hydrogen embrittlement of a cyclically deformed high strength Al alloy

Abstract

High cycle fatigue experiments have been performed on a 7075 Al alloy, principally in the T6 temper in dry air, distilled water 0.5N NaCl and 0.5N Na₂SO₄ solutions as functions of cathodic charging and catalyst poisoning of the hydrogen evolution reaction. All aqueous solutions appreciably lowered fatigue resistance with Cl^-ion producing the greatest reduction in resistance and SO₄ ⁼ion behaving essentially in the same manner as distilled water. Under cathodic charging conditions fatigue resistance is significantly reduced and both Cl^- and SO₄ ⁼solutions produce similar fatigue lives. A catalyst poison (As) added to Cl^- solutions reduces fatigue resistance relative to neutral Cl^- solutions. Fractography of specimens fatigued in aqueous environments shows that a significant amount of cleavage and quasicleavage occurs, the extent of these features being apparently a function of hydrogen available to the alloy free surface and to the tips of growing cracks. On the basis of these observations, it is suggested that corrosion fatigue of 7075 alloy is essentially a hydrogen embrittlement phenomenon where the low diffusivity of hydrogen is counterbalanced by the fact that hydrogen need only be present in the alloy free surface for crack initiation and in the plastic zone of growing cracks for propagation.