p-Type and n-Type Behavior of Chromium Oxide as a Function of the Applied Potential

Thermodynamic data as well as physical and electrochemical studies demonstrate that the composit ion of the passive layer on chromium depends on the potential and the electrolyte pH. The electrochemical propert ies of Cr203 electrodes were compared with the anodic behavior of chromium in borate buffer solutions. The feasibil ity of the solid-phase trans-formation of a surface layer under anodic polarization was investigated. The conduct iv i ty type and deviation from the stoi-chiometr ic composit ion of the chromium oxide was determined in situ using the photopolarization method. Depending on the polarization potential, stoichiometric point defects in Cr~O: ~ result in the p- or n-type behavior of the oxide. The for-mula can be expressed as Cr,,,O,_~. (or Cr,,,+~O,, where x ~> 0). Photopolar izat ion measurements suggest hat Cr203 behave l ike a p-type semiconductor at low anodic potentials, but it shows n-type behavior at high anodic potentials. The transit ion from p-type to n-type conduct iv i ty is a pH-dependent revers ib le process, occurr ing at potent ia ls corresponding to the transformation of Cr203 (p-type) to CrO ~ (n-type) via the solid-state mechanism. Chromium and alloys contain ing chromium are of con-s iderab le techno log ica l s igni f icance (1). Chromium is one of the most important ant ieorrosive substances used in many kinds of commerc ia l stainless steel. It has bee