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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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In Situ Near Crack Tip pH Measurements to Confirm Alkaline Crack Conditions Causing Crack Arrest in Cathodically Polarized AA5456-H116
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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- Award ID(s):
- 1943870
- PAR ID:
- 10590549
- Publisher / Repository:
- AMPP
- Date Published:
- Journal Name:
- Corrosion
- Volume:
- 80
- Issue:
- 8
- ISSN:
- 0010-9312
- Page Range / eLocation ID:
- 828 to 835
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.5006/4591
