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The combination of CeCl3 and H2O2 in cerium-based conversion coating (CeCC) solutions causes the dissolution of the aluminum alloy substrate, resulting in formation of voids a few microns below the surface during deposition. Localized, excessive dissolution of aluminum occurs near intermetallic particles over approximately 10% of the substrate surface. Alternate deposition methods were explored whereby either H2O2 or chloride content was reduced to minimize dissolution. An alternative oxidant, NaClO4, added to offset the impact of lower H2O2 content. CeCCs were deposited without sub-surface voids, but the resulting coatings had poor corrosion protection compared to CeCCs deposited with H2O2 alone. Coatings prepared with alternate Ce3+ salts using different ratios of CeCl3 and Ce(NO3)3, along with higher concentrations of H2O2, were thicker and more uniform than coatings with lower H2O2 content. However, CeCCs deposited from solutions containing H2O2 and CeCl3 had the highest overall electrochemical impedance and exhibited the best corrosion protection.

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Cerium-based conversion coating solutions were prepared from chloride and nitrate cerium salts, and coatings were deposited on Al 7075-T6 substrates. Solutions with a fixed Ce(III) ion concentration of 0.11M were prepared using CenormalCl3 or Ce(NnormalO3)3 individually as well as in combination. Coatings produced from solutions using all chloride precursor ( 0.33M Cl− concentration) and normalH2normalO2 (1M) were ∼450nm thick, had an impedance of ∼95kΩcm2 , and a corrosion current density (normalicorr) of 0.288μA∕cm2 . However, the combination of chloride ions and normalH2normalO2 in the deposition solution led to the formation of subsurface crevices in the aluminum alloy substrate. To prevent or reduce subsurface crevice formation, cerium nitrate was substituted for the chloride precursor. Using a solution containing only the nitrate precursor and normalH2normalO2 (1M) , the coatings thickness decreased to ∼60nm , the impedance decreased to ∼5kΩcm2 , and the icorr increased to 7.07μA∕cm2 . Thus, the coating thickness and corrosion performance were directly related to the chloride ion content in the coating solution. Decreasing the chloride ion content by substituting nitrate ions made the deposition solution less aggressive, hindering the reactions necessary for coating deposition to take place and subsequently reducing the formation of subsurface crevices.