Abstract The mechanism of impurity-induced discharges in plasma electrolysis (PE) was elucidated on the basis of a novel microstructural interpretation. We identified the presence of impurity compounds decorating the rim of the micro-pores, which were ‘fingerprint’ of plasma discharge activities. The formation mechanism of inorganic layer by PE was suggested based on the distribution of impurities and a transient diffusion pattern accompanying plasma discharges.