Environmental Effects on the High-Temperature Corrosion of Superalloys in Present and Future Gas Turbines

Abstract

Effects of temperature and contaminant levels on the high-temperature corrosion of superalloys used in gas turbines were investigated using pressurized passages which simulate the operating conditions of present-day gas turbines. The alloys were tested in a cooled configuration realistically simulating the air-cooled vanes and blades of a gas turbine. Conclusions are drawn as to the permissible level of contaminants and the effect of metal cooling on high-temperature corrosion. It is shown that the surface temperature of a blade or vane rather than the gas-stream temperature is the critical factor in determining the amount of attack to be expected at a given contaminant level and the amount of attack is an exponential function of this temperature. Furthermore, in a dynamic-type test no decrease in corrosion rate is noted at higher temperatures. It was concluded that the use of a 5 ppm Na/2 ppm V fuel would result in an excessive amount of attack with a metal surface temperature of 1500 deg F.

Temperature Estimation and Life Prediction of Turbine Blades Using Post Service Oxidation Measurements

“…This was considered to be reasonable in light of some metal loss data obtained in a three atmosphere combustor rig on pin samples of three other alloys (IN-713C, E14-738 and U-500) of varying aluminum and chromium compositions [12]. The relative performance of the three alloys and their aluminum and chromium contents are listed in Table 4.…”

Diagnostics and Instrumentation Requirements for Advanced Power Systems

“…For example, it is important to measure local material surface temperature and composition or relative changes in surface composition with test time. These points have been emphasized by Dils (48), Dils and Follansbee (49,50), Lee et al (44), and, more recently, by HTTT investigators at Curtiss-Wright (12,13) and General Electric (14,15). These studies support the need for extensive gas turbine materials testing under realistic conditions which simulate the environments expected in coal-fired systems.…”

“…For the uncleaned exhaust, these levels of Va and Na are much higher than presently realized in aircraft or industrial gas turbines. At present, unacceptably high corrosion rates are observed for firing with Type 3-GT heavy liquid fuel containing 5 ppm Na and 2 ppm V (43,44). For turbines fired with liquid hydrocarbons, higher air bypass ratios are used and overall equivalence ratios of less than 0.2 are obtained For these systems we estimate product gas concentrations of Na and V of 65 ppb and 26 ppb, respectively.…”

“…3 This will be necessary in order to minimize blade corrosion due to higher concentrations of sodium, potassium, vanadium, sulfur, and chlorine in these lower-grade fuels. It is observed (43,44) that corrosion rates are exponentially dependent upon blade metal temperature rather than upon the hot gas temperature. Present turbines operate with metal temperatures approaching 1500-1800 F (1100-1260 K).…”