Impact of hydrogen incorporation on electronic and magnetic structure of X2CrNi18-9 stainless steel

Hydrogen absorption significantly alters the mechanical properties of steel. However, absorbed hydrogen also influences its electronic and magneto-structural properties, helping to interpret how hydrogen is incorporated. This study therefore investigates the influence of hydrogen incorporation on the electronic and magneto-structural properties of X2CrNi18-9 stainless steel in different microstructural states. Microstructural characterization included analytic electron microscopy mapping, X-ray diffraction and thermodynamic stability maps to evaluate grain size, dislocation density and chemical homogeneity. The electronic properties were characterized using the Seebeck coefficient, while the magneto-structural properties were investigated using diffuse neutron scattering and small-angle neutron scattering (SANS). Hydrogen incorporation showed clear changes in the Seebeck coefficients. Magnetic SANS in conjunction with diffuse neutron scattering indicates the existence of nanoscale inhomogeneities with the same fcc structure as the bulk, but with correlation lengths of a few nanometres. The size of these inhomogeneities increased with hydrogen incorporation, suggesting that hydrogen preferentially accumulates in their vicinity. However, no direct correlation between the electronic and magneto-structural properties and the dislocation density could be demonstrated. We suggest that studies such as these will lead in the medium term to the development of guidelines for material design to make steels more resistant to hydrogen.

💡 Research Summary

This paper investigates how hydrogen incorporation influences the electronic and magnetic‑structural properties of X2CrNi18‑9 (AISI 304L) stainless steel in three distinct microstructural states. The authors prepared three sets of samples with identical bulk chemistry but different processing routes: (1) conventional casting, forging and solution annealing (CON‑SA); (2) laser powder‑bed fusion followed by solution annealing (PBF‑SA); and (3) laser powder‑bed fusion in the as‑built condition without further heat treatment (PBF‑AB). Detailed microstructural characterization was performed using back‑scattered electron imaging, X‑ray diffraction, energy‑dispersive spectroscopy (EDS) mapping, and thermodynamic ΔG stability maps derived from Thermo‑Calc. Grain size varied from ~28 µm (CON‑SA) to ~9–10 µm (PBF‑SA, PBF‑AB). Chemical homogeneity, quantified by the standard deviation of ΔG, was highest for CON‑SA (≈141 J mol⁻¹) and markedly lower for the PBF samples (≈88 J mol⁻¹), indicating that additive manufacturing yields a more uniform alloy at the mesoscale. Dislocation densities, extracted from X‑ray line broadening, were 3.04 × 10¹³ m⁻² (CON‑SA), 2.97 × 10¹³ m⁻² (PBF‑SA) and 3.88 × 10¹³ m⁻² (PBF‑AB), with the as‑built PBF material showing the highest density due to rapid solidification stresses.

Hydrogen charging was carried out at 300 °C under 10 bar H₂, achieving ~11 wt ppm hydrogen in all specimens. The Seebeck coefficient was measured from 300 K to 345 K. All samples displayed negative Seebeck values, confirming electrons as the dominant charge carriers. Hydrogen incorporation consistently reduced the absolute magnitude of the Seebeck coefficient, indicating an increase in conduction‑electron concentration and a more metallic character. Notably, the PBF‑SA and PBF‑AB specimens exhibited distinct Seebeck values even before hydrogen loading, which the authors attribute to their superior chemical homogeneity compared with CON‑SA. No systematic correlation was found between Seebeck changes and dislocation density.

Magnetic and structural information was obtained via diffuse neutron scattering and magnetic small‑angle neutron scattering (SANS). Diffuse scattering confirmed that the overall face‑centered cubic (fcc) lattice is retained after hydrogen charging. SANS revealed nanoscale inhomogeneities—still fcc in nature—with correlation lengths of a few nanometres. Importantly, the size of these inhomogeneities grew with increasing hydrogen content, suggesting that hydrogen preferentially accumulates near existing chemical or defect‑related heterogeneities. However, the magnitude of these magnetic nanostructures did not correlate with the measured dislocation densities, implying that dislocations are not the primary hydrogen trapping sites in these steels.

The authors conclude that hydrogen subtly modifies the electronic density of states near the Fermi level (as sensed by the Seebeck coefficient) and induces nanoscale magnetic inhomogeneities, but these effects are governed more by chemical uniformity and existing mesoscale heterogeneities than by dislocation density. Consequently, strategies to improve hydrogen‑embrittlement resistance should focus on enhancing chemical homogeneity and controlling nanoscale segregation during processing, rather than solely reducing dislocation density. The study provides a framework for linking hydrogen‑induced electronic and magnetic signatures to microstructural features, paving the way for rational design of hydrogen‑resistant steels.