Real-time mapping of hydrogen diffusion in steel

Hydrogen absorption in structural alloys can induce degradation through mechanisms that depend on its transport, distribution, and interaction with microstructure. Yet, these processes have been accessible only through indirect, bulk-averaged, or post-mortem measurements. Here, we present an operando high-energy X-ray diffraction microscopy approach that enables the quantification of hydrogen solubility and diffusivity from real-time lattice-strain evolution with micrometre-scale spatial resolution. The method, demonstrated on a double precipitation-hardened martensitic steel, provides spatiotemporal maps of hydrogen ingress across entire specimens, enabling simultaneous determination of local concentration and transport kinetics from a single experiment. Complementary electrochemical permeation measurements reveal analogous multi-stage transients, directly visualising the transition from lattice-fast to trap-controlled regimes and validating the diffraction-derived diffusivity hierarchy. The results show that hydrogen diffusivity is not a fixed material property but rather evolves during uptake. Precipitation-rich microstructures confine hydrogen near the surface, whereas precipitation-free martensite permits rapid penetration, establishing hydrogen transport as a state-dependent, time-variant process.