The effect of the chemical composition of the substrate and Na2WO4 additive on the microstructure and corrosion behavior of PEO coatings of aluminum-silicon alloys

Subject Areas : journal of New Materials

Masoud Moshrefifar ¹

1 - Assistant prof. Department of Mining and Metallurgical Engineering, Yazd University, 8915818411, Yazd, Iran

Received: 2024-10-30 Accepted : 2025-01-13 Published : 2024-05-04

Keywords: Aluminum-Silicon alloy, Coating, Plasma electrolytic oxidation, Cor-rosion resistance,

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: The results indicate that the main phase of the coatings is γ-Al₂O₃, and by adding sodium tungstate dihydrate, the constituent components of the electrolyte are introduced into the coating, and tungsten is one of the phases forming the coating along with Al₂O₃. Examining the coated specimens showed a pancake structure and a volcanic crater with irregular micro-cracks and micro-porosity. The participation of tungsten in the coating caused an increase in the thickness of the outer layer, so the increase in the thickness of the coating due to the participation of tungsten increases the corrosion resistance of the outer layer compared to the equivalent samples without additives. 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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