Conduction mechanism of the anodic film on chromium in acidic sulphate solutions

The passive state of Cr in 1 M sulphate solutions (pH 0 and 5) was studied with a combination of electrochemical techniques - impedance spectroscopy, photoelectrochemistry and de resistance measurements by the contact electric resistance (CER) technique. Passive film growth was found to be associated with an exponential increase of the film resistance probably due to the simultaneous dehydration/oxidation of Cr(II) to Cr(III) via a solid state electrochemical reaction. A description of the steady state passive film as a thin insulator layer with small intrinsic conductivity was consistent with the experimental results, Polarization of Cr, on which a steady state film had been formed, to positive and negative potentials led to substantial increase of the conductivity of the film. These changes can be attributed to the generation of lower and higher valency Cr species in solid state electrochemical reactions: At low potentials lower valency species are formed in the first layers adjacent to the metal/film interface, while at high potentials higher-valency species are formed in the first layers adjacent to the film/electrolyte interface. At sufficiently high positive (or low negative) potentials the film was concluded to be transformed into a conductor allowing transpassive (or active) dissolution to take place. (C) 1998 Elsevier Science Ltd. All rights reserved.