This study systematically investigates the hydrogen permeation behavior of palladium (Pd)-coated 316L stainless steel (SS316L) substrates with varying surface states. To align with reader expectations and provide a clear foundation for understanding the experimental design, characteristics of untreated (original) SS316L samples are first described. The untreated SS316L substrate naturally develops an oxide layer during fabrication and exposure to ambient conditions. Surface characterization reveals that this oxide film is approximately 25 nm thick, composed primarily of iron and chromium oxides. These intrinsic properties serve as a baseline for evaluating the impact of subsequent surface modifications. The original samples are retained in the experiment to simulate the actual working conditions of the material, providing valuable insights into its performance without additional surface treatments.
The research explores the influence of three distinct surface treatments—Ar plasma etching, mechanical polishing, and thermal oxidation—on the hydrogen permeation performance of Pd-coated SS316L. Untreated SS316L samples are retained as a reference to highlight the effects of these modifications. Surface treatments are specifically designed to alter the thickness, composition, and morphology of the oxide layer. Palladium films, approximately 2.5 μm thick, are deposited on all substrates using an unbalanced magnetron sputtering technique to ensure uniformity and comparable film quality.
Hydrogen permeation behavior is evaluated through gas-phase-driven permeation tests at temperature ranging from 400 to 550 ℃, under upstream hydrogen pressure of 100 kPa. Additionally, long-term stability tests are conducted at 400 ℃ to assess the durability of the Pd films over extended exposure to high-purity hydrogen. Surface and structural analyses of the substrates and Pd-coated samples are performed by X-ray diffraction (XRD), glow discharge optical emission spectroscopy (GDOES), scanning electron microscopy (SEM), and transmission electron microscopy (TEM).
The results demonstrate that the surface treatment significantly influences both the hydrogen permeation flux and the long-term stability of the Pd membranes. Untreated SS316L samples exhibit moderate hydrogen permeation flux due to the presence of their natural oxide layer, which acts as a diffusion barrier. In contrast, Ar plasma etching and mechanical polishing effectively remove the oxide layer, resulting in a smoother and more reactive surface for Pd deposition. These treated samples exhibit the highest hydrogen permeation flux, reaching 2.016 × 10-6 mol/(m2·s) at 550 ℃ for the Ar plasma-etched substrate. Mechanically polished substrates show similarly high flux values, demonstrating the effectiveness of oxide removal in enhancing hydrogen diffusion.
Thermally oxidized substrates, which exhibit thicker oxide layers (~50 nm), show the lowest hydrogen permeation flux (1.88×10-7 mol/(m2·s) at 550 ℃). The presence of this thicker oxide layer hinders hydrogen atom diffusion and acts as a barrier to permeation. Long-term stability tests reveal that untreated and thermally oxidized substrates suffer from significant degradation phenomena, such as surface bubble formation and delamination, during prolonged exposure to hydrogen. Thermally oxidized samples fail within 10 hours of testing due to extensive bubble formation and flux decline. Conversely, samples treated with Ar plasma etching and mechanical polishing maintain superior hydrogen permeation stability over 30 hours, exhibiting negligible flux reduction and no evidence of structural failure.
In conclusion, the original sample's characteristics provide essential contexts for understanding the modifications' impact. The findings underscore the critical role of surface preparation in optimizing the hydrogen permeation performance and stability of Pd membranes. By reducing surface oxidation, treatments such as Ar plasma etching and mechanical polishing significantly enhance membrane performance, making them become promising techniques for improving Pd membranes' efficiency and durability in hydrogen separation and space nuclear power systems.
Key words
palladium film /
316L stainless steel /
Ar ion etching /
thermal oxidation treatment /
hydrogen permeation behavior /
surface structure
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Funding
Central Guided Local Science and Technology Development Funds (24ZYQA042); National Natural Science Foundation of China (12305226, 11875305); Gansu Provincial Basic Research Innovation Group Project (20JR10RA062); Gansu Provincial Major Science and Technology Special Project (22ZD6GA002); Shandong Provincial Laboratory of Advanced Materials and Green Manufacturing Scientific Research Funding Project (SYS202203)
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