Incorporation of multi-walled carbon nanotubes into the oxide layer on a 7075 Al alloy coated by plasma electrolytic oxidation: Coating structure and corrosion properties

The influence of incorporating multi-walled carbon nanotubes (MWCNTs) on the coating structure and the corrosion response of a 7075 Al alloy coated by plasma electrolytic oxidation (PEO) was investigated. For this purpose, a series of PEO coatings were prepared on 7075 alloy specimens in a silicate electrolyte with MWCNTs, and the results were compared to those without MWCNTs. Microstructure observations revealed that when MWCNTs were added to the electrolyte, the pore size and fraction of the oxide layer coated in the electrolyte incorporating MWCNTs were respectively smaller. Since the number of MWCNTs having negative potential within the present electrolyte moved into the oxide layer, which acts as an anode, some micro pores were filled effectively with MWCNTs, resulting in a denser coating layer. From the results of the electrochemical tests, the corrosion resistance of the present sample obtained by PEO coating in the electrolyte with MWCNTs was significantly improved. The electrochemical mechanism underlying the high corrosion resistance of the present sample was discussed based on the equivalent circuit model.