C-Mn (L485MB or X70 steel: 0.09%C, 1.61%Mn) and Cr-Mo (0.32%C, 0.99% Cr, 0.91%Mo) steels, widely used in natural gas pipelines, are being considered for hydrogen transport and storage (C-Mn) or storage (Cr-Mo), but can be susceptible to hydrogen embrittlement (HE). A key factor in mitigating HE is the oxide layer formed on the steel surface, which can act as a barrier to hydrogen ingress [1-3]. The long-term durability of these facilities depends on the stability over time of this oxide layer under hydrogen exposure.
To better understand hydrogen-oxide-steel interactions, it is crucial to identify the rate-limiting step for hydrogen entry to infer its distribution in the steel over usage duration. The present work focuses on determining the rate-limiting step using deuterium tracer experiments on the Cr-Mo steel. The following stages will be considered: adsorption/absorption on/by the oxide layer, diffusion through the different oxide layers, interfaces crossing, and absorption into the metal.
As the native oxides present on the freshly-prepared steels are very thin (few nm), diffusion-driven studies are not possible. Therefore work has been done on laboratory-grown oxides with controlled composition and thickness to enable analysis of the deuterium distribution in the {oxide + metal} system. The characterization of these oxide layers was done by various techniques such as XPS for native oxides, SEM, EDX, and RAMAN analysis for the thermally grown oxides, to assess their microstructure and compositions.
The specimens were then exposed to deuterium gas under different temperatures (from room temperature to 200 °C) and durations in a dedicated set-up at CEA. TDS was used to quantify the amount of deuterium that was absorbed during the exposure phase, while the deuterium distribution in the oxide layer was characterized by GDOES and SIMS depth profiling, as well as SKPFM. Based on these results, attempts to identify the rate limiting step for hydrogen entry into the system under the different investigated conditions will be discussed.
[1] Y. Ishikawa et al., Vacuum 47 (1996) pp. 701–704
[2] M. Wetegrove et al., Hydrogen 4 (2023) pp. 307–322
[3] Y. Hatano, in Tritium: Fuel of Fusion Reactors, T. Tanabe (ed.), pp. 207-229, ISBN 978-4-431-56460-7 (eBook)
This work has been funded by the French Government “France 2030” in the framework of the PEPR-H2 ‘HYPERSTOCK’ grant.
Steel, Oxide layer, Hydrogen, Barrier effect, Rate limiting step, Oxide Degradation