Estimation of useful life in turbines blades with cracks in corrosive environment

Geothermal turbines of 110 MW were installed in the Federal Electricity Commission in Cerro Prieto Mexico, which operating time exceeds 150,000 h. Therefore, the critical components which determine the useful life of the turbine should be evaluated to determine the rehabilitation or replacement of them. The critical components are the blades of the last stage in the steam turbine. It has been observed that different blades of the turbine of 110 MW with cracks presented corrosion products, which resulted in a failure for corrosion fatigue mechanisms. In this paper, it was studied the effect of crack propagation produced in a geothermal turbine blade of the last stage, L-0, which is made of stainless steel AISI 410 exposed to corrosion under a sea water solution. The corrosion phenomena including localized corrosion suffered by the cracking sample were studied through the electrochemical noise technique in current and potential and polarization curves. The tests were conducted on pieces of blades subjected to fatigue. The results indicated that the exposure to the corrosion solution modified the width and the length of the cracks. Using a scanning electron microscopy (SEM), the surface of the crack was observed, showing that the corrosion mechanism produced a significant increment of the velocity of crack propagation and therefore, a decrement of the useful life of the material. This research will allow us to understand the corrosion process in addition to estimate the useful life of the blades when they are subjected to load cycles.