The effect of Ni in the electrochemical properties of oxide layers grown on stainless steels

This study analyses the influence of Ni in the electrochemical behaviour of three different stainless steels in alkaline medium. The studied steels have increasing Ni content: AISI 430, AISI 304L and AISI 316. The obtained results are compared with those of a nickel base alloy (Ni > 42%, w/w) and a pure Ni electrode. Electrochemical impedance spectroscopy and cyclic voltammetry have been the main tools used to study the growth and evolution of the passive layers formed on those materials in alkaline medium. XPS and SEM have been employed for chemical and morphological characterization of the developed passive films.The presence of Ni promotes the formation of thinner and more protecting passive films. This fact noticed in the XPS analysis is reflected in the cyclic voltammograms by an important decrease of the magnetite formation peak current as well as that corresponding to Cr3+/Cr6+ oxidation. The low frequency limit of complex plane impedance plots also increases with the Ni content. In order to better characterise the resistivity of the electrochemically formed films, a more detailed impedance analysis in the high frequency range (1 kHz–10 MHz) has been performed. The analysis of the registered spectra indicates that Ni modifies the conductivity of the oxide layers, promoting the formation of more resistive oxide films.