Influence of Dichromate Ions on Corrosion Processes on Pure Magnesium
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Date
2003
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Abstract
The corrosion behavior of Mg is of interest because of its growing use as an alloy in the transportation
industry and also because it is a major component of some intermetallic phases in Al alloys, such as the deleterious
S (Al2CuMg)-phase found in AA2024-T3. Pure Mg corrodes rapidly in a chloride-containing solution and even
dissolves in water if the surface hydroxide is damaged by scratching the surface, for example. Uniform dissolution is
drastically reduced in NaCl solutions (from 0.01 to 0.5 M) with the addition of very dilute concentrations of
dichromate (10-4 M). However, it is replaced by a strong localized attack in the form of fast filiform-like attack. On a
large-grained sample with a defined defect structure, the attack can be seen to propagate at twin boundaries.
Orientation imaging microscopy analysis found that corrosion was limited to planes near {0001} orientations with
propagation being in prismatic directions. Auger electron spectroscopy analysis shows that interaction of chromate
with the Mg hydroxide results in incorporation of reduced chromium ions in the hydroxide surface layer. Formation
of a more resistant surface film could explain the very local nature of the corrosion in this case. The interaction
between dichromate ions and Mg hydroxide can also explain the higher corrosion resistance of S-phase particles in
chloride solutions containing dilute dichromate, although differences in the surface film formed compared to pure
Mg are observed. Sputter-etching of the surface in order to assess the depth of the attack revealed that very hard or
isolating corrosion products difficult to sputter are produced along the filiform path and that chromium compounds
are not integrated in the corrosion products. Focused ion beam sectioning followed by scanning electron microscopy
investigation of the sectioned area, demonstrates the presence of a continuous protective surface film. Adhesion
between the Mg hydroxide and the metal is lost at the location of the corrosion filament, suggesting that the
mechanism of propagation is similar to filiform corrosion under a coating. The depth of attack is a couple of
micrometers with large cracks present within the corroded area that could induce severe surface damage.
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Keywords
pure magnesium, dichromate ions, corrosion behavior
Citation
Schmutz, P.; Guillaumin, V.; Frankel, G. S.; Lillard, R. S.; Lillard, J. A. "Influence of Dichromate Ions on Corrosion Processes on Pure Magnesium," Journal of The Electrochemical Society, v. 150, no. 4, 2003, pp. B99-B110.