Comparative corrosion studies of composite coatings by impedance spectroscopy method. 1 - Theoretical aspects of the impedance spectroscopy method incorrosion studies

This paper describes an investigation of the comparative corrosion behaviou r of metal and metal matrix composite with ZrO2, TiO2, Al2O3 and SiC embedd ed as disperse phase during nickel electrodeposition from a sulphate-platin g bath. Electrochemical Impedance Spectroscopy was used to determine the po larisation resistance, the double layer capacitance and the true surface ar ea in sulphuric acid and sulphate solutions. The Faradaic distortion method was used to determine the corrosion rates. The results were in good agreem ent with those determined by polarisation resistance using the Stem Geary f ormula. A mechanism with adsorbed intermediates for metal corrosion in sulp huric acid solution and sulphate solution is proposed. The influence of dis perse phases like oxides and silicon carbides in nickel,. copper and zinc c oatings on polarisation resistance and corrosion resistance are discussed. Several types of composite coatings were prepared and tested by comparison with pure metal coating. The activating of the cathodic reaction of the hyd rogen evolution was verified by cathodic polarisation curves. The first par t of the work, entitled "Theoretical Considerations", presents the method o f evaluating impedance spectra from experimental data.