Electrochemical determination of the porosity of single and duplex PVD coatings of titanium and titanium nitride on brass

For the development of optimized coating-substrate systems one needs fast and reliable methods which permit the characterization of both intrinsic corrosion resistance and porosity. In the present paper electrochemical procedures for the determination of porosity are developed and applied to the study of Ti and TiN coatings on brass and of TiN/Ti duplex coatings. In a first approach the porosity was obtained from a comparison of the de polarization resistance of the uncoated and coated substrate at constant potential. In another approach the ratio of current maxima observed during dissolution of the brass substrate in presence and absence of a coating was taken as a measure of porosity. The third method used impedance spectroscopy for the determination of the polarization resistance in the presence and absence of a coating. All methods gave consistent results that were well reproducible. The lowest porosity was found for duplex coatings and for titanium coatings deposited under application of a bias voltage. The electrochemical methods used in this study are well suited for the characterization of the porosity of passive physically vapor deposited coatings deposited on electrochemically active substrates.