Improving energy efficiency and corrosion resistance of PEO coating on Zr alloys through a two-frequency stepped strategy

Abstract

It was reported that high frequencies can improve energy efficiency and corrosion resistance of plasma electrolytic oxidation (PEO) coating by accelerating its growth rate and refining discharge pores, respectively. In this study, we observed that the application of ultra-high frequency (∼10⁴ Hz) to produce a PEO coating on Zr alloy resulted in a notable increase in its growth rate at a later stage (after about 470 V), yet was deleterious to its growth at an initial stage (before about 470 V). It is postulated that the former was attributed to an increased concentration and specific surface area of reactants, while the latter was caused by a decreased reaction temperature. To further increase the energy efficiency of PEO coating, a two-frequency stepped method was proposed, whereby a low frequency (∼10² Hz) and an ultra-high frequency were respectively employed at the initial and the later stages. For comparison, two PEO counterparts at low frequency were prepared through either prolonging oxidation time or increasing current density to achieve the same thickness as the two-frequency stepped PEO coating. Results reveal that the proposed approach enables up to 41.8 % and 44.1 % energy saving in comparison to the two regular counterparts. Additionally, the resulting coating exhibited an order of magnitude lower corrosion current density in NaCl solution and a higher passivation tendency in HCl solution than its counterparts owing to the presence of a continuous intermediate dense layer.