Mojca Slemnik, Valter Doleček, and Miran Gaberšček*
Faculty of Chemistry and Chemical Engineering, Smetanova 17, 2000 Maribor, Slovenia
*National Institute of Chemistry, Hajdrihova 19, 61115 Ljubljana, Slovenia
 

Abstract
The behaviour of differently heat-treated X20Cr13 steels in 0.1 M H2SO4 was studied using the classical potentiodynamic method and electrochemical impedance spectroscopy. On the base of the potentiodynamic curve the constant potentials have been chosen at which the impedance spectra were recorded: at the active peak, the potential next to the active peak and in the active - passive region. All impedance spectra show typical shapes: the high frequency portions have typical semicircular shapes of complex plane plots, whereas at low frequencies the plots deviate into semicircles running in opposite direction and yielding the so-called negative resistance as the frequency approaches 0 Hz. These spectra are interpreted in terms of a model by R.D. Armstrong describing, in a general way, the electrochemical reaction at interfaces with adsorbed intermediates.10 Applying the model to the measured impedance spectra, we have been able to distinguish between the cases in which the observed charge transfer resistance is solely determined by nature of metal surface and the cases in which, beyond this inherent metal property, the charge transfer is determined by the degree of surface coverage with adsorbed intermediates. For example, while the oil-quenched X20Cr13 steel shows the lowest inherent charge transfer resistance of bare surface, the air-quenched sample exhibits the largest resistance due to surface passivation by intermediates. All results of impedance analysis are well-correlated to the measured potentiodynamic curves.