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.