CHLORIDATION-OXIDATION OF FE-CR AND NI-CR ALLOYS AT 800-DEGREES-C

The chloridation-oxidation behavior of Fe-Cr (0-25 wt.%Cr) and Ni-Cr ( 0-20 wt.%Cr) alloys was studied at 800 degrees C in three different H- 2-HCl-H2O(v) environments. In a low-HCl and low-H2O(v) environment, wh ere Cr2O3 is thermodynamically stable, the corrosion resistance of the Fe-Cr alloys increased with increasing Cr content in the alloys, In a high-HCl and high H2O(v) environment, where FeCr2O4 is stable and CrC l2 is metastable, the corrosion resistance of the Fe-CI alloys depende d similarly on the Cr content, Low-Cr-Fe-Cr alloys exhibited large wei ght losses, while Fe-Cr alloys with higher than 19 wt.%Cr showed good corrosion resistance. In an environment of high-HCl in the absence of H2O(v), the evaporative corrosion rate was fast and limited by gas-pha se diffusion, and independent of the Cr content in the Fe-Cr alloys. N i and Ni-Cr alloys generally showed good corrosion resistance in the e nvironments of high H2O(v) because of the tow NiCl2 vapor pressure and formation of a protective Cr2O3 scale. However, in the environment of high HCl in the absence of H2O(v), selective formation and evaporatio n of CrCl2 occur, which results in Cr depletion and networks of voids for even a high-Cr Ni-Cr alloy.