The Ti21Nb24Mo23Hf17Ta15 biocompatible high entropy alloy (HEA) was analyzed for different air-exposure times before corrosion tests in simulated body fluid (SBF), simulating implant air storage before implantation, presenting tremendous surface passivation capacity. Electrochemical impedance spectroscopy indicates layers thickening during air exposure with a very capacitive behavior. Two-time constant relaxation confirmed the formation of a double layer and oxide one. The equivalent Relectrolite+ (Coxide)+ (R//CPE) circuit model simulated the EIS behavior. Atoxic inert oxides were observed by X-ray photoelectron spectroscopy before and after SBF immersion for seven days, not impairing biocompatibility and producing a stable passive film.