Hydrogen embrittlement is one of the critical barriers to the widespread adoption of hydrogen technologies, and a key aspect in understanding this phenomenon lies in accurately measuring hydrogen diffusivity in metals. In our recent study, on the relative efficacy of electropermeation and isothermal desorption approaches for measuring hydrogen diffusivity, we compared two established methods – electrochemical permeation (EP) and isothermal desorption spectroscopy (ITDS) – to assess their accuracy, repeatability and applicability across different materials.
ITDS measures hydrogen desorption from pre-charged samples under ultra-high vacuum (UHV) at constant temperature, with hydrogen diffusivity obtained by fitting finite element simulations to the desorption profile. In contrast, EP relies on electrochemical charging and diffusion across a membrane, but is prone to scatter due to variable surface conditions and unstable electrochemical boundary effects. Our experiments on cold-rolled pure iron revealed that ITDS provided much greater repeatability than EP, reducing data scatter that in EP can be up to five-fold depending on boundary conditions and diffusion model assumptions.
Figure 1. Dr. Alfredo Zafra (left) & Prof. Emilio Martinez-Pañeda (right).
Furthermore, ITDS can access higher temperature ranges than EP, which is limited by the boiling point of the electrolyte, enabling a more complete Arrhenius analysis and robust modelling of temperature-dependent diffusion. These advantages stem from the controlled environment of ITDS, which eliminates artefacts from surface interactions and ensures stable boundary conditions. However, achieving reliable ITDS results requires advanced instrumentation capable of sustaining UHV, minimising contamination and detecting hydrogen at extremely low levels.
high accuracy throughout the tests. These features allowed us to collect highly consistent datasets and extend ITDS measurements to a broad range of alloys, including nickel-based alloys, austenitic stainless steels and high-entropy alloys, where our results suggest that some prior electropermeation data may have been influenced by electrochemical artefacts.
Here, Hiden Analytical’s UHV-TDS system was indispensable. Its rapid pumping and stable vacuum design allowed accurate measurements on thin (<1 mm) specimens of fast-diffusing metals that had previously been regarded as incompatible with ITDS. The exceptional sensitivity of the mass spectrometer, capable of detecting hydrogen fluxes as low as 4.4 × 10⁻⁶ wppm/s, combined with precise temperature control through conduction heating (up to 80 °C/min), ensured minimal hydrogen loss and
Figure 3. Image credit to University of Oxford.
While EP retains certain advantages, such as enabling directional studies or coating assessments, ITDS has emerged as a more robust and repeatable method for measuring diffusivity across many structural metals, particularly those central to hydrogen infrastructure. Looking forward, we anticipate further improvements by integrating cryogenic pre-stages to prevent hydrogen loss before testing and combining the system with permeation modules to study directional diffusion with unprecedented sensitivity. These developments could make ITDS one of the most powerful and versatile tools for probing hydrogen–metal interactions, offering critical insights for the design of hydrogen-resistant materials and supporting the development of safe, durable components for the hydrogen economy.
Project Summary by: Alfredo Zafra, University of Oxford, Oxford, England.
Paper Reference: Zafra, A., Harris, Z., Evzen Korec and Martínez-Pañeda, E. (2022) ‘On the relative efficacy of electropermeation and isothermal desorption approaches for measuring hydrogen diffusivity.’ International Journal of Hydrogen Energy. Elsevier BV, 48(3) pp. 1218–1233. DOI: 10.1016/j.ijhydene.2022.10.025.
Hiden Product: TPD Workstation (now known as TDS Lab Series).
