Matthew J. O'Keefe

Characterization of cerium-based conversion coatings for corrosion protection of aluminum alloys

Cerium-based conversion coatings were formed by a spontaneous reaction between a water-based solution containing CeCl3 and aluminum alloy 7075-T6 substrates. Coating performance was evaluated in neutral salt fog according to ASTM B117. Coating microstructure and thickness were observed by scanning electron microscopy (SEM). Coating composition and the cerium oxidation state were characterized using energy-dispersive spectroscopy (EDS) and X-ray photoelectron spectroscopy (XPS) respectively. The morphology and salt fog performance of cerium conversion coatings were affected by pre-treatment of the panel prior to coating. The best pre-treatment consisted of desmutting, degreasing, and acid activation. After immersion in the coating solution for 30 s, Ce-rich deposits formed on the 7075 surface. After 5 min, coatings consisted of Ce-rich particles in a Ce-containing matrix. Immersion times of 5 min or longer produced coatings that could pass published military requirements for conversion coating performance in neutral salt fog. XPS analysis showed that the coatings contained Ce4+.

Deposition and characterization of Cerium Oxide conversion coatings on Aluminum Alloy 7075-T6

Abstract An improved process has been developed for the spontaneous deposition of cerium oxide conversion coatings for corrosion protection of aluminum alloy 7075-T6. Approximately 80% of panels prepared using the improved process inhibited corrosion up to two weeks (336 h) in ASTM B117 salt fog testing, compared to approximately 20% for previous processes. Coatings were deposited from water-based solutions of CeCl3 and other compounds. Coating thickness, surface morphology, and performance in salt fog testing were sensitive to process parameters including surface preparation prior to coating, immersion time in the coating solution, and post-treatment. Surface preparation of the alloy was a critical first step in the deposition process. Desmutting followed by degreasing in a water solution of a commercial alkaline cleaner at a specific temperature was found to be an acceptable pre-treatment. Coating thickness, as determined by Auger electron spectroscopy depth profiling, varied depending on the surface pre-treatment and time that the panel was immersed in the coating solution. Immersion of desmutted, degreased panels for 15 min produced coatings 200 nm thick. Post-treatment, which converted Ce4+ in the as-deposited coating to Ce3+ as shown by X-Ray photoelectron spectroscopy, consisted of immersion in a phosphate sealing solution. Transmission electron microscopy analysis indicated that the as-deposited coatings were composed of a heretofore unidentified nano-crystalline cerium compound, presumably a hydrated Ce4+ oxide or Ce4+ hydroxide.

Effect of Phosphate Source on Post-Treatment of Cerium-Based Conversion Coatings on Al 2024-T3

The surface morphology, electrochemical characteristics, and salt spray corrosion performance were studied for cerium-based conversion coatings on Al 2024-T3 that were post-treated in heated aqueous solutions of orthophosphate, pyrophosphate, and polyphosphate compounds. Phosphate post-treatment reduced cracking in the coatings, which resulted in better corrosion protection compared to coatings that were highly cracked or contained other defects. In addition, post-treatment in orthophosphate solutions converted the as-deposited hydrated cerium oxide to hydrated CeP0 4 , which further improved corrosion protection. Electrochemical analyses showed that cerium-based conversion coatings that contained hydrated cerium phosphate after post-treatment had the highest resistance (∼100 kΩ cm 2 ), the most noble pitting potentials (∼―270 mV), and the best corrosion protection of the post-treatments that were tested. While pyrophosphate and polyphosphate post-treatments reduced cracks in the coatings, they did not promote the formation of hydrated cerium phosphate. The results suggest that the corrosion protection of cerium-based conversion coatings on Al 2024-T3 is dependent on both the surface morphology and the phase of the coating.

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.

Influence of Surface Pretreatment on Coating Morphology and Corrosion Performance of Cerium-Based Conversion Coatings on AZ91D Alloy

Abstract The corrosion protection of cerium-based conversion coatings (CeCC) formed on AZ91D magnesium alloy has been studied using potentiodynamic polarization measurements, electrochemical impeda...

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.

Electrochemical Cu nanoparticle deposition on TaSiN diffusion barrier films

High-density Cu nanoparticles were spontaneously deposited on TaSiN diffusion barrier layers using organic solutions. These activated surfaces were then plated with Cu using electroless deposition. The organic deposition solution was composed of conventional solvent extractants that are very poor electrolytic conductors but can sustain short range spontaneous reactions. The process proceeds by an electrochemical displacement mechanism and effective Cu seed layers could be obtained at 35°C in 10 to 20 s. Additives consisting of low formula weight organics were used to enhance the Cu nanoparticle deposition. Other operating procedures, such as substrate etching, solution concentration, additives, agitation, and deposition time, were evaluated to determine their effects on particle density, morphology, and uniformity. The Cu-seeded TaSiN surfaces were then built up using a standard electroless Cu process. A continuous, pore free, smooth, and adherent Cu film was attained after 2 min of electroless deposition.

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.

A Comprehensive Multi-Modal NDE Data Fusion Approach for Failure Assessment in Aircraft Lap-Joint Mimics

Multi-modal data fusion techniques are commonly used to enhance decision-making processes. In previous research, a comprehensive structural analysis process was developed for quantizing and evaluating characteristics of defects in aircraft lap-joint mimics using eddy current (EC) nondestructive evaluation (NDE) data collected for structural health monitoring. In this research, a comprehensive multi-modal structural analysis process is presented that includes intra- and inter-modal NDE data fusion based on EC, millimeter wave (MW), and ultrasonic (UT) data obtained from five lap-joint mimic test panels. The process includes defect detection, defect characterization, and finite-element modeling-based simulated fatigue loading for structural analysis. The multi-modal structural analysis process is evaluated using four test panels with corroded patches at different layers of the lap joints and one painted pristine panel used as a reference. The test panels are subjected to two rounds of mechanical loading, preceded by multi-modal NDE data obtained before each round. Different NDE modality combinations are examined for test panel modeling, including: 1) EC, 2) UT, 3) MW, 4) EC and UT, 5) EC and MW, and 6) EC, UT, and MW. Experiments are performed to compare the simulated fatigue loading, based on models determined from the different modality combinations, and the mechanical loading results to find susceptible-to-failure areas in the test panels. Experimental results showed that the EC and UT modality combination yielded a correct vulnerable (crack) location recognition rate of 98.8%, an improvement of 14.7% over any individual modality, demonstrating the potential for multi-modal data fusion for characterizing corrosion and defects.

Formation of cerium oxide/hydroxide on copper substrates by a spontaneous immersion process

A spontaneous immersion process has been developed to deposit adherent cerium oxide/hydroxide coatings on copper foil from an aqueous bath consisting of cerium nitrate, hydrogen peroxide, and sodium chloride. The spontaneous cerium oxide/hydroxide deposition exhibited characteristics consistent with an immersion process, with the deposition rate decreasing as the copper substrate was coated. A deposition mechanism is proposed in which the reaction of copper with hydrogen peroxide produced near-surface-pH conditions conducive to cerium oxide/hydroxide precipitation on the copper surface. Hydrogen peroxide concentration was varied between 0 and 1 vol % percent (as a 30% solution), with 0.2% producing the maximum deposition rate of 8-10 nm/min. Hydrogen peroxide concentrations above the optimum concentration resulted in substrate passivation due to the stabilization of copper oxide, which retarded coating formation. Parts-per-million concentrations of chloride ion were found to increase deposition kinetics without affecting the copper dissolution rate, possibly by destabilizing copper oxide formation. Autocatalytic hydrogen peroxide decomposition on the coating surface did not appear to play an important role in the deposition mechanism.