Understanding corrosion fatigue crack tip pH differences between AA2024 and AA7075 using in situ measurements

At low fatigue loading frequencies of 0.1 Hz, the near crack tip pH of AA2024-T8 and AA7075-T6 diverge strongly, whereas at higher loading frequencies of 10 Hz they approach the bulk solution pH. In-situ micro pH probes positioned in line with crack plane show AA7075-T6 develops acidic crack tip pH ~3.5, whereas AA2024-T8 becomes strongly alkaline with pH ~11 at 0.1 Hz. This pH difference aligns with corrosion fatigue (CF) behavior where AA7075-T6 exhibits higher crack growth rates (da/dN) compared to air while AA2024-T8 exhibits da/dN near that of lab air. As frequency increases, both alloys display pH converging toward the neutral bulk pH of 0.6 M NaCl and both da/dN higher than lab air. Results are evaluated against a hypothesis in the literature suggesting alloy-specific crack wake cathodic reactions may set local pH and thereby influence CF kinetics in AA2xxx and AA7xxx Al alloys. Other possible hypotheses are evaluated.