Abstract
The diffusion couples consisting of δ1 specimens with an equilibrium concentration on the zinc-rich side and electrodeposited iron were annealed in the temperature range of 468 to 525°C. In the diffusion zone, δ1, Γ1 and Γ phases appeared, while α solid solution of zinc in iron could not be detected in any couple examined. The δ1 phase is composed of two layers with different microstructures; the same structure layer as the palisade δ1 in hot dip galvanizing and the same granular structure layer as the δ1 couple half in this experiment. The Kirkendall markers exist at the interface between the two δ1 phase layers, where the intrinsic diffusion coefficient of zinc is far greater than that of iron. This probably means that the granular δ1 phase layer is formed as a result of the one-sided diffusion of zinc atoms from the δ1 couple half and the palisade δ1 phase layer is formed by the reaction of zinc atoms with iron atoms in the iron couple half.
The interdiffusion coefficient of δ1 phase reaches a minimum value in the concentration range of 10.5 to 12.5 at%Fe where the concentration gradient in the penetration curve is very steep. The concentration gap between the compact and palisade δ1 phase layers formed in the hot dip galvanizing process ranges from 10 to 12 at%Fe, which approximately corresponds to the range of the steep concentration gradient mentioned above. The steep gradient has so far been presumably misunderstood as the concentration gap.
