The passive state of Cr in 1 M sulphate solutions (pH 0 and 5) was stu
died with a combination of electrochemical techniques - impedance spec
troscopy, photoelectrochemistry and de resistance measurements by the
contact electric resistance (CER) technique. Passive film growth was f
ound to be associated with an exponential increase of the film resista
nce probably due to the simultaneous dehydration/oxidation of Cr(II) t
o Cr(III) via a solid state electrochemical reaction. A description of
the steady state passive film as a thin insulator layer with small in
trinsic conductivity was consistent with the experimental results, Pol
arization of Cr, on which a steady state film had been formed, to posi
tive and negative potentials led to substantial increase of the conduc
tivity 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 f
irst layers adjacent to the metal/film interface, while at high potent
ials 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.
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