Absorption of hydrogen in tensile strained iron and high-carbon steel studied by electrochemical permeation and desorption techniques

Absorption of hydrogen in gradually tensile strained Armco iron and high-carbon steel, cathodically charged in 0.1 M NaOH solution, was studied using the electrochemical permeation and desorption techniques. Measurements of hydrogen permeation through specimens in the form of a membrane allowed determining the lattice diffusivity and concentration of hydrogen (diffusible hydrogen). The lattice diffusivity of hydrogen in iron (D = 6.2 × 10−5 cm2/s) was about 280 times higher than that in high-carbon steel (D = 2.2 × 10−7 cm2/s). In turn, a detailed analysis of the desorption rate of hydrogen from previously hydrogen charged and strained, cylindrical specimens made it possible to characterize hydrogen reversibly attached to traps. This trapped hydrogen made nearly a whole and a majority (from 70% to 85%, depending on strain) of the reversibly absorbed hydrogen in iron and high-carbon steel, respectively. In both studied materials, the amount of the trapped hydrogen strongly increased with strain. Moreover, in contrast to the diffusible hydrogen, evenly distributed in the charged specimen, the trapped hydrogen was mainly located within a subsurface region of the specimen. The estimated thickness of this subsurface region in iron was about 0.44 mm, whereas that in high-carbon steel was only about 0.017 mm. Consequently, the subsurface concentration of hydrogen in high-carbon steel was extremely high. It may be one of the reasons for more intensive hydrogen embrittlement of high-carbon (high-strength) steels in comparison with that of iron.