Electrochemical behavior of Ni+Ti, Ni-P+Ti composite layers in an alkaline environment
Andrzej Serek, Antoni Budniok Uniwersytet Śląski, ul. Bankowa 12, 40-007 Katowice
Annals 3 No. 6, 2003 pages 58-63
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abstract The subject of this work is electrochemical characterization of composite Ni+Ti and Ni-P+Ti layers obtained by codeposition of crystalline or amorphous nickel with titanium from the bath containing titanium powder suspension. The layers were electrodeposited under galvanostatic conditions (j = 300 mA/cm2) on a steel substrate (St3S). Ni+Ti layers contained 25wt.% of Ti and Ni-P+Ti layers contained 17wt.% of Ti. The obtained layers were subjected to thermal treatment at the temperature of 800oC in argon or nitrogen atmosphere for 24 h. Electrochemical characteristic was carried out in alkaline environment (6M KOH) using classical method (voltammetry). Potentiodynamic j = f(E) curves were recorded in the range ±200 mV starting from j = 0. On this basis some parameters like cathodic and anodic Tafel slope, corrosion potential and current or corrosion wear were determined. Electrochemical impedance spectroscopy was also used to study the electrochemical properties of the layers. On the base of obtained Nyquist plots Z” = f(Z’) the equivalent circuit was approximated. The results of EIS investigation show, that the ac behaviour of Ni+Ti and Ni-P+Ti layers could be described by CPE1 model, consising of the solution resistance Rs in series with a parallel connection of the CPE element and the charge-transfer resistance - Rct. As a result of approximation of experimental data in the case of CPE1 model the following parameters were determined: Rs, Rct, T - capacity parameters and φ - dimensionless parameter related to the constant phase angle. On the basis of these parameters the double-layer capacitance and the value of roughness factor - Rf were calculated. It was stated that the greatest corrosion resisance in comparison with raw layers exhibit the Ni+Ti and Ni-P+Ti layers subjected to nitriding. The reason of such behaviour could be the presence of products of nitriding on the electrode surface as well as their more developed surface, which enhances their corrosion resistance. Key words: nickel, titanium, electrolytic composite layers, corrosion resistance, impedance spectroscopy