Join Emilio Martinez-Pañeda and Alfredo Zafra from the University of Oxford as they unlock the secrets of isothermal desorption spectroscopy with Hiden Analytical.
Could you briefly explain the principle of isothermal desorption spectroscopy and how it compares to other methods, such as electrochemical permeation, for measuring hydrogen diffusivity in materials?
Isothermal desorption spectroscopy (ITDS) measures hydrogen desorption from pre-charged materials under ultra-high vacuum (UHV) conditions at a constant temperature, tracking the hydrogen release rate to calculate diffusivity. The hydrogen diffusion coefficient is ultimately determined by fitting a simple numerical simulation of hydrogen transport to the experimentally measured ITDS profile.
Unlike electrochemical permeation (EP), which depends on electrochemical hydrogen generation and diffusion through a membrane, ITDS offers more precise control over boundary conditions, eliminating errors related to electrochemical surface effects.
Additionally, ITDS is well-suited for fast-diffusing materials where EP shows variability in hydrogen diffusion coefficients. In short, it is a much cleaner method than any other existing method used for measuring hydrogen diffusivity.
Your research involves the application of ITDS in studying hydrogen diffusion in cold-rolled pure iron. What specific insights has ITDS provided that were not achievable with traditional EP methods?
ITDS provided several critical insights that were not achievable with traditional EP methods, particularly in fast-diffusing materials like pure iron. First, ITDS minimized the variability caused by electrochemical boundary conditions, a common issue in EP experiments.
Our results suggest that the increased scatter in EP data is largely driven by variations in electrochemical boundary conditions due to surface conditions. Depending on the diffusion model used, this can cause up to a fivefold difference in the calculated diffusivity.
This inconsistency is further exacerbated by hydrogen trapping at microstructural defects, which introduces additional variability and distorts the accuracy of EP results.
By contrast, ITDS operates in a controlled UHV environment, eliminating the influence of surface interactions and electrochemical potentials, which are difficult to regulate in EP. As a result, ITDS offers a much clearer picture of hydrogen diffusion and provides more consistent repeatability across experiments.
ITDS has a distinct advantage when it comes to temperature. In EP, the achievable temperatures are limited by the boiling point of the electrolyte solution, typically capping at around 70–80 ºC, which restricts its usefulness for measuring diffusivity at higher temperatures.
In contrast, ITDS allows us to measure diffusivity at much higher temperatures without these constraints, offering deeper insights into how temperature influences hydrogen diffusion, where EP methods often struggle due to unstable boundary conditions and solution boiling.
Image Credit: University of Oxford
Your recent study highlights the improved repeatability of ITDS measurements compared to EP techniques. Could you elaborate on the factors that contribute to the enhanced accuracy and lower variability observed in ITDS?
ITDS’s superior accuracy and consistency stem from its controlled environment and refined analysis approach. Unlike EP, which is prone to variability due to surface effects, trapping phenomena, and unstable electrochemical boundary conditions, ITDS operates under UHV, effectively minimizing these issues.
A key advantage of ITDS is its more sensitive analysis method. It calculates hydrogen diffusivity based on the hydrogen desorption rate rather than relying on the concentration over time as EP does.
This approach allows ITDS to be more responsive to subtle changes in diffusivity, leading to improved accuracy. Moreover, the well-defined boundary conditions in ITDS, along with tight control over experimental variables such as specimen thickness and timing, further enhance its repeatability and reliability.
As a result, ITDS is significantly less prone to the scatter commonly observed in EP experiments, providing more consistent and precise hydrogen diffusivity measurements, especially in fast-diffusing materials like pure iron.
What were some of the key challenges you faced in conducting ITDS experiments, particularly in achieving UHV conditions, and how did Hiden Analytical’s equipment help overcome these challenges?
One key challenge in conducting ITDS experiments is achieving and maintaining UHV conditions, as even minimal external contamination can significantly alter results. Hiden Analytical’s UHV-TDS system played a vital role in overcoming this challenge by providing the stability necessary to maintain these conditions.
The advanced vacuum design allowed for rapid sample introduction, minimizing the time before desorption began and effectively reducing hydrogen egress.
Can you describe the role that Hiden Analytical’s technology played in your research? Specifically, how did their TPD Workstation* contribute to the success of your ITDS experiments?
*this system has now been upgraded to the TDSLab Series.
Historically, ITDS has been limited to thick samples of materials with very slow diffusivities. However, the capabilities of Hiden’s system have enabled us to successfully measure hydrogen diffusivity in very thin (<1 mm) samples of fast-diffusing metals like pure iron.
Its low pumping times and ability to quickly reach and sustain stable UHV for extended periods—sometimes even days—are key factors in achieving accurate results. Additionally, the exceptional sensitivity of Hiden’s mass spectrometer allowed for hydrogen detection with a resolution as low as 4.4x10-6 wppm/s, leading to remarkably consistent data.
