An optimal nickel difluoride (NiF2) film-formation technology has been investigated for electroless Ni-P deposited films, where highly sensitive electrochemical anodic polarization measurement and thin-film x-ray diffraction (XRD) amorphous structure as deposited which is converted into crystalline structure such as nickel phosphides (Ni2P or Ni3P) with heating, this is undesirable for the formation of homogeneous fluorine passivation films. Low incident angle XRD allows us to determine the fluoridation conditions where the NiF2 films little include these nickel phosphides crystals. Further, anodic polarization measurement curves support the film-formation conditions (temperature, fluorine concentration) of nickel difluoride and the effect of heat-treatment after fluoridation. The low fluoridation temperature at ca. 300-degrees-C is not sufficient and low fluorine concentration is also unfavorable for the fluoridation of the electroless Ni-P deposited films. We conducted several evaluations such as corrosion resistance, outgassing characteristics, and plasma resistance using the nickel difluoride films formed by the most favourable condition.