Scanning Kelvin probe force microscopy study of the effect of thermal oxide layers on the hydrogen release - Experiments and finite element method modelling

Scanning Kelvin Probe Force Microscopy (SKPFM) was used to study the hydrogen diffusion through a surface designed for simultaneous mapping of multiple areas containing different thermal oxides, all covered with Pd. Potential maps were obtained simultaneously on an area of bare iron as the reference, an area covered with a bilayer oxide (inner magnetite and outer hematite) and an area covered with a magnetite layer (obtained by removing the outer hematite layer of a bilayer oxide). After hydrogen charging at the bottom side of the specimen, a contrast was obtained in the potential mapping on the covering Pd layer due to differences in hydrogen release through these distinct areas on the specimen surface. A finite element method (FEM) model of hydrogen diffusion across the different phases was developed to simulate the experiment. The modelling showed that both a lower diffusion coefficient and a lower solubility in the oxide can explain the contrast obtained in SKPFM. Cross diffusion in the ferritic bulk underneath the thermal oxide was found to have an influence on the spatial distribution of the hydrogen release.