Meaning
Rate constants governing the transport velocity of atomic hydrogen through a solid metal matrix establish kinetic thresholds for environmental degradation. Permeation rates depend directly on lattice structure, temperature, and localized defect density. Measurements of hydrogen diffusivity determine the incubation period required for subcritical crack growth under cathodic protection.
Application boundaries apply exclusively to atomic species mobile within the interstitial sites of the host metal.
Transport Kinetics
Interstitial jumps between adjacent octahedral and tetrahedral sites drive atomic transport through the crystal lattice. Trapping sites such as vacancies and inclusions temporarily delay atomic movement through the bulk volume. Electrochemical permeation tests measure hydrogen diffusivity across membrane specimens under controlled charging currents.
Assuming rapid diffusion based on room-temperature literature values leads to inadequate bake-out times during heat treatment cycles.
Temperature Dependence
Thermal energy increases interstitial jump frequencies according to Arrhenius relationships. Elevated temperatures accelerate degassing procedures for heavy forged sections. Holding times must account for thick cross-sections during stress relief operations.
Trap Saturation
High trap densities reduce effective diffusivity until interstitial sites become fully occupied under high fugacity. Microstructural modifications tailor trap populations to slow atomic mobility. Verification audits confirm low effective transport rates before field installation in sour service environments.
