مواد نوین (Oct 2023)
Investigating the effect of silicon content on structure and corrosion resistance by plasma electrolytic oxidation on aluminum-silicon alloys
Abstract
Abstract Introduction: Plasma electrolytic oxidation is a new and upgraded method of anodizing process to improve the corrosion resistance of aluminum alloys by creating a ceramic coating on their surface. Methods: One of the parameters affecting of PEO process as well as the performance of the prepared coatings is the composition of the substrate. In this study, the effect of increasing the percentage silicon of substrate on the plasma electrolytic oxidation process with bipolar pulse current in a silicate-based electrolytic bath was investigated. Scanning electron microscopy was used to evaluate the morphology and structure of the coating and X-ray diffraction test was used for phase detection. Coating corrosion behavior was evaluated by electrochemical tests after 1 hour immersion in 3.5% NaCl solution with the adjustment of pH 4. Findings: coatings had a pancake structure with craters with irregular micro-cracks and micro-porosity. Investigations showed that with an increase in the percentage of silicon in the substrate, the thickness and porosity of the coatings decreased, and the volcanic morphology prevailed over the pancake morphology in the growth of the coating. Analysis showed the coatings mainly contain a mixture of γ- Al2O3, η- Al2O3, δ-Al2O3, SiO2, a small amount of mullite and some amorphous phases. Tofel polarization test revealed, in addition to reducing the corrosion current density up to 9% of the substrate silicon, we have seen an increase in the polarization resistance with the increase in the silicon percentage of the substrate after coating. The electrochemical spectroscopy test revealed that with the increase in the silicon percentage of the substrate, the coating forms a physical barrier against charge transfer substrate and the resistance of the outer layer decreases, but the resistance of the inner layer increases.
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