Localized corrosion inhibition of AA7075-T6 by calcium sulfate

Pitichon Klomjit, R. G. Buchheit

Research output: Contribution to journalArticlepeer-review

10 Scopus citations


Aluminum alloy 7075-T6 is used in aerospace applications as a result of its high strength-to-weight ratio, but this material is susceptible to localized corrosion in condensing atmospheric exposure conditions and is usually protected with an anticorrosion coating system. CaSO4 is sometimes found as an extender component in corrosion inhibition primer pigment systems, such as commercial Pr-rich primer, which has been qualified for military use. The role CaSO4 might play in corrosion protection afforded by the coating system has not been addressed thoroughly in the scientific literature. The goal of this study was to isolate and characterize any inhibiting effect of CaSO4 on AA7075-T6 to understand how it might play a role in non-chromate primer coatings. Samples of AA7075-T6 were exposed to chloride solutions with varying pH via static immersion exposure with and without CaSO4 as an inhibitor for times ranging up to 30 d. The solution pH was varied from 3 to 10 and the chloride concentration was fixed at 0.1 M. Pitting corrosion damage was characterized by optical profilometry to find pit depth and pit area. Potentiodynamic polarization and electrochemical impedance spectroscopy (EIS), were used to characterize the extent of corrosion. Surface analysis was performed by x-ray photoelectron spectroscopy (XPS). Results showed that sulfate suppressed pitting corrosion under acidic conditions. Calcium did not appear to contribute to inhibition in any of the evaluations performed. EIS results showed an increase in polarization resistance in the presence of sulfate. Thermodynamic calculation and XPS data suggest that the protective film formed in the presence of CaSO4 is Al(OH)SO4.

Original languageEnglish
Pages (from-to)486-499
Number of pages14
Issue number4
StatePublished - Apr 2016

Bibliographical note

Publisher Copyright:
© 2016, NACE International.


  • Aerospace
  • Aluminum alloy
  • Corrosion inhibitor
  • Electrochemical impedance spectroscopy
  • Localized corrosion
  • Pitting
  • Polarization

ASJC Scopus subject areas

  • Chemistry (all)
  • Chemical Engineering (all)
  • Materials Science (all)


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