Corrosion Fatigue Crack Initiation in Aluminum Alloys 7075 and 7050

Mechanisms of corrosion fatigue crack initiation by pitting in high-strength, 7000-series aluminum alloys were studied. The alloys used were Al 7075-T7351 (UNS A97075) and Al 7050-T7451 (UNS A7050). Corrosion pits were formed in a 0.6-M sodium chloride (NaCl) solution by the electrochemical reaction between the constituent particles and the surrounding matrix. Depending on whether these particles were cathodic (C-type) or anodic (A-type) relative to the alloy matrix, pits formed as a consequence of matrix or particle dissolution. In the Al 7075 alloy, C-type and A-type particles were present. In the Al 7050 alloy, only C-type particles were present. The C-type particles were identified, using transmission electron microscopic techniques, as Al₂₃CuFe₄ in the Al 7075 alloy and Al₇Cu₂Fe in the Al 7050 alloy. Because these particles tend to cluster parallel to the rolling plane in the rolling direction, significant pit growth from pit coalescence was observed following prolonged exposure to an aerated 0.6 M NaCl solution. The effect of preexisting corrosion pits on fatigue crack initiation was investigated in an ambient air environment (20°C and 42% relative humidity) using blunt-notch fracture mechanics specimens oriented in the short-transverse direction to obtain cracks in the same orientation as those that grow from rivet holes. Results to date indicate that the presence of corrosion pits can significantly shorten the fatigue crack initiation life and decrease the threshold (ΔK/√ρ)_th of the alloy by as much as 50%. Post initiation analyses further confirmed that, when corrosion pits were present, fatigue cracks always initiated from these pits. In the absence of pits, fatigue cracks initiated from large inclusions. Identification of constituent particles, mechanism of pit formation and growth, and analyses of fatigue crack initiation kinetics were discussed.