Thermodynamic aspects of the effect of small palladium additions on the solubility of hydrogen and flaking in steels

The danger of the formation of flakes has been considered from the position of the estimation of the maximum pressure of gaseous hydrogen in micropores of the alloy, whose magnitude is substantially affected by the solubility of hydrogen at P = 1 atm. The hydrogen solubility can be increased sharply by the introduction of additions of elements which strongly interact with hydrogen. In our article this effect is examined based on the example of additions of palladium to steel, which has for the first time been experimentally studied by Arkharov. A statistical model of a ternary alloy with strongly interacting substitutional and interstitial atoms has been developed. An equation of hydrogen solubility has been derived, which contains, besides the concentration of substitutional atoms, a single energy parameter ε equal to the difference in the energies of interaction of Pd-H and Fe-H pairs. A comparison of the theoretical expression for the solubility with the experimental data made it possible to determine that ε = -22.5 kJ/mol. A specific feature of hydrogen solubility in Fe-Pd alloys consists in the fact that the reduction in the solubility below 400 K, which is common for iron and steels, is changed by a solubility increase caused by the accumulation of hydrogen atoms near palladium atoms (acting as traps), which is the main factor responsible for an increase in the hydrogen solubility and a sharp decrease in the hydrogen pressure in micropores. © 2009 Pleiades Publishing, Ltd.