Polarization microscopic investigations were utilized to establish that a natural gas turbine was operated with an intake of very polluted surrounding air for the combustion of the natural gas and that the filter plant efficiency was insufficient. Specific optical microscopic and scanning electron microscopic investigations into the structure of the blades proved that over a certain time period, overheating of the blades to temperatures higher than 1100 degree C took place and thus evaporation of the Cr as CrO//3. Cement minerals, i. e. , Portland clinker and blast-furnace slag, which attacked the upper matrix protecting coating with the simultaneous reducing effect of the flue gas, destroyed the IN 939 turbine stator blades within 7565 operating hours with 336 stop-start cycles. The overheating of the blades resulted, among other things, in the formation of alpha , beta , eta and sigma phases, increased carbonization and nitrogen absorption, which produced embrittlement. This meant that the blades were, in places, mechanically overloaded and that material particles could break off. Moreover, the bearing of the turbine became damaged by imbalance as well as overheating.
