In situ observations of silver-decoration evolution under hydrogen permeation: Effects of grain boundary misorientation on hydrogen flux in pure iron

Abstract

An in situ silver decoration technique using a light microscope was developed. Hydrogen was introduced from the bottom surface of an annealed pure iron specimen, and its top surface was covered with a chemical solution containing silver ions. Samples were exposed for sufficient time for hydrogen permeation to occur from the back to the top surface. The resultant in situ silver decoration visualized that the hydrogen flux through low-angle grain boundaries is lower than that observed at high-angle grain boundaries. The in situ silver decoration technique can be used for kinetic analysis of solute hydrogen in ferritic iron and steels.

Hydrogen Grain Boundary Segregation and Migration in the Alpha-Iron

“…Therefore, the delay in hydrogen visualization observed in [11] can be interpreted as an indirect evidence for retarded hydrogen diffusion due to grain boundaries.…”

“…First, it is worth noting that the calculations of the expected hydrogen permeation in their experiment were performed by the authors of [11] for the bulk migration of hydrogen atoms, which ignores the existence of grain boundaries. Therefore, the delay in hydrogen visualization observed in [11] can be interpreted as an indirect evidence for retarded hydrogen diffusion due to grain boundaries.…”

“…Second, in their statements concerning hydrogen diffusivity at grain boundaries and in the grain interior, the authors [11] proceed from the data by Hagi et al [16] where the bulk diffusivity of hydrogen in the pure iron is claimed to be independent of the existence of grain boundaries at ambient temperature.…”

“…Recently, Koyama et al [11] have studied hydrogen location throughout the surface of a hydrogen-charged polycrystalline iron using a technique of silver decoration earlier developed in [12][13][14][15][16]. In contrast to previous studies, silver decoration was used directly in the course of hydrogen permeation.…”

“…In contrast to previous studies, silver decoration was used directly in the course of hydrogen permeation. Hydrogen visualization was needed to correlate the cracking events with presence of local hydrogen because, as it is claimed by the authors [11], 'the factors causing the cracking are preferential diffusion and the segregation of hydrogen on or in the vicinity of the grain boundaries'.…”