Effect of Process Parameters on the Corrosion Resistance Properties of PEO Coatings Produced on AZ31B Magnesium Alloy
Authors: Savguira Yuri , Ni Qing , Sobrinho Pedro H. , North Tom H. , Thorpe Steven J. 
Published: February 2020

The automotive sector is particularly interested in magnesium alloys, which can decrease the weight of the vehicle leading to improved fuel efficiency and decreased emissions. However, poor corrosion resistance, especially in solutions containing chlorides, is a major limitation for its widespread exploitation in exposed automotive applications. Plasma electrolytic oxidation (PEO) coatings have been shown as a promising technology to improve the corrosion resistance of magnesium alloys 1. The properties of PEO synthesized coatings have been shown to significantly dependent on the process parameters employed to produce them2,3. Understanding the influence of process parameters on the overall corrosion resistance of AZ31 is essential if PEO technology is to be used in an industrial application. The present investigation examines the effect of processing time, current density, and electrolyte temperature on structural morphology of PEO coatings made on AZ31B linked to their corrosion resistance.

PEO coatings were produced using a sodium silicate basic electrolyte using current densities ranging between 10 mA/cm2 and 20 mA/cm2. The temperature of the electrolyte was varied between 10-40ºC, while the processing time was varied between 15 and 30 minutes. The overall corrosion rate of PEO-coated samples was evaluated using mass loss testing and electrochemical impedance spectroscopy (EIS), while the composition and morphology of the PEO coatings were analyzed using a combination of x-ray diffraction (XRD), electron microscopy, and white light profilometry.

The phase composition of the synthesized PEO coatings was analysed using XRD, see Figure 1. Spectra indicated that the PEO coating comprised two main phases, namely magnesium oxide (MgO) and forsterite (Mg2SiO4). The ratio between magnesium oxide and magnesium silicate was estimated via the reference intensity ratio (RIR) analysis. The mass ratio (MgO/Mg2SiO4) for PEO coatings made on AZ31B decreased from 0.63 (10 mA/cm2) to 0.11 (20 mA/cm2). The observed increase in the weight fraction of forsterite when higher current densities were is related to polymerization of silicate ions during the deposition process. It has been previously reported that the extremely high energy generated by the plasma discharges promote polymerization of the silicate4. Increasingly favorable polymerization resulted in greater incorporation of silicates into the coating, ultimately leading to higher weight fraction of forsterite and lower weight fraction of magnesium oxide.

The corrosion rates of the two coated specimens (10 mA/cm2 and 20 mA/cm2) in addition to the bare metal AZ31 substrate were measured by 5-day mass loss testing in a 0.086M NaCl solution, see Figure 2. Both PEO coatings exhibited significantly improved corrosion resistance properties compared to as-received AZ31. The corrosion rates of coatings produced using an applied current density of 10 mA/cm2 were significantly lower than those of coating produced using a current density of 20 mA/cm2. The current research effort is focusing on providing explanations for observed differences in corrosion resistance properties of PEO produced when applying different current density values. The effect of changes in processing time and electrolyte temperature is also under investigation.

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Figure 1