The nucleation process of hydrogen blister in metals was investigated through experiments and the mechanism was discussed. Small hydrogen blister in charged Ni-P amorphous coating and steel was studied using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The thermodynamics and kinetics of hydrogen and vacancies in metals are analyzed. Further, an approach of the nucleation mechanism of hydrogen blister is proposed as follows. Atomic hydrogen can induce superabundant vacancies in metals. The superabundant vacancies and hydrogen aggregate into a hydrogen-vacancy cluster (small cavity). The hydrogen atoms in the hydrogen-vacancy cluster become hydrogen molecules that can stabilize the cluster. And the hydrogen blister nucleates. The pressure in the small cavity increases as the hydrogen atoms enter the cavity. The cluster, that is, the hydrogen blister nucleus, grows through vacancies diffusing into it under the action of cluster-hydrogen binding energy and hydrogen pressure. When the blister nucleus grows to a critical size C cr cracks will initiate from the wall of the cavity due to the internal hydrogen pressure.

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