On the regularities of electrolytic hydrogenation of ARMCO iron

The results of electrolytic hydrogenation of 5 zones of vertically mounted 30x20x2 mm ARMCO-iron samples, differing in different locations on the central vertical of the sample (from bottom to top through 5 mm), in 5% H2SO4 solution with the addition of a 1.5 g/l CS(NH2)2 promoter at a cathode current density of 10; 17.5 and 25 mA/cm2 and the time of hydrogenation t is up to 35 minutes. It was found that, regardless of the measurement zone and current density, the change in the hardness of HV samples over time has the same form: an increase within 7.5 – 10 minutes from the initial (HV0 ~ 1,475 MPa) to the maximum value (HVmax up to 2,090 MPa), and then a decrease, with a significant difference between the values of ΔHVmaх = HVmax - HV0 for the lowest zone 1 (ΔHVmax = 615 MPa) and the highest 5 (ΔHVmax = 372 MPa). Zone 1 of the sample surface corresponds to the maximum number of “bubbles” (“blisters”), on some of which dark lines – cracks are recorded. The type of the established dependence ΔHV = f(t) is caused by the combined effect of two factors: distortion of the crystal lattice due to the introduction of hydrogen atoms into it and its compression by the pressure of molecular hydrogen located in micro-voids. Micro-voids are located in the surface layer of metal, When the pressure of hydrogen reaches a critical value in the “blisters”, they crack with the release of hydrogen into the atmosphere. This process is accompanied by a decrease in the compressive forces exerted by the blisters on the crystal lattice, with a decrease in its deformation and, as a consequence, the recorded hardness. The value of the maximum increment of hardness ΔHVmax in each zone of the sample depends on the number of “blisters” formed in this zone, and for this reason, decreases as the measurement zone rises upwards. A slight difference in the values of ΔHVmax of the samples after the completion of hydrogenation and after 30 days of exposure to air, sufficient for desorption of dissolved hydrogen from the metal, indicates that molecular hydrogen makes the main contribution to the distortion of the crystal lattice and the increase in the hardness of ARMCO-iron during hydrogenation. The studies were carried out with the involvement of the laboratory facilities of the Center for Collective Use of the Mining University.