Corrosion fatigue is one of the most important issues involved in operating compressors in gas turbine power plants securely. To address this issue, we investigated corrosion pit growth as well as crack growth behavior initiated from corrosion pits. We also developed a method for evaluating corrosion fatigue life under service conditions.
Although we did not observe any obvious relationship between operative stress and corrosion pit sizes of operated compressor blades, our investigation results suggested that corrosion pit growth was dependent on operational time. We therefore calculated and propose an estimation curve for corrosion pit growth. Under a corrosive environment with highly concentrated Na+, Cl-, and SO42-, the fatigue limit decreased to 40% of in-air strength. It also tended to decrease in inverse proportion to artificial defect size. We calculated crack growth threshold stress range σth from the relationship between stress and artificial defect size and found that the crack growth threshold stress intensity factor range ΔKth was 2.4MPa √m under a corrosive environment.
We also measured operative stress in compressor blades under service conditions and calculated equivalent stress σeq from stress frequency distribution. We found out that corrosion pit size corresponding to crack initiation was 0.58mm from obtained σeq and ΔKth and developed an evaluation diagram for calculating operational time from our estimation curve for corrosion pit growth. Our results suggest that we can measure operative stress to evaluate corrosion fatigue life quantitatively and formulate allowable stress.
