Abstract
	The presented work was focused on the evaluation of hydrogen diffusion coefficient in micro alloy X52 and X60 API steels (tubes) having different microstructures - banded ferrite and pearlite after hot rolling and the mixture of tempered  bainite and martensite after quenching and tempering. The specimens, which had the form of thin discs, were taken in two distinct orientations – perpendicular or parallel to the tube surface. 
	The hydrogen diffusion coefficient was evaluated by the electrochemical permeation method. The permeation tests were performed at ambient temperature in a permeation cell composed of two glass compartments separated by the specimen (working electrode). The hydrogen entry side of the cell was polarized galvanostatically at a constant current density in 0.05 M H2SO4. The hydrogen exit side of the cell was held at a constant potential of 0.28 VNHE in 0.1 M NaOH. The exit side of specimens was previously coated with a thin layer of Pd. The anodic current recorded at the hydrogen exit side was the measure of hydrogen permeation rate. 
	The calculated values of hydrogen diffusion coefficient varied from (0.3 to 10.2).10-6 cm2.s-1. Lower values were found for the specimens after quenching and tempering. The role of orientation was important only for specimens after hot rolling, where higher values of hydrogen diffusion coefficient were detected for the specimens perpendicular to the tube surface. These results could be correlated to the band orientation.