Particle-Doped Organic-Inorganic Hybrid Coatings as Corrosion Inhibiting Surface Treatments for Aluminum Alloys
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Particle-Doped Organic-Inorganic Hybrid Coatings as Corrosion Inhibiting Surface Treatments for Aluminum Alloys Tammy L. Metroke, Olga Kachurina, and Edward T. Knobbe Oklahoma State University, Environmental Institute, 003 Life Sciences East, Stillwater, OK 74075 Abstract Organically-modified silicate (Ormosil) films have been found to provide good corrosion resistance for metal substrates and are potential replacement systems for hexavalent chromiumbased conversion coatings. Significant enhancement in corrosion resistance properties were observed upon addition of sub-micron to micron-sized particles (TiO2, Al2O3, SiN, TiN) to the Ormosil coating as determined using accelerated salt spray and electrochemical analysis. Rcorr values derived from potentiodynamic polarization curves were found to increase from 1520 to (3800 - 4680) kΩcm2 upon addition of various concentrations of TiO2, Al2O3, SiN or TiN particles to the Ormosil thin film. Similarly, Epit values were found to range from +0.2 V for the non-doped Ormosil to (0.6 – 1.9) V for particle-doped Ormosil thin films. Introduction Stricter environmental regulations have mandated the near term removal of Cr(VI)containing compounds from corrosion inhibition packages used for the protection of aluminumskinned aircraft.1 A wide variety of novel coating processes have been developed as potential replacements for hexavalent chromium based conversion coatings as described in various reviews2. Thin films derived from organically modified silicates (Ormosils), hybrid organicinorganic materials formed through the hydrolysis and condensation of organically modified silanes with traditional alkoxide precursors3, are among the potential replacement systems. Ormosil films have found use in various high-performance applications including abrasion resistant, anti-soiling and anti-fogging coatings on various substrates; further, they have been found to provide good corrosion resistance for metal substrates based on their ability to form dense barriers to the penetration of corrosion initiators4. Review articles by Guglielmi5, Metroke and Knobbe6, and Twite and Bierwagen7, and the references presented therein, indicate that solgel derived coatings are of interest for corrosion resistance of various substrates. In recent work, Kato studied inorganic SiO2 films derived from Si(OC2H5)4-C2H5OH-H2O solutions for the corrosion resistance of aluminum.8 Van Ooij et al investigated the use of silane based pretreatments (aminopropyltrimethoxysilane, vinyltrimethoxysilane, 1,2-bis-triethoxysilyl ethane) for the corrosion protection of steel and aluminum alloy substrates9. Kasemann and Schmidt found that thermally-cured epoxy silane/bisphenol A coatings exhibited high scratch, abrasion, and corrosion resistance on aluminum, magnesium, and silver4. Donley and Vreugdenhal described the use of sol-gel derived self-assembled, nano-phase (SNAP)10 and organically modified zirconate11 coatings for the corrosion protection of 2024-T3 aluminum alloy. Osborne et al found that a series of water-based sol-
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