|
Evaluation
of Molybdate Conversion Coatings for Aluminum Alloys by Electrochemical
Impedance Spectroscopy
June 2001
By: Luz
Calle & Louis MacDowell
Abstract
Electrochemical
impedance spectroscopy (EIS) was used to investigate the corrosion
inhibiting properties of proprietary molybdate conversion coatings on
aluminum alloy 2024-T3 in aerated 5% (w/w) NaCl. Corrosion potential and EIS measurements were gathered for six
formulations at several immersion times for two weeks. The conversion-coated alloy panels showed an increase in the
corrosion potential during the first 24 hours of immersion that later
subsided and approached a steady value. The EIS spectra of the conversion-coated alloy were
characterized by impedance that is higher than the impedance of the
bare alloy at all the immersion times. The low frequency impedance, Zlf, for the
conversion-coated alloy was higher at all the immersion times than
that of the bare panel. This indicates improvement of corrosion resistance with
addition of the molybdate conversion coating. Scanning electron microscopy (SEM) revealed the presence of
cracks in the coating and the presence of cubic crystals believed to
be calcium carbonate. Energy dispersive spectroscopy (EDS) revealed the presence of
high levels of aluminum, oxygen, and calcium but did not detect the
presence of molybdenum on the test panels. X-ray photoelectron spectroscopy (XPS) indicated the presence
of less than 0.01 atomic percent molybdenum on the surface of the
coating.
For additional information, a
complete copy of this study is available as NASA Report. Send requests for copies to corrosion@ksc.nasa.gov.
To download a
full copy of this report in Adobe's pdf format for local printing, click here.
(0.16MB)
Return
to Publications Page
|
