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Effect of Zinc on the Sulphidation Reaction in Marine Gas Turbines

Abstract : One of the major factors controlling the lifetime of marine gas turbines is the rapid degradation of the turbine blades by a process known as sulphidation. In this form of accelerated corrosion attack, the salt ingested with the intake air combines with the sulphur from the fuel and reacts with the blade alloy at high temperature. However, metallurgical examination of turbine blades from a P&W FT-12 gas turbine showed that no degradation of the blades had occurred after 3500 hours of operation in a DDH 280 class destroyer. Similarly, blades from the Solar Saturn gas turbines were sulphidation free after as many as 11,200 hours of operation although degradation due to other causes was observed. The blade surfaces were partially covered by a white deposit identified as ZnSO4. The source of the Zn is the zinc pigmented protective coating in the fuel tanks. Sulphidation of turbine blades was induced in the laboratory where it was found that in the majority of the tests the blades were completely oxidized after 16 - 90 hrs. However, with the addition of 2.5 or 5 percent ZnSO4 to the 90/10 Na2SO4/NaCl salt mixture, sulphidation was prevented. These results, along with the observation that sulphidation has not occurred in service, suggest that the presence of Zn in the fuel may have prevented the sulphidation process. (Author)