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In situ crack tip pH measurements for corrosion fatigue (CF) cracks in sensitized AA5456-H116 loaded under low loading frequencies show that cathodic polarization can arrest actively growing stress corrosion cracking (SCC) and CF cracks and produce a local alkaline crack tip pH. A method for measuring crack tip pH in situ was developed. For AA5456-H116 under a single level of high sensitization, CF experiments while loading in the Paris regime at a loading frequency of 0.1 Hz were conducted under freely corroding conditions and a cathodic polarization of −1.3 VSCE. Results show that under freely corroding conditions the crack actively grows, and the crack tip pH is slightly acidic, while at −1.3 VSCE an alkaline crack tip develops with a pH of 10 to 12. The findings of this study support the earlier published hypothesis that crack arrest of SCC and low loading frequency CF cracks is due to corrosion-induced blunting after the development of highly alkaline conditions that cause corrosion of the crack tip region blunting and halting the crack.
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This content will become publicly available on November 10, 2026
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 0.6MNaCl 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.
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- Award ID(s):
- 1943870
- PAR ID:
- 10656266
- Publisher / Repository:
- Nature
- Date Published:
- Journal Name:
- npj Materials Degradation
- Volume:
- 9
- Issue:
- 1
- ISSN:
- 2397-2106
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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