Possibilities of hydrogen atom trapping in steels by ferrite / cementite interfaces. 2. Adsorption theory

Ferrite / carbide interfaces in steels are known to be possible trapping sites for hydrogen atoms. The present work deals with hydrogen adsorption at ferrite / cementite interfaces in carbon steels. Experiments on hydrogenation of 1.0 pct C steel specimens having coarse and thin-plate lamellar pearlite structure (interlamellar spacing S 0,36 and 0,085 m) were carried out in H2S-containing solution according to NACE Standard TM 0177-2005. After hydrogenation the specimens were held in air for 4 months, and then hydrogen concentration was evaluated using LECO RH402 hydrogen analyzer. It was found to increase from 6.85 to 8.4 and from 8.45 to 13.5 wt. ppm (related to specimens not subjected to hydrogenising treatment) for the coarse and thin-plate pearlite specimens correspondingly. TEM study showed that hydrogenation changes the state of cementite lamellae in pearlite colonies: they look thicker due to visual blurring of interfaces, and a non-uniform contrast appears along them. These changes are to be ascribed to hydrogen pickup by the interphase interfaces.Hydrogen adsorption at ferrite / cementite interfaces is treated theoretically in terms of Langmuir theory. Accepting the estimate of trapping site density at interphase interfaces of 6.627·10 14 cm-2 made in the first part of the work from crystal geometry considerations and comparing it with experimentally observed amounts of absorbed hydrogen one can find that the fraction of occupied trapping sites is much less than unity (0.15-0.20). Binding energy of hydrogen at the interface is 12.66 kJ/mole according to existing data, so from experimental results the pre-exponential factor in the adsorption equation may be evaluated to 2.2·10 -4 / S. This means that effective solubility of hydrogen in disperse lamellar pearlite may increase 1.5-2 times. The same analysis can be applied to hydrogen trapping at different types of interfaces. It lies in the framework of Oriani’s approach and can be combined with it to evaluate the total amount of bound hydrogen in steel with the account of all kinds of traps.