By means of X-ray diffraction (XRD), scanning electronic microscopy/energy dispersion spectroscopy observation, thermogravimetric analysis (TGA), differential scanning carorimeter (DSC), and cathodic polarization tests, the mechanism of electrolytic yttrium hydroxide thin-film deposition on IN617 superalloy in Y(NO3)(3) aqueous solution was investigated. The cathodic polarization curve in Y(NO3)(3) aqueous solution can be divided into two portions: (i) the reduction of H+ and O-2, i.e., 2H(+)+2e(-)-> H-2 and O-2+4H(+)+4e(-)-> 2H(2)O at -0.2 V to -0.95 V (vs saturated Ag/AgCl); (ii) the reduction of H2O and/or (Y+3)(H2O)(6), i.e., 2H(2)O+2e(-)-> H-2+2 OH- and 2Y(+3)(H2O)(6)+6e(-)-> 2Y(OH)(3)center dot 3H(2)O+3H(2) at -0.95 V to -3 V. The best deposition condition of Y(OH)(3)center dot 3H(2)O was conducted at the second portion. From the XRD, DSC, and TGA analyses, it was found that the as-coated film was hydrated Y(OH)(3)center dot 3H(2)O, dehydrated into Y(OH)(3)center dot H2O at 122 degrees C, further dehydrated into Y(OH)(3) at 366 degrees C, condensed into YOOH at 480 degrees C, and finally transformed into Y2O3 at 550 degrees C. Also, the electrolytic Y2O3 coating film revealed the efficient retarding effects on the oxidation of IN617 superalloy in air.