Role of sodium chloride in low temperaturs hot corrosion of superalloys.

Abstract

In this thesis, the influence of NaC1 on hot corrosion behaviour of IN 738 and Mar-M 509 at 750°C in static air, under thermal cyclic conditions has been studied. Salt-coating technique was employed, in which alloy samples were coated with approx. 2.5 mg/cm² of salt. Hot corroded sample of IN 738 and Mar-M 509 have been analysed using SEM, point EDS, X-ray mapping, ESCA and XRD techniques and correlated with corrosion kinetics. The results indicated that Na₂SO₄+25%NaCl was very aggressive compared to that of either pure Na₂SO₄+1%NaCl. All samples exhibited enhanced internal sulfidation in the presence of sulfate/chloride salt mixture. It has been concluded that continuous replenishment of Na₂SO₄+25%NaCl mixture on both Ni-base and Co-base alloys resulted in a typical LTHC morphological features during the propagation stage. Basic fluxing model has been proposed for low temperature hot corrosion of IN 738 in Na₂SO₄+25%NaCl mixture during the propagation stage. The initiation and propagation stage of Mar-M 509 is attributed to the attack along W -rich phase and sulfidation/oxidation of Cr. The role of NaCl on hot corrosion behaviour of IN738 and Mar-M 509 has been discussed in the light of the above experimental study.