Breakdown mechanisms of preformed Al/sub 2/O/sub 3/, Cr/sub 2/O/sub 3/ and SiO/sub 2/ scales in H/sub 2//H/sub 2/O/H/sub 2/S environments at 950/degree/C. [Fe-18Cr-6Al-1Hf; Ni-20Si; Ni-30Cr]

Technical Report · Wed Jun 01 04:00:00 EDT 1988

OSTI ID:6911210

Kim, G M; Meier, G H

The breakdown mechanisms of preformed oxide scales, Cr/sub 2/O/sub 3/, Al/sub 2/O/sub 3/ and SiO/sub 2/, have been studied at 950/degree/C at low oxygen and high sulfur potentials. For Cr/sub 2/O/sub 3/ on Ni-30Cr, sulfur penetration occurs through thin areas in the Cr/sub 2/O/sub 3/ scale. These thinner areas were formed over voids formed during preoxidation because the voids act as chromium diffusion barriers. Internal sulfides form at the rim of the voids where the chromium activity is high. The filling of voids by sulfides and the progressive formation of sulfides through the scale establish sulfide rich channels which are easy paths for cation transport. The breakdown occurs once the channels are introduced. For Al/sub 2/O/sub 3/ on Fe-18Cr-6Al-1Hf, sulfur reacts with metallic iron on the surface and HfO/sub 2/ extending through the Al/sub 2/O/sub 3/ scale to form iron sulfides and hafnium rich sulfides, respectively. These sulfides provide easy paths for inward diffusion of sulfur ions and outward diffusion of base metal ions to form internal sulfides at the craters around Hf rich phases, and external sulfides at the scale/gas interface, respectively. For SiO/sub 2/ on Ni-20Si, although porosity can form at the scale/gas interface due to the high vapor pressures of SiS and SiO, the formation of a thin layer of vitreous silica and good scale adherence provide better resistance to sulfur penetration than do Cr/sub 2/O/sub 3/ and Al/sub 2/O/sub 3/. 33 refs., 30 figs., 2 tabs.

• Research Organization: Oak Ridge National Lab., TN (USA); Pittsburgh Univ., PA (USA). Dept. of Materials Science and Engineering
• DOE Contract Number: AC05-84OR21400
• OSTI ID: 6911210
• Report Number(s): ORNL/Sub-83-43346/01; ON: DE88013705
• Country of Publication: United States
• Language: English

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36 MATERIALS SCIENCE
360105* -- Metals & Alloys-- Corrosion & Erosion
ALLOYS
ALUMINIUM COMPOUNDS
ALUMINIUM OXIDES
CHALCOGENIDES
CHEMICAL REACTIONS
CHROMIUM COMPOUNDS
CHROMIUM OXIDES
COHERENT SCATTERING
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EXPERIMENTAL DATA
HYDROGEN
HYDROGEN COMPOUNDS
HYDROGEN SULFIDES
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MINERALS
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NICKEL BASE ALLOYS
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VERY HIGH TEMPERATURE
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