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- Oernek, Cem, et al.
(author)
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Understanding passive film degradation and its effect on hydrogen embrittlement of super duplex stainless steel-Synchrotron X-ray and electrochemical measurements combined with CalPhaD and ab-initio computational studies
- 2023
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In: Applied Surface Science. - : Elsevier BV. - 0169-4332 .- 1873-5584. ; 628
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Journal article (peer-reviewed)abstract
- The passive film stability on stainless steel can be affected by hydrogen absorption and lead to microstructure embrittlement. This work shows that the absorption of hydrogen results in surface degradation due to oxide reduction and ionic defect generation within the passive film, which decomposes and eventually vanishes. The passive film provides a barrier to entering hydrogen, but when hydrogen is formed, atomic hydrogen infuses into the lattices of the austenite and ferrite phases, causing strain evolution, as shown by synchrotron x-ray diffraction data. The vacancy concentration and hence the strains increase with increasing electrochemical cathodic po-larization. Under cathodic polarization, the surface oxides are thermodynamically unstable, but the complete reduction is kinetically restrained. As a result, surface oxides remain present under excessive cathodic polari-zation, contesting the classical assumption that oxides are easily removed. Density-functional theory calculations have shown that the degradation of the passive film is a reduction sequence of iron and chromium oxide, which causes thinning and change of the semiconductor properties of the passive film from n-type to p-type. As a result, the surface loses its passivity after long cathodic polarization and becomes only a weak barrier to hydrogen absorption and hence hydrogen embrittlement.
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| 3. |
- Ornek, Cem, et al.
(author)
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The causation of hydrogen embrittlement of duplex stainless steel : Phase instability of the austenite phase and ductile-to-brittle transition of the ferrite phase-Synergy between experiments and modelling
- 2023
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In: Corrosion Science. - : Elsevier BV. - 0010-938X .- 1879-0496. ; 217, s. 111140-
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Journal article (peer-reviewed)abstract
- Various mechanisms have been proposed for hydrogen embrittlement of duplex stainless steel, but the causation of hydrogen-induced material degradation has remained unclear. This work shows that phase instability (decomposition) of the austenite phase and ductile-to-brittle transition of the ferrite phase precedes hydrogen embrittlement. In-situ diffraction measurements revealed that Ni-rich sites of the austenite phase decompose into metastable hydrides. Hydride formation is possible by increasing the hydrogen chemical potential during electrochemical charging and low defect formation energy of hydrogen interstitials. Our findings demonstrate that hydrogen embrittlement can only be understood if measured in situ and in real-time during the embrittlement process.
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| 4. |
- Örnek, Cem, et al.
(author)
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Time-resolved grazing-incidence X-ray diffraction measurement to understand the effect of hydrogen on surface strain development in super duplex stainless steel
- 2020
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In: Scripta Materialia. - : Elsevier BV. - 1359-6462 .- 1872-8456. ; 187, s. 63-67
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Journal article (peer-reviewed)abstract
- Grazing-incidence x-ray diffraction was employed to measure, operando, during electrochemical hydrogen charging, the lattice strain development of the near-surface in super duplex stainless steel under applied tensile load. Hydrogen absorption led to the formation of tensile strains in both the austenite (gamma) and ferrite (delta) phases perpendicular to the loading axis, whereas compressive strains were formed in the ferrite phase parallel to the loading direction, despite the acting tensile load. The earliest stages of degradation are discussed in light of understanding hydrogen embrittlement.
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