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Subsurface microstructural changes of alloy 625 related to oxidation at 900 °C in lab air and humid air.

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Title: Subsurface microstructural changes of alloy 625 related to oxidation at 900 °C in lab air and humid air.
Authors: Jacquard, B.1,2 (AUTHOR) bruno.jacquard@univ-lorraine.fr, Mathieu, S.1 (AUTHOR) stephane.mathieu@univ-lorraine.fr, Steciuk, G.1 (AUTHOR) gwladys.steciuk@univ-lorraine.fr, Migot, S.1 (AUTHOR) sylvie.migot@univ-lorraine.fr, Bonina, D.1 (AUTHOR) david.bonina@univ-lorraine.fr, Perez, T.3 (AUTHOR) thomas.perez@safrangroup.com, Epifano, E.4 (AUTHOR) enrica.epifano@toulouse-inp.fr, Monceau, D.4 (AUTHOR) daniel.monceau@toulouse-inp.fr, Vande Put, A.4 (AUTHOR) aurelie.vandeput@ensiacet.fr
Source: Corrosion Science. Jul2026, Vol. 267, pN.PAG-N.PAG. 1p.
Subjects: Oxidation, Precipitation (Chemistry), Chemical decomposition, Molybdenum oxides, Kirkendall effect
Abstract: Alloy 625 was oxidized at 900 °C in lab air and in air + 20%vol. H 2 O for 400 h to estimate the microstructural changes experienced by the alloy during exposure at high temperature. The specimen mass loss in air + 20%vol. H 2 O evolved linearly after a very short period of exposure due to the formation of volatile species from the protective Cr 2 O 3 layer. The chromium loss rate was in the order of 5.5 × 10−7 mg.cm-2.s-1 (evaluated by direct mass variation measurements) and 7.5 × 10−7 mg.cm-2.s-1 (evaluated from Cr-mass balance) in air + 20%vol. H 2 O in the present conditions. Pronounced chemical and microstructural changes were observed in the subsurface for samples exposed in air + 20%vol. H2O. The recession of the alloy interface due to chromium removal was insufficient to explain the pronounced accumulation of δ-Ni 3 (Nb,Mo) phase at the alloy-oxide interface, demonstrating the significant uphill diffusion of Nb. Chemical breakaway on the sample edges involved the oxidation of δ-Ni 3 (Nb,Mo) phase, forming powdery oxides. • Linear mass loss evolution is recorded for alloy 625 in air + 20%vol. H 2 O at 900 °C. • Chromia volatilization rate estimated at 10−6 mg.cm−2.s−1 (gas velocity 0.7 cm.s−1). • Precipitation at the metal-oxide interface of δ-Ni 3 (Nb,Mo) phase is quantified. • Significant uphill diffusion of Nb when alloy 625 oxidizes in air + 20%vol. H 2 O. • MoO 3 (g) and powdery oxides form at the sample edges when chemical breakaway occurs. [ABSTRACT FROM AUTHOR]
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Database: Engineering Source
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Abstract:Alloy 625 was oxidized at 900 °C in lab air and in air + 20%vol. H 2 O for 400 h to estimate the microstructural changes experienced by the alloy during exposure at high temperature. The specimen mass loss in air + 20%vol. H 2 O evolved linearly after a very short period of exposure due to the formation of volatile species from the protective Cr 2 O 3 layer. The chromium loss rate was in the order of 5.5 × 10−7 mg.cm-2.s-1 (evaluated by direct mass variation measurements) and 7.5 × 10−7 mg.cm-2.s-1 (evaluated from Cr-mass balance) in air + 20%vol. H 2 O in the present conditions. Pronounced chemical and microstructural changes were observed in the subsurface for samples exposed in air + 20%vol. H2O. The recession of the alloy interface due to chromium removal was insufficient to explain the pronounced accumulation of δ-Ni 3 (Nb,Mo) phase at the alloy-oxide interface, demonstrating the significant uphill diffusion of Nb. Chemical breakaway on the sample edges involved the oxidation of δ-Ni 3 (Nb,Mo) phase, forming powdery oxides. • Linear mass loss evolution is recorded for alloy 625 in air + 20%vol. H 2 O at 900 °C. • Chromia volatilization rate estimated at 10−6 mg.cm−2.s−1 (gas velocity 0.7 cm.s−1). • Precipitation at the metal-oxide interface of δ-Ni 3 (Nb,Mo) phase is quantified. • Significant uphill diffusion of Nb when alloy 625 oxidizes in air + 20%vol. H 2 O. • MoO 3 (g) and powdery oxides form at the sample edges when chemical breakaway occurs. [ABSTRACT FROM AUTHOR]
ISSN:0010938X
DOI:10.1016/j.corsci.2026.113815