Preoxidation treatment of MA 956 superalloy at 1100 degrees C produces a fine and tightly adherent alpha-alumina layer at the surface, which provides the alloy with an excellent barrier against a great variety of aggressive environments. In this work the protective capacity of the alumina/alloy system is evaluated in a physiological medium by means of electrochemical impedance spectroscopy. The electrochemical response of the material is modelled by equivalent circuits which provide the most relevant corrosion and protection parameters applicable to MA 956 in both preoxidized and as-received conditions (passivated state). The high protective capacity of preoxidized MA 956 superalloy holds for long-term tests, which indicates that the corrosion phenomena, if any, would be characterized by very slow kinetics. The corrosion resistance of the preoxidized material is at least two orders of magnitude higher than that of the non-treated alloy.