Interfacial factors in corrosion protection: an EIS study of model systems

The effects of the liquid-coating interface and coating-metal interfaces on corrosion protection of a coated aluminum sheet (Alclad 7075-T6) were investigated using electrochemical impedance spectroscopy (EIS) of selected model systems. Parylene C was deposited on a solvent-cleaned aluminum sheet to prepare the coated aluminum sheet. Parylene C does not adhere well to most smooth surfaces: a freestanding film can easily be peeled off the substrate, although it does not peel off by itself during EIS measurements. When an ultra-thin layer of plasma polymer (ca. 50 nm) is deposited on the substrate, however, Parylene C adheres very strongly to the sheet. The top surface of Parylene C coating was modified via deposition of an ultra-thin layer of either a very hydrophobic or a very hydrophilic plasma polymer. The EIS Bode plot for the first run showed a typical good barrier coating regardless of the nature of the two interfaces under consideration. However, it was found that the impedance value in the lower frequency region decreases with increased immersion time, and the time-dependent change is greatly influenced by the nature of the two interfaces. A hydrophilic top surface increased the rate of time-dependent change, while good adhesion of Parylene C to the metal decreased the extent of time-dependent change. With a hydrophobic top surface and good adhesion to the substrate, EIS characteristics of a Parylene C coated sheet remained unaffected for 18 days (the longest time employed in this study). The decrease in impedance in the low frequency region converges to the Bode plot for a freestanding Parylene C film. The early decline of EIS characteristics can be correlated to the (pre-corrosion) micro-delamination of the coating. (C) 2001 Elsevier Science B.V. All rights reserved.