Tailored thin plasma polymers for the corrosion protection of metals

The barrier properties of thin plasma polymer layers on metals were characterised by means of electrochemical impedance spectroscopy (EIS). Tailored microwave plasma polymer layers were deposited on pure iron substrates. The evaluated resistance and capacitance values were correlated with the morphology and chemical structure of the plasma polymer. The chemical structure of the films were determined by means of infrared reflection absorption spectroscopy (IRRAS) and X-ray photoelectron spectroscopy (XPS). The morphology of the plasma polymer surface and its interface to the metal substrate were characterised using atomic force microscopy (AFM). It could be shown that, at higher pressure, nanoscopic particles which adsorb on the metal surface led to nanoscopic voids at the metal/polymer interface. These void-containing films also consisted of a high amount of trimethylsilyl groups. By lowering the pressure and thereby the deposition rate, the plasma polymer perfectly imitates the substrate roughness and leads to excellent barrier properties even at a thickness below 50 nm. Moreover, the ratio between siloxane and trimethylsilyl groups is increased promoting a better adhesion to the oxide covered metal substrate. It is clearly shown that EIS can be used to quantify the barrier properties and that the obtained measured data can be correlated with the morphology and chemical structure of the ultra-thin films.