Techniques for High Temperature Fatigue Testing

The influence of small concentrations of SO₂, HCl or Na₂SO₄ vapour in air on the corrosion of the uncoated nickel-based superalloys IN 792 DS + Hf, CMSX-6 and MA 760 ODS and the coatings RT 22 and LCO 22 has been investigated at 1000 and 1100°C. A level of 1% SO₂ in air somewhat increased the oxidation of IN 792 DS + Hf and favoured the scale spallation of CMSX-6 at 1000°C. Some precipitates of Ti sulphide were found in the subsurface zones of both alloys. No influence was observed at 1100°C. The oxidation of MA 760 ODS and the two coatings was not affected by SO₂ at 1000 and 1100°C. The addition of 100ppm HCl to air favoured the spallation of the scales of IN 792 DS + Hf and CMSX-6 at 1000°C. Again, no influence could be observed with these alloys at 1100°C nor with MA 760 ODS or the coatings at either of these temperatures. Contrary to expectations, no synergistic effect was found in air with 1% SO₂ and 100ppm HCl, but SO₂ reduced the negative effect of HCl. Catastrophic corrosion occurred with IN 792 DS + Hf and CMSX-6 in the presence of Na₂SO₄ vapour in air at 1000 and 1100°C after an incubation period of more than 100 h. The mechanism of this rapid corrosion could not be clarified 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 between Na₂SO₄ vapour and the scale, thus altering the transport properties of the oxides. MA 760 ODS and the coatings were not tested. Constant extension rate tests with IN 792 DS + Hf and CMSX-6 at 1000°C did not reveal any influence of 100 ppm HCl on the mechanical properties at strain rates of 1 × 10^?6 and 3 × 10^?8 S^?1.