ABSTRACT
<p><b>OBJECTIVE</b>To investigate the properties of passive film of three Cr alloys and to analyze their corrosion resistance in the artificial saliva with different NaCl mass fraction in vitro.</p><p><b>METHODS</b>Artificial saliva with 1%, 2% and 3% NaCl mass fraction was prepared. Cobalt-chromium alloy, nickel-chromium alloy and Ti-Ni-Cr alloy were employed as the working electrode in the artificial saliva. Semi-conductor properties of passive film on Cr alloy was analyzed by Mott-Schottky plots. In addition, the factors which affect the semi-conductive compact characteristic of the passive film was also discussed.</p><p><b>RESULTS</b>The passive film exhibits n-type semi-conductive characters. With the increasing of NaCl mass fraction, the carrier density of the Cr alloys also increased. The carrier density of the cobalt-chromium alloy, nickel-chromium alloy and Ti-Ni-Cr alloy are 3.71 x 10⁸, 2.34 x 10⁹, and 7.66 x 10⁹/cm³ respectively. This decreases its donor density and its film stability.</p><p><b>CONCLUSIONS</b>When exposed to saliva environment with high concentration of chlorine ion, corrosion resistance of the three types of Cr alloys decrease. This will reduce the service life of Cr alloy prosthesis.</p>