Disclosed is a diagnostic method for estimating the corrosion fatigue life of a metal material used in a service environment even if the service environment would not allow a direct evaluation of the life by a corrosion fatigue test. The method is intended so that reliable devices which satisfy required specifications are developed and offered in a short period of time at low costs. One aspect of the diagnostic method for estimating the corrosion fatigue life of a metal material is characterized by the following steps. The method includes the step (step 1) of measuring the critical pitting corrosion temperature of the metal material in an environment in which the metal material is used; the step (step 2) of comparing the maximum temperature of the service environment to the critical pitting corrosion temperature; the step (step 3a) of measuring the pitting corrosion depth resulting from immersing the metal material in the service environment for a predetermined time; the step (step 4a) of determining a stress intensity factor range in the pitting corrosion during the immersion time; the step (step 5) of setting the lower limit of fatigue crack growth of the metal material in the service environment; the step (step 6a) of comparing the stress intensity factor range to the lower limit of fatigue crack growth; and the step (step 7) of determining the time at which the stress intensity factor range is equivalent to the lower limit of fatigue crack growth.
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