Cyclic voltammetric and potentiodynamic studies were carried out on 300W carbon steel in Bayer plant solution, at 100 degreesC, with different alumina concentrations. Alumina behaves as an anodic inhibitor, shifting the critical passivation potentials positively and decreasing the critical passivation current with increasing concentration. Increase in alumina concentration promotes the formation of a uniform and less porous. film. The pore resistance model describes the properties of the oxide. films. Aluminium was found in all oxides formed, supporting the formation of a mixed oxide Fe3-xAlxO4. Thermodynamic calculation of some equilibrium potentials was carried out using the Fe(OH)(-)(3) ion rather than HFeO2- ion. Moreover, the Al(OH)(-)(4) ion was considered instead of AlO2- ion in the oxidation process.