  In binary metallic systems like the Pd--Er alloys charged with hydrogen the
observed structure evolution exhibits complex dynamics. It is characterized by
non-monotonic time variations in an Er-rich fraction respect with an Er-poor
fraction observed experimentally. The present paper proposes a qualitative
model for this non-monotonic structure relaxation. We assume that the alloy
have crystalline defects which trap (or emit) an additional amount of Er atoms.
Hydrogen atoms into the alloy disturb the phase equilibrium as well as change
the defects capacity with respect to Er atoms. Both of these factors lead to
the spatial redistribution of Er atoms and cause the interface between the
Er-rich and the Er-poor phase to move. The competition of diffusion fluxes in
system is responsible for non-monotonic time variations, for example, in the
relative volume of the enriched phase. We have found the conditions when the
interface motion can change its direction several times during the system
relaxation to a new equilibrium state. From our point of view this effect is
the essence of the hydrogen induced non-monotonic relaxation observed in such
systems. The numerical simulation confirms the basic assumptions.
