This study delves into the underlying causes of the atypical 'double rise' shape observed in hydrogen permeation rising transients on pure iron and low alloy steels. Electrochemical permeation experiments on pure iron reveal a fast initial rise, a short pseudo-plateau, and a slow second rise. Similar patterns emerge in the decaying transients. The micro-porosity present in material appears to act as reversible traps, affecting hydrogen diffusion. Surface damage, confirmed by SEM analysis, exacerbates the issue. Utilizing numerical simulations, an FEM model effectively replicates the 'double rise' behavior, attributed to limited recombination/dissociation kinetics at bulk-cavity interfaces. Overall, micro-porosity is identified as the primary factor behind this unique permeation curve shape.