The conduction-based heating system also precisely controlled heating rates of up to 70–80 ºC/min, ensuring minimal hydrogen loss during the heating phase.
In essence, Hiden Analytical’s technology was indispensable for our ITDS experiments, providing the accuracy, repeatability, and robustness needed for our hydrogen diffusivity measurements in fast-diffusing materials.
Image Credit: University of Oxford
How has the introduction of advanced ITDS techniques impacted your understanding of hydrogen diffusion in materials? Have there been any surprising findings that contradicted prior assumptions based on EP data?
The introduction of advanced ITDS techniques has significantly deepened our understanding of hydrogen diffusion in materials, particularly regarding temperature effects. Unlike EP, which is limited to around 70–80 ºC due to the boiling point of the electrolyte solution, ITDS enables us to measure diffusivity at much higher temperatures.
The TDS system’s rapid heating response allows us to quickly reach and maintain stable high isothermal temperatures for extended periods, facilitating accurate measurements across a wide range of temperatures.
This capability allows us to apply Arrhenius’ law for diffusivity, enabling data interpolation and extrapolation to determine diffusivity at any temperature. This approach provides valuable insights into hydrogen-material interactions and contributes to predictive simulations for hydrogen-assisted cracking.
Using this technique, we have successfully obtained diffusivity measurements for various slow-diffusing metals, including nickel-based alloys like Inconel 718, austenitic stainless steels like 316L, and high-entropy alloys.
Notably, our findings have challenged some existing literature on EP for these alloys, suggesting that certain data may be incorrect or influenced by surface effects or other artifacts. This highlights the importance of ITDS in providing a more accurate understanding of hydrogen diffusion in materials.
Your research also involved a comparative analysis of ITDS and EP methods. Could you share any key takeaways from this comparison, particularly regarding the applicability of these techniques to different materials and conditions?
In our comparative analysis of ITDS and EP methods for determining hydrogen diffusivity, several key takeaways emerged regarding their applicability to different materials and conditions. Our results confirm the utility of the ITDS approach, even for fast-diffusing materials like pure iron in thin sheet geometries (<1 mm), which has historically been seen as incompatible with ITDS.
Notably, both ITDS and EP experiments yielded similar average hydrogen diffusivities, highlighting the robustness of ITDS for this application.
However, we observed that the scatter in diffusivity measurements from EP experiments was consistently at least two times higher than that from ITDS, regardless of the analysis strategy used. This increased scatter in EP data is likely driven by variations in electrochemical boundary conditions due to surface conditions and hydrogen trapping effects.
Our comparison of different diffusion models for EP revealed significant discrepancies, with boundary conditions causing variations of up to five-fold in diffusivity. These findings underscore the importance of conducting EP experiments with potentiostatic approaches.
In contrast, ITDS provides more stable and reliable measurements, making it well-suited for assessing hydrogen diffusivity across a broad range of materials, including very slow diffusion alloys such as nickel-based alloys and high-entropy alloys.
Looking forward, how do you envision the application of ITDS in future research on hydrogen-metal interactions? Are there any specific materials or conditions where you see ITDS providing critical insights?
Looking forward, I see significant potential for leveraging ITDS in future research on hydrogen-metal interactions in multiple areas. For example, there is an opportunity to enhance the ITDS technique itself by incorporating a cryo stage. This addition would help prevent any hydrogen loss before initiating the ITDS test, thereby improving the accuracy and reliability of the measurements even more.
About the Speakers
Prof Emilio Martinez-Pañeda is an Associate Professor at the University of Oxford. Prior to joining Oxford, he was a Reader (Associate Professor) at Imperial College London, where he led an interdisciplinary research group from 2019 to 2023 (2019: Lecturer, 2021: Senior Lecturer, 2023: Reader). Prof Emilio Martinez-Paneda’s research spans a wide range of challenges lying at the interface between mechanics and other disciplines such as biology, geology, chemistry and materials science, being particularly known for his pioneering contributions to the area of environmentally assisted cracking.
Alfredo is a senior postdoctoral researcher on hydrogen embrittlement and environment-assisted cracking in the "Hydrogen embrittlement and SCC testing lab", at the Department of Engineering Science, University of Oxford. Before this, he was a research associate in the Department of Civil and Environmental Engineering at Imperial College London (2021-2023) and a graduate researcher at the University of Oviedo, where he obtained his PhD in Materials Science in 2021. He is currently dedicated to the development of a new generation of hydrogen-resistant metals using advanced manufacturing techniques, including additive manufacturing and nanofabrication.
This information has been sourced, reviewed, and adapted from materials provided by Hiden Analytical.
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