Chemical and Structural Analyses of Subsurface Crevices Formed During Spontaneous Deposition of Cerium-based Conversion Coatings
Subsurface crevices formed during the deposition of cerium-based conversion coatings were analyzed in cross-section to assess the effect of deposition and post-treatment on the structure and chemistry of phases present. An Al–O containing phase, believed to be amorphous Al(OH)3, was formed in crevices during coating deposition. Analysis by energy dispersive X-ray spectroscopy revealed the presence of up to 1.6 at.% chlorine within the Al–O phase, which was likely a product of soluble chlorides that were present in the coating solution. Cerium was not detected within crevices. After post-treatment in an 85 °C aqueous phosphate solution, the chloride concentration was reduced to ≤ 0.30 at.% and electron diffraction of the Al–O phase produced ring patterns, indicating it had crystallized. Some diffraction patterns could be indexed to gibbsite (Al(OH)3), but others are believed to be a combination of hydrated aluminum hydroxides and/or oxides. Aluminum phosphate was not identified. Separately from its effect on cerium-based conversion coatings, phosphate post-treatment improved the corrosion resistance of Al 2024-T3 substrates by acting to crystallize Al(OH)3 present on interior surfaces of crevices and by reducing the chloride concentration in this phase.
D. K. Heller et al., "Chemical and Structural Analyses of Subsurface Crevices Formed During Spontaneous Deposition of Cerium-based Conversion Coatings," Materials Characterization, Elsevier, Nov 2011.
The definitive version is available at http://dx.doi.org/10.1016/j.matchar.2011.07.020
Materials Science and Engineering
Keywords and Phrases
Corrosion; Aluminum; Cerium; Post-Treatment; Crevices; Conversion Coatings
Article - Journal
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