Date Approved

2016

Degree Type

Open Access Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

College of Technology

Committee Member

Vijay Mannari

Committee Member

John Dugger

Committee Member

Subhas Ghosh

Committee Member

Weidian Shen

Abstract

Pretreatments are thin, adherent, and protective layers deposited directly on cleaned metal surfaces to enhance the performance of the multi-coat finishing system on metal and alloy substrates. Regulatory and environmental considerations dictate that industries replace the extant toxic and hazardous chromate-based pretreatments used for aluminum alloys with more sustainable ones. Two new families of organic-inorganic hybrid (OIH) pretreatments for aluminum alloys have been developed using customized silane-type sol-gel precursors—Epoxy-silane and Bis-ureasil—as primary components. Two series of sol-gel compositions varying in concentrations of silane precursor, organic corrosion inhibitor, and nano-silica particles have been prepared and used to deposit OIH pretreatments on aluminum alloy substrates (AA-2024-T3). The corrosion resistance performance of these OIH pretreatments has been studied by electrochemical impedance spectroscopy (EIS), direct current (DC) polarization resistance, and neutral salt spray test and compared with the resistance performance conventional hexavalent chromium-based pretreatment, as a benchmark and industry standard for Al alloys. This research specifically investigates the effects of key components of sol-gel compositions on the corrosion performance of the OIH pretreatments on Al-2024-T3. Using the Box-Behnken Design of Experiment (DoE) methodology and Minitab software for analysis, the key factors affecting corrosion performance have been identified and sol-gel compositions optimized. This study revealed that for both families of OIH pretreatments, the film deposited in the sol-gel bath containing 25% by weight of precursor provided the best performance. Furthermore, incorporation of nano-silica particles to an optimum level of 5% by weight led to a remarkable improvement of the corrosion-resistance performance of the pretreatments

for both families. This study has also very clearly shown that the use of the organic corrosion inhibitor mercaptobenzthiazole is effective in enhancing the corrosion-resistance performance of both families of pretreatments when used at the optimum level of 3% by weight. A comparison of the corrosion-resistance performance of the two families of OIH pretreatments with chromate conversion coating (control) showed that pretreatments based on epoxy-silane precursor when used at optimum compositions clearly outperform chromate conversion coatings used as a control in this study. Furthermore, OIH based on a bis-ureasil precursor, while showing a slightly lower performance than its epoxy-silane counterpart, is comparable or slightly superior to the commercial conversion coating when used at optimum compositions.

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