A general model for partitioning of gases between a metal and its plasma environment

When a metal or an alloy is exposed to a pure diatomic gas such as nitrogen, the concentration of the species in the material can be predicted from Sieverts law. However, such calculations cannot be used to understand processes in which the gas phase near the metal contains excited molecules, atoms, ions, and electrons, in addition to the diatomic gases. The presence of plasma leads to species concentrations in the metal that are much higher than the values predicted by the Sieverts law. At present, there is no unified theoretical model to understand the partition of nitrogen, oxygen, and hydrogen between a metal and its plasma environment. In this work, a theoretical model has been developed to understand the enhanced solubility of gases in metals exposed to plasma environment. The model has been applied to explain several different sets of independent experimental results available in the literature. The analysis of the data shows that the enhanced solubilities can be explained on the basis of superequilibrium concentration of atomic species near the metal surface. Both the enhanced solubilities and the maximum species concentrations predicted by the model are in good agreement with the independent experimental data for various systems.