Characterization of Cerium-Based Conversion Coatings on Al 7075-T6 Deposited from Chloride and Nitrate Salt Solutions

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 0.11 M were prepared using CeCl3 or Ce(NO3)(3) individually as well as in combination. Coatings produced from solutions using all chloride precursor (0.33 M Cl- concentration) and H2O2 (1 M) were similar to 450 nm thick, had an impedance of similar to 95 k Omega cm(2), and a corrosion current density (i(corr)) of 0.288 mu A/cm(2). However, the combination of chloride ions and H2O2 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 H2O2 (1 M), the coatings thickness decreased to similar to 60 nm, the impedance decreased to similar to 5 k Omega cm(2), and the icorr increased to 7.07 mu A/cm(2). 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. (C) 2011 The Electrochemical Society. [DOI: 10.1149/1.3537821] All rights reserved.