DIFFUSION-COEFFICIENT OF HYDROGEN IN IRON FREE FROM TRAPPING BY DISLOCATIONS AND IMPURITIES

In order to obtain the diffusion coefficients of hydrogen in iron free from dislocation and impurity trapping, electrochemical permeation me asurements have been done using high purity iron (99.99 mass%Fe), pure iron (99.99 mass%Fe, high substitutional and low interstitial impurit y contents), pure iron (99.94 mass%Fe), and commercially pure iron (99 .83 mass%Fe) between 278 and 318 K.Though these specimens, in which di slocation densities are (0.6-4.4) x 10(11) m-2, have been annealed at high temperatures (973-1223 K), the diffusion coefficients measured (D (a)) are influenced by the trapping effect of dislocations.Therefore, the diffusion coefficients free from dislocation trapping (D(df)), and free from dislocation and impurity trapping (D(H)) have been calculat ed from D(a) on the basis of the trapping theory. In the iron specimen s of 99.99 mass% purity, the values of D(H) are nearly equal to those of D(df). This indicates that D(a) obtained in the 99.99 mass% purity iron specimens is little influenced by the impurity atoms. The tempera ture dependence of D(H) has been determined as follows: D(H)(m2/s)=(5. 8 +/- 0.1) x 10(-8) exp (-(4.5 +/- 0.2) (kJ/mol)/RT).