Simon Joshi

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 CeCl 3 or Ce(NO 3 ) 3 individually as well as in combination. Coatings produced from solutions using all chloride precursor (0.33 M Cl- concentration) and H 2 O 2 (1 M) were ∼450 nm thick, had an impedance of ∼95 kΩ cm 2 , and a corrosion current density (i corr ) of 0.288 μA/cm 2 . However, the combination of chloride ions and H 2 0 2 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 H 2 0 2 (1 M), the coatings thickness decreased to ∼60 nm, the impedance decreased to ∼5 kΩ cm 2 , and the i corr increased to 7.07 μA/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.

Characterization of Localized Surface States of Al 7075-T6 during Deposition of Cerium-Based Conversion Coatings

The combination of chloride ions and H 2 O 2 in solutions used to deposit cerium-based conversion coatings led to localized dissolution of aluminum alloy 7075-T6 substrates. Potentiodynamic scans indicated that exposure of the alloy to a solution containing 0.3 M chloride ions and 1 M H 2 O 2 led to active dissolution. This process resulted in selective etching of the aluminum alloy substrate that produced a nonuniform surface and voids that penetrated a few micrometers into the alloy. When H 2 O 2 was replaced by an alternative oxidizing agent, NaClO 4 , cerium-based conversion coatings were deposited without substrate dissolution. Chloride ions and H 2 O 2 selectively etch aluminum alloy 7075-T6 due to electrochemical reactions that take place. The reactions readily dissolved the native oxide that was present, allowing for the aluminum substrate along with embedded intermetallic particles to be exposed to the electrolyte, which propagated localized dissolution.

Cerium-based Conversion Coatings as Alternatives to Hex Chrome: Rare-earth Compounds Provide Resistance Against Corrosion for Aluminum Alloys in Military Applications

Summary Rare-earth compounds are corrosion inhibitors for aluminum alloys that show promise as replacements for hexavalent chromate compounds. Cerium-based conversion coatings can be applied with spontaneous spray or immersion processes as well as an electrolytic process. The coatings can meet military standards by providing corrosion protection for up to 14 days in ASTM B117 salt spray. As with any surface finishing operation, the performance is a function of the parameters used in the deposition, but the process is reproducible and robust. Development efforts are continuing to optimize the deposition process, to enhance the long-term corrosion resistance of chromate-free systems, and to integrate CeCCs into non-chromate coating systems that include primers, topcoats, or multifunctional UV-curable coatings in addition to CeCCs.