The effect of sulfur segregated at grain boundaries on the behavior of hydrogen permeation through iron has been studied by using the gas phase charging and electrochemical detection method. The amount of sulfur at grain boundaries was controlled by varying the heat treatment and determined by Auger electron spectroscopy(AES)after in-situ fracture under the ultra-high vacuum condition. Input hydrogen pressure was varied by mixing with nitrogen. The values of time lag increased with increasing concentration of sulfur at grain boundaries, while decreased with increasing input hydrogen pressure. These results indicate that the segregated sulfur at grain boundaries were represented as saturable trap sites. The trap density and trap binding energy for hydrogen were obtained from($t_T/t_L$)-1vs. $P^{-1/2}_{H_2}$ plot. The trap density for annealed specimen and tempered specimen after the annealing treatment was found to be about $1.1\times10^{-7}mol/cm^3$ and $3.0\times10^{-7}mol/cm^3$, respectively. Also, the trap binding energy for both specimens was determined to be about -57kJ/mol. From these results it is suggested that the segregated sulfur at grain boundaries act as deep trap sites of hydrogen.
