THERMODYNAMICS OF SOLUBILITY AND PERMEATION OF HYDROGEN IN METALS AT HIGH TEMPERATURE AND LOW PRESSURE

The solubility and permeation of hydrogen in metals is analyzed on the basis of classical thermodynamics plus a minimum of kinetic theory. Expressions are derived for x, the number of H atoms absorbed per solid atom, and J, the permeation rate, in the limiting cases of continuum and free-molecule flow. For most conditions of temperature (T) and total pressure (pt=pH+pH 2)> the expressions reduce to the familiar equations for solubility and permeation, i.e., a curve for constant pt appears as a straight line on Arrhenius plots of logx vs 1/T and of logJ vs 1/T. However, for extreme conditions of high temperature and low pressure, the expressions predict that a curve of constant pt will become increasingly nonlinear as T increases, and it may even pass through a maximum when T becomes sufficiently high to produce almost complete dissociation of the gaseous H 2. Also, under certain conditions neither x nor J is proportional to the square root of the total pressure. The analysis provides a simple explanation of existing experimental data on the solubility and permeation of hydrogen in tungsten and in molybdenum at high temperature and low pressure.