Abstract
A concept of two steps current-decreasing mode derived from constant current mode was developed to fabricate micro-arc oxidation (MAO) coatings on ZK60 Magnesium (Mg) alloy in a dual electrolyte system. The growth characteristics of coatings were analyzed by voltage-time curves, and the coating microstructures were characterized by scanning electron microscopy (SEM). Meanwhile, the roughness, corrosion behavior and microhardness of MAO coatings were investigated. The results showed that the MAO coatings exhibit a smooth and compact surface and have improved hardness and thickness, smaller roughness, uneven distribution of holes and even low electrical energy consumption. Such positive characteristics results in an improved corrosion resistance of MAO coatings. The coating produced under the two steps current mode of “1.2-0.6A” showed a smaller corrosion rate of 0.1559 g/m2h as compared to the counterpart produced under the other modes. The nanoscratch tests results showed that the coating fabricated by “1.2-0.6A” mode has strong bond strength with the substrate. Under this optimized mode, the MAO process also had the lowest energy consumption of 49.8 W·(dm2·μm)-1.
Highlights
For their good mechanical and thermal properties, magnesium and its alloys have attracted more and more attention in many industrial fields, such as automobile, aerospace, and medical instruments (Pan et al, 2018; Ramalingam et al, 2019; Venkatraman and Swamiappan, 2020)
To examine the effect of process parameters on coatings produced under a constant current mode, the voltage output is measured during the micro-arc oxidation (MAO) process
The phase constituents of four coatings mainly consist of Mg2SiO4, Mg, Mg2Si, and MgO phases, which are in agreement with previously reported results processed in a silicate-containing electrolyte (Durdu et al, 2013)
Summary
For their good mechanical and thermal properties, magnesium and its alloys have attracted more and more attention in many industrial fields, such as automobile, aerospace, and medical instruments (Pan et al, 2018; Ramalingam et al, 2019; Venkatraman and Swamiappan, 2020). The application of Mg alloys is largely limited due to its poor corrosion resistance. In order to expand their applications, an available solution should be worked out to solve the problem of the alloy’s poor corrosion resistance (Goli and Aghajani, 2018; Ren et al, 2018; Yang et al, 2018; Pan et al, 2019, 2020; Kuang et al, 2020; Sun et al, 2020). In the past several decades, many surface modification technologies have been used on magnesium alloys, including anodic oxidation, ion implantation, electroless plating, techniques of chemical conversion coatings. Cai et al (2009) produced an in-depth study about anodic oxidation of magnesium including the corrosion mechanism of active dissolution, passivation and secondary oxidation of magnesium in 6 M KOH solution.
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