This paper addresses the effects of corrosion on the high cycle fatigue (HCF) strength of a high mechanical strength martensitic stainless steel (X12CrNiMoV12-3) that is used in aeronautic applications. HCF tests (between 105 and 107 cycles) were carried out in two environments: (i) in air and (ii) in an aqueous solution (pH = 6) of 0.1 M NaCl + 0.044 M Na2SO4 at a loading frequency of 120 Hz. Surface crack initiation is observed in air, whereas in solution, the crack initiated at corrosion defects. The decrease observed in the corrosion fatigue strength of specimens at 107 cycles was 33% of that of similar specimens tested in air. The electrochemical behaviour of the passive film was investigated during fatigue testing in a NaCl aqueous solution by monitoring the free potential and through electrochemical impedance spectroscopy (EIS). Based on fractography analysis and the EIS tests results, fatigue crack initiation mechanisms in air and in the NaCl aqueous solution were identified. A scenario for fatigue crack initiation is proposed based on the physical evidence. Local passive film ruptures are the cause of the corrosion fatigue crack initiation. The film is broken by material strain from the applied cyclic stress during fatigue testing, causing increased local corrosive attack. This effect has been shown at 10 Hz and 120 Hz because the characteristic time for repassivation for the steel is very low compared to the period of the cyclic loading.Read less <