Causes of the anomalous variation in the parameters of diffusion of hydrogen in iron at temperatures near 200°C

Conclusions

• 1. The anomalous behavior of hydrogen in technical iron at temperatures below 300°C is a result of a change in the character of diffusion of the grain-boundary component of the hydrogen flow in iron.

• 2. Even a short-term exposure of technical iron at elevated temperatures to hydrogen at a low pressure leads to the formation of a network of through submicroscopic cracks and pores permeable by inert gases.

• 3. Changes in the linear dimensions of cementite as a result of the magnetic transformation near 200°C lead to a large increase in the size of micropores at temperatures below 200°C.

• 4. The character of the diffusion of hydrogen along grain boundaries in technical iron varies with temperature. At temperatures below 200°C the grain-boundary component of the diffusion constitutes largely a viscous laminar flow of molecular hydrogen through large-diameter pores. At temperatures above 200°C the grain-boundary flow of hydrogen in iron constitutes thermally activated molecular flow of a single layer of partially dissociated hydrogen migrating along the walls of small-diameter submicroscopic pores. With rising temperature and increasing degree of dissociation of hydrogen the dependence of the grain-boundary flow on the gas pressure on the specimen inlet side approaches more and more closely the square root law.