[Paper Review] Exploring Hydride Formation in Stainless Steel Revisits Theory of Hydrogen Embrittlement

Cem Örnek, Alfred Larsson+11 more|arXiv (Cornell University)|Sep 20, 2022

Hydrogen embrittlement and corrosion behaviors in metals4 citations

TL;DR

This study reveals that metastable hydrides form in stainless steel under electrochemical hydrogen charging, challenging the long-held view that stainless steel is non-hydride-forming. Using in-situ diffraction, the authors demonstrate that hydrogen embrittlement arises from phase instability due to high hydrogen chemical potential and low interstitial defect formation energy, necessitating real-time, in-situ observation to fully understand degradation mechanisms.

ABSTRACT

Various mechanisms have been proposed for hydrogen embrittlement, but the causation of hydrogen-induced material degradation has remained unclear. This work shows hydrogen embrittlement due to phase instability (decomposition). In-situ diffraction measurements revealed metastable hydrides formed in stainless steel, typically declared as a non-hydride forming material. 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-induced material degradation can only be understood if measured in situ and in real-time during the embrittlement process.

Motivation & Objective

• To investigate whether stainless steel, traditionally considered non-hydride-forming, can form hydrides under specific conditions.
• To understand the role of hydrogen chemical potential and interstitial defect formation energy in enabling hydride phase formation.
• To examine the dynamic evolution of hydrogen-induced phase transformations in real time during embrittlement.
• To reassess the theoretical basis of hydrogen embrittlement by identifying previously overlooked phase instability mechanisms.
• To establish the necessity of in-situ, real-time measurements for accurate characterization of hydrogen-induced degradation processes.

Proposed method

• Employed in-situ X-ray diffraction (XRD) to monitor structural changes in stainless steel during electrochemical hydrogen charging.
• Controlled hydrogen chemical potential via electrochemical charging to promote hydride nucleation and growth.
• Analyzed diffraction patterns to detect the formation of metastable hydride phases not previously identified in stainless steel.
• Calculated interstitial defect formation energy for hydrogen in the lattice to assess thermodynamic feasibility of hydride formation.
• Used real-time data acquisition to track phase evolution and correlate structural changes with embrittlement onset.
• Combined experimental observations with materials science principles to evaluate phase stability under high hydrogen activity.

Experimental results

Research questions

• RQ1 Can metastable hydrides form in stainless steel under electrochemical hydrogen charging, despite its classification as non-hydride-forming?
• RQ2 What role does hydrogen chemical potential play in enabling the formation of hydride phases in stainless steel?
• RQ3 How does the defect formation energy of hydrogen interstitials influence the stability and nucleation of hydride phases?
• RQ4 To what extent does phase instability contribute to hydrogen embrittlement in stainless steel?
• RQ5 Is in-situ, real-time observation essential for accurately capturing the mechanisms of hydrogen-induced degradation?

Key findings

• Metastable hydride phases were directly observed in stainless steel using in-situ XRD during electrochemical hydrogen charging.
• The formation of these hydrides is enabled by a high hydrogen chemical potential and low interstitial defect formation energy.
• The study challenges the conventional theory that stainless steel is immune to hydride formation, revealing a previously overlooked phase instability pathway.
• Hydrogen embrittlement in this context is driven by the decomposition of the solid solution into hydride phases, indicating a phase transformation mechanism.
• The results underscore the necessity of real-time, in-situ measurements to capture dynamic degradation processes that are missed in ex-situ analyses.
• The observed hydride formation occurs under conditions where thermodynamic stability is not the sole determinant, highlighting kinetic and defect-mediated pathways.