Cai et al. described the phase content and electrochemical performance of oxide layers processed in a phosphate and silicate electrolytes separately. Duan et al. [15] evaluated the formation mechanism of oxide layer processed in an alkaline silicate electrolyte. It has been already reported that the PEO coating is mainly composed of two layers such as inner dense layer and outer loose layer [16]. When metal cations diffuse to the coating/electrolyte interface, generally forms loose layer, while anions diffuse to metal/coating interface,forms dense layer [17]. Arrabal et al.