Effects of a Carbon Nanotube Additive on the Corrosion-Resistance and Heat-Dissipation Properties of Plasma Electrolytic Oxidation on AZ31 Magnesium Alloy

Abstract

Plasma electrolytic oxidation (PEO) coating was obtained on AZ31 Mg alloy using a direct current in a sodium silicate-based electrolyte with and without a carbon nanotube (CNT) additive. The surface morphology and phase composition of the PEO coatings were investigated through field emission scanning electron microscopy (SEM), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS). The corrosion-resistance properties of the PEO coatings were evaluated using potentiodynamic polarization measurements and electrochemical impedance spectroscopy (EIS) in a 3.5 wt.% NaCl solution. Furthermore, the heat-dissipation property was evaluated by a heat-flux measurement setup using a modified steady-state method and Fourier transform infrared spectroscopy (FT-IR). The results demonstrate that, by increasing the concentration of CNT additive in the electrolyte, the micropores and cracks of the PEO coatings are greatly decreased. In addition, the anticorrosion performance of the PEO coatings that incorporated CNT for the protection of the Mg substrate was improved. Finally, the coating’s heat-dissipation property was improved by the incorporation of CNT with high thermal conductivity and high thermal emissivity.

Plasma Electrolytic Oxidation (PEO) Process—Processing, Properties, and Applications

“… Potentiodynamic polarization curves of PEO coatings developed. Reproduced with permission from [ 129 ]. Copyright MDPI, 2018.…”

“… Heat flux of AZ31 Mg substrate and PEO coatings with various CNT concentration. Adapted from [ 129 ]. …”

“…Moreover, the K 2 ZrF 6 additives showed better corrosion resistance of the coating compared to other additives. Hwang et al [ 129 ] investigated the effects of carbon nanotubes (CNT) additives (0, 2.5, 5, 10 g/L CNT) in 2 g/L KOH, 2 g/L KF, and 6 g/L Na 2 SiO 3 electrolytes on AZ31 Mg alloy subjected to the PEO process [ 129 ]. The authors found that with increasing concentration of CNT, there was a gradual increase in the conductivity value of the samples.…”

Chromate-Free Corrosion Protection Strategies for Magnesium Alloys—A Review: Part II—PEO and Anodizing

“…For instance, graphene oxide (GO) was successfully introduced into a PEO coating [ 242 , 243 , 244 ], reducing the number of micropores and improving the corrosion resistance due to the increased tortuosity of the electrolyte species diffusion pathway. Similarly, the addition of graphite [ 237 , 245 , 246 ], multi-walled carbon nanotubes (CNT) [ 247 , 248 ], or carbon spheres (CS) [ 249 ] into the electrolyte produced a coating densifying effect, thereby increasing the corrosion resistance while the CNT oxidized during PEO (new). However, the primary benefits of the incorporation of carbon-based particles are enhanced hardness, wear resistance, and heat dissipation [ 247 , 248 , 249 ].…”

“…Similarly, the addition of graphite [ 237 , 245 , 246 ], multi-walled carbon nanotubes (CNT) [ 247 , 248 ], or carbon spheres (CS) [ 249 ] into the electrolyte produced a coating densifying effect, thereby increasing the corrosion resistance while the CNT oxidized during PEO (new). However, the primary benefits of the incorporation of carbon-based particles are enhanced hardness, wear resistance, and heat dissipation [ 247 , 248 , 249 ]. Another innovative idea is the in situ incorporation of nanocontainers loaded with corrosion inhibitors (e.g., halloysite or aluminosilicate nanotubes loaded with benzotriazole, molybdate, or vanadate salts or 8-hydroxyquinoline) [ 250 , 251 , 252 ].…”