Hydrogen can be dissolved and diffused into steel/metal only when it is in atom form (H) and interact with steel surface is complicated. Some elements or impurities on steel/metal surface may stop or delay the absorption of H into steel surface. There are some studies indicated hydrogen activity on metal surface is strongly affected by inhibitor and promoters in the electrolyte and by the present of alloying elements and impurities in metal surface at/around room temperature (below 100 C). However there is less work on the influence of alloying elements and impurities in steel/metal surface on the absorption of hydrogen in steel (metal) at high temperature (above 750 C) . Would you please provide more information if your research is in this area.
As, diffusion is directly related to the temperature.....
As H atom is the smallest spices in the atomic level ....due to its small size it can be easily diffuses through the grain boundaries " in case of crystalline materials"...... now it can diffuse in to the metal but not going to stay there.... until or unless it can make some hydrides "like TiH"..... at the high temperature these hydrides were dissolve and hydrogen diffuses outside from the metal surface..... so if the surface having the hydride forming ability then it can absorb more hydrogen ... which can be diffuses at the high temperature from the metal surface.... this kind of behaviour used for the "Hydrogen storage material".....to get energy in space or in remote area with little heatting....
I have some experiences with diffusible hydrogen in welds, the literature says that the Hydrogen atoms that enters in steel, progress into steel depending of the temperature principally, phenomenons like corrosion are medias to add H, sobresaturated H atmospheres, coatings, etc; is reported too that the microstructure of the surface of steel (Fe alpha, Fe gamma) presents differences in H absortion (Fe gamma more than Fe alpha, obviously depending of the temperature) and some alloy atoms (prefereably elements with a high atomic radius) have certain influence or minus resistance to permit the pass the hydrogen into the steel.
At grain boundaries containing certain dopant or impurities water splitting can catalytically occurs to produce hydrogen.. This hydrogen may accumulate at voids. This hydrogen may also be oxidized to produce hydrogen ions due to the low hydrogen over-voltage of dopant or impurities at the surface of metal or within the grain boundaries.Hydrogen gas can also diffuse through the grain boundaries and react with some impurities to produce hydride which are gases at even standard conditions. There are solid hydrides and there are gases hydrides.
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