On the Nature of Critical Potentials for Aluminum Repassivation

Repassivation (E_rp) and pitting transition potential (E_ptp)^1,2 are considered critical parameters in aluminum pitting, because they have been associated with repassivation phenomena^3,4. The physical meaning of E_rp has long been well-established in the literature as the threshold between stable pitting and complete repassivation of the material^5–7. The nature of E_ptp, however, has not been thoroughly explained in the current literature apart from the primary assumption that it represents the commencement of repassivation⁴. Moreover, experimental investigation and evidence to expand this argument as well as to explore the relationship between these two potentials has been scarce⁴.

The present work aims to address these open questions with cyclic potentiodynamic polarization (CPP) and potentiostatic experiments conducted on high purity Al in NaCl. Repassivation trends at and below E_ptp were explained based on the assumption of actively corroding fractions which were dependent on the applied potential. Morphological transformation was also observed during polarization as evidenced by electron microscopy and X-ray microtomography after the electrochemical tests. The observation of a distinct E_rp separate from the E_ptp during slow CPP scans was related to the charge passed and was hypothesized to be the net effect of the gradually decreasing actively corroding area at potentials below E_ptp, which resulted in morphology that restricted mass transport out of the pits, which delayed complete repassivation. SNL is managed and operated by NTESS under DOE NNSA contract DE-NA0003525.

References

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2. K. L. Moore, J. M. Sykes, and P. S. Grant, Corrosion Science, 50, 3233–3240 (2008).

3. S. T. Pride, J. R. Scully, and J. L. Hudson, J. Electrochem. Soc., 141, 3028 (1994).

4. M. Yasuda, F. Weinberg, and D. Tromans, J. Electrochem. Soc., 137, 3708 (1990).

5. A. Broli and H. Holtan, Corrosion Science, 13, 237–246 (1973).

6. G. S. Frankel, J. R. Scully, and C. V. Jahnes, J. Electrochem. Soc., 143, 1834 (1996).

7. G. S. Frankel, J. Electrochem. Soc., 145, 2186 (1998).