Corrosion protection of copper by silane sol-gel coatings

The corrosion protection of three silane-based coatings on copper substrate was investigated in 3.5 % NaCl solution. For this purpose, tetraethoxysilane (TEOS), 3-tri-methoxysilyl-1-propanethiol (TPS) and a hybrid TEOS + TPS were coated onto a Cu substrate by the sol-gel deposition method. The corrosion behavior of uncoated and coated copper specimens was evaluated using potentiodynamic polarization, electrochemical impedance spectroscopy and immersion tests. Furthermore, the surface morphologies and chemical compositions of the coatings were assessed using atomic force microscopy (AFM), scanning electron microscopy and Fourier transform infrared spectrometer (ATR-FTIR). The results indicated that hybrid TEOS + TPS coating exhibited the best corrosion performance as compared with the copper substrate and single TEOS and TPS coatings. This behavior was ascribed to the simultaneous formation of strong Cu-S and cross-link Si-O-Si bonds according to the ATR-FTIR observations. The surface topography studies by AFM after 120-h immersion in 3.5 % NaCl demonstrated the heavy degradation of the coating in the case of TEOS. In contrast, hybrid TEOS + TPS coating produced a homogenous and highly adherent protective nanostructure coating (75-250 nm) on the copper substrates. The three-dimensional AFM topography images of TEOS-, TPS- and TEOS + TPS-coated copper surface (a, c and e) before and (b, d and f) after immersion in 3.5 % (w) NaCl solution for 120 h.