Processing and Properties of Environmentally-Friendly Corrosion Resistant Hybrid Nanocomposite Coatings for Aluminum Alloy AA2024
Environmentally friendly sol-gel nanocomposite coatings have been processed and characterized for corrosion protection of aerospace alloy AA2024-T3. The hybrid nanocomposite coatings were processed in solution using acid-catalyzed hydrolysis and condensation of precursors Glycidoxypropyl trimethoxysilane (GPTMS) and Tetramethoxysilane (TMOS) forming self assembled nanoscale composite followed by curing using a one step and a two step process. Nanometer scale montmorillonite clay, Cloisite 15A was dispersed in the precursor solution prior to film coating. The structure and composition of the hybrid coatings was determined using Fourier Transform Infrared Spectroscopy (FTIR) and X-ray Diffraction Spectrometry (XRD). Hydrolysis and condensation kinetics were tracked using FTIR and a semi-quantitative equation was developed. Viscosity studies performed to understand the rheology of the hybrid nanocomposite, shows two distinct regimes for hydrolysis and curing. XRD results indicate a physical structure that is mostly amorphous with selective crystalline regions due to siloxane bridge formations. The Cloisite 15A clay was exfoliated, as determined by the disappearance of clay peak. Atomic Force Microscopy (AFM) was used to study the surface morphology, showing increase in surface roughness with the addition of clay. Corrosion performance was determined using DC Polarization and Electrochemical Impedance Spectroscopy (EIS). The DCP results were used to determine the optimal composition of the hybrid nanocomposite and optimal time of reaction for the first layer. The corrosion current and hence the corrosion rate for the two-step system was around three orders of magnitude lower than the one-step system, with the corrosion potential being correspondingly higher. EIS results show that the two-step system has a corrosion resistance two orders of magnitude higher than the mono-layer. The impedance was in the order of E+6 Ohms, which remained consistent over 8 weeks of testing, with a low water uptake of around 20%. Modification of the coating with diamine curing agent and clay provided significantly better results with higher resistance (E+6 Ohms) and water uptake values as low as 3-5% over 6 weeks.
School:University of Cincinnati
School Location:USA - Ohio
Source Type:Master's Thesis
Keywords:environmentally friendly sol gel nanocomposite corrosion coatings
Date of Publication:01/01/2006