THE INFLUENCE OF GASEOUS IMPURITIES IN AIR ON THE HIGH-TEMPERATURE CORROSION OF COATED AND UNCOATED NICKEL-BASED SUPERALLOYS

The influence of small concentrations of SO2, HCl or Na2SO4 vapour in air on the corrosion of the uncoated nickel-based superalloys IN 792 D S + Hf, CMSX-6 and MA 760 ODS and the coatings RT 22 and LCO 22 has be en investigated at 1000 and 1100 degrees C. A level of 1% SO2 in air s omewhat increased the oxidation of IN 792 DS + Hf and favoured the sca le spallation of CMSX-6 at 1000 degrees C. Some precipitates of Ti sul phide were found in the subsurface zones of both alloys. No influence was observed at 1100 degrees C. The oxidation of MA 760 ODS and the tw o coatings was not affected by SO2 at 1000 and 1100 degrees C. The add ition of 100 ppm HCl to air favoured the spallation of the scales of I N 792 DS + Hf and CMSX-6 at 1000 degrees C. Again, no influence could be observed with these alloys at 1100 degrees C nor with MA 760 ODS or the coatings at either of these temperatures. Contrary to expectation s, no synergistic effect was found in air with 1% SO2 and 100 ppm HCl, but SO2 reduced the negative effect of HCl. Catastrophic corrosion oc curred with IN 792 DS + Hf and CMSX-6 in the presence of Na2SO4 vapour in air at 1000 and 1100 degrees C after an incubation period of more than 100 h. The mechanism of this rapid corrosion could not be clarifi ed and it is still an open question whether the corrosion is caused by acidic fluxing of a liquid Na2SO4-MoO3 film or by the reaction betwee n Na2SO4 vapour and the scale, thus altering the transport properties of the oxides. MA 760 ODS and the coatings were not tested. Constant e xtension rate tests with IN 792 DS + Hf and CMSX-6 at 1000 degrees C d id not reveal any influence of 100 ppm HCl on the mechanical propertie s at strain rates of 1 x 10(-6) and 3 x 10(-8) s(-1).