Effect of electrolyte composition on the formation of PEO coatings on AA2024 aluminium alloy

Since the electrolyte composition plays a crucial role in the plasma electrolytic oxidation (PEO) coating formation process, a systematic and in-depth study was proposed to identify an electrolyte composition for fast PEO coating growth on AA2024 alloy. Different concentration ratios of mixed alkaline electrolytes (hydroxide, silicate and phosphate) were investigated. PEO process was conducted at low constant current density of 50 mA/cm2, which is intended for lowering the energy consumption. Results demonstrated that the breakdown voltage of PEO coatings is directly proportional to the logarithm of electrolyte resistivity. The coating growth mechanism showed two main directions. The thickening of the coating mainly depends on the rapid deposition of electrolyte compounds in Si-based electrolyte, and the coating growth occurs mainly towards the electrolyte/coating interface. Contrary, in OH-, and P-based electrolytes, the inward coating growth was dominating mainly by substrate oxidation. A variety of phases as a function of different electrolyte compositions and final voltages were observed. With high final voltages (over 470 V) for coatings produced in mixed electrolytes with low concentrations of hydroxide, silicate or phosphate (2 and 6 g/L), gamma-Al2O3 crystalline phase predominates in the PEO layer composition. However, only a low efficiency of coating growth can be reached. In the electrolytes with high silicate concentrations accompanied by an increase of phosphate concentration, the final voltage is around 455 V and the coating composition is dominated by an amorphous phase in combination with crystalline mullite and gamma-Al2O3. High silicate-phosphate contents (18-24 g/L) in mixed electrolytes with low final voltages of about 360 V results in a fully amorphous PEO layer and significantly increases coating thickness. A combination of low content of hydroxide, high silicate with increasing content of phosphate in mixed electrolyte increases the coating thickness, and improves the density and uniformity of the overall PEO layers.