Hydrogen Transport, Grain-Boundary Trapping, and Cohesive Degradation in Structural Steel A Thermodynamically Consistent Multiscale Framework

Hydrogen-assisted degradation in structural steels is governed by coupled processes across multiple length scales. Hydrogen enters the material, diffuses through interstitial lattice sites, interacts with microstructural traps, and may accumulate at grain boundaries and crack-tip regions. Mechanical loading can modify the hydrogen chemical potential and redistribute hydrogen toward regions of high hydrostatic tension. At sufficiently high local hydrogen coverage, interfacial cohesion and fracture resistance may decrease; however, the magnitude andcharacter of the response depend on boundary structure, alloy chemistry, trap populations, temperature, and loading history.This paper formulates a thermodynamically motivated framework for investigating hydrogen transport, grain-boundary segregation, trap occupancy, and cohesive degradation in structural steel. The framework combines a stress-dependent chemical potential, a lattice–trap mass balance, a McLean-type segregation relation, and a hydrogen-dependent traction–separation law. A proposed damage indicator is introduced to connect interfacial hydrogen coverage with cohesive energy loss. The proposed constitutive extensions are hypotheses for calibration and validation; they are not presented as universal laws or as experimentally established coefficients. The framework is intended to guide future studies using permeation experiments, thermal desorption spectroscopy, fracture testing,microscopy, and atomistic calculations.