Hydrogen in α-iron: role of phonons in the diffusion of interstitials at high temperature

Abstract

Ab-initio Density Functional Theory has been used to compute phonons for interstitial hydrogen in α -iron. In the harmonic approximation, these phonons yield Helmholtz’s free energy as a function of temperature, which can be used to obtain diffusion barriers from an Arrhenius plot. By comparing with the experimental database compiled by Kiuchi and McLellan, we show that the role of phonons is crucial to understand the diffusion of interstitial hydrogen at T > 300 K. The computed specific heat for Fe 16 H and Fe behaves quite differently due to the appearance of optical modes and could be used to calibrate the amount of interstitials in the iron matrix.

Diffusion and trapping of hydrogen in carbon steel at different temperatures

“…For the calculation of phonons, the internal electronic total energy has been converged further to 10 − 10 eV. Standard statistical physics yields the entropic contribution of lattice vibrations to Helmholtz free energy which, in turn, leads to a simplified expression for the diffusion coefficient as a function of temperature, D (T) [17].…”

“…A highfrequency mode associated with the octahedral position has been found to cause a significant correction to the zero-point energy difference between both configurations. The calculation of Helmholtz free energies for both sites including the effect of phonons yields a variation with the temperature of diffusion barriers due to the entropic contribution (the electronic contribution is considered separately) [17]. The tetrahedral site gains stability compared to the octahedral one as temperature increases.…”

“…Accordingly, the diffusion coefficient for Hydrogen in the iron lattice can be obtained from the following equation [17]:…”

Comportamiento frágil de las armaduras corroídas

“…La corrosión de las armaduras puede producir diversos tipos de deterioros en las estructuras, como por ejemplo la pérdida de adherencia de las armaduras [1,2]. Desde el punto de vista estructural una de las consecuencias de la corrosión es el posible comportamiento frágil de las armaduras [3][4][5][6][7][8] que puede producir fallos repentinos en componentes. Existen diversos mecanismos de corrosión que pueden tener lugar en las estructuras de hormigón armado, dando lugar a diferentes tipos de corrosión: corrosión homogénea, corrosión por picaduras, corrosión bajo tensión, etc.…”

“…Por ejemplo, la aparición de un defecto puede provocar un fallo frágil por concentración de tensiones de forma que a escala microscópica pueda tener un comportamiento dúctil, mientras que en otros casos se puede producir un comportamiento frágil a escala microscópica. Un ejemplo de este último es el fenómeno de fragilización por hidrógeno [3,[9][10][11][12].…”