Development Of Rare Earth Based Conversion Coating With Improved Corrosion Resistance Of High Strength Aluminium Alloys

Date

2026-4

Author

Betin, Tilbe

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Aluminium alloys are widely used in industry due to their low density and high mechanical strength achieved through precipitation hardening. However, the heterogeneous microstructure formed by alloying elements increases susceptibility to localized corrosion. Although hexavalent chromium-based conversion coatings provide effective corrosion protection, their toxicity and increasingly strict environmental regulations have encouraged the development of environmentally friendly alternatives. In this thesis, rare-earth-based conversion coatings are investigated as environmentally friendly solutions, focusing on immersion-type lanthanum-based coatings applied to high-strength aluminum alloys. The study is conducted on AA6061 and AA7075, with bare substrates evaluated for comparison. The effects of coating parameters are investigated by varying bath pH values (2, 3.8, and 5) and temperatures (25 °C and 45 °C). The formed surface films are characterized using Scanning Electron Microscopy (SEM), X-ray Diffraction (XRD), Energy Dispersive Spectroscopy (EDS), and elemental mapping to evaluate morphology, phase composition, and elemental distribution. Corrosion resistance is assessed using Tafel polarization and Electrochemical Impedance Spectroscopy (EIS). In addition, coating thickness, Scribed Salt Spray Testing, and Pull-Off test are conducted to evaluate coating performance and adhesion properties. The results demonstrate that bath condition parameters significantly influence electrochemical performance and adhesion behavior. Among all conditions, the best corrosion resistance is obtained at a bath pH of 3.8, particularly at 45 °C, with improved performance also observed at 25 °C. However, adhesion performance remained comparable to the bare substrate. Overall, the electrochemical results indicate that lanthanum-based coatings show promising potential as environmentally friendly alternatives for high-strength aluminum alloys and represent a promising direction for future studies.

Subject Keywords

Corrosion, Conversion Coating, Aluminum, Aluminum Alloys, Corrosion Resistance

URI

https://hdl.handle.net/11511/119140

Collections

Graduate School of Natural and Applied Sciences, Thesis