Effect of ambient aging on inhibition of oxygen reduction by chromate conversion coatings

W. Zhang, R. G. Buchheit

Research output: Contribution to journalArticlepeer-review

14 Scopus citations

Abstract

Cathodic polarization curves were collected from chromate- conversion-coated 2024-T3 (UNS A92024) electrodes subject to prior aging under ambient indoor conditions. Selected exposures at elevated temperatures were made for comparison. Results showed that chromate conversion coatings (CCC) aged for <48 h at ambient temperatures effectively inhibited the oxygen reduction reaction (ORR). However, with increasing aging time, ORR inhibition gradually disappeared. In these experiments, evidence of ORR inhibition was absent after aging for 168 h. These findings indicate that the ability of CCCs to inhibit oxygen reduction is temporary and likely lasts for no more than a week under ambient atmospheric conditions. Examination of coating cross sections by electron microscopy showed that aging led to the formation of shrinkage cracks that extended to and propagated along the coating- substrate interface exposing unprotected substrate. It is speculated that this cracking permits solution contact with the substrate, leading to increased ORR rates. Heat treatment of CCC at elevated temperature did not necessarily result in the loss of ORR inhibition but did cause immobilization of Cr6+. This result suggests that Cr6+ immobilization is not strongly related to the loss in ORR inhibition. Rather, the loss of ORR inhibition is associated primarily to the development of shrinkage cracking in the conversion coating.

Original languageEnglish
Pages (from-to)356-362
Number of pages7
JournalCorrosion
Volume59
Issue number4
DOIs
StatePublished - Apr 2003

Keywords

  • AA2024-T3
  • Aging
  • Cathodic inhibition
  • Chromate conversion coatings
  • Oxygen reduction reaction

ASJC Scopus subject areas

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

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