The influence of the electrolytic bath on the hard anodizing of diecast Al-Si-Cu alloys

In the present work, the influence of the temperature and concentration of the electrolyte on the anodizing response of a high-pressure diecast AlSi11Cu2(Fe) alloy was studied. Sandblasted components, produced by high pressure diecasting, were anodized at −4.5 °C or +5 °C in an electrolyte with a free sulfuric acid concentration of 168 g/L or 218 g/L. Metallographic and image analysis techniques were carried out to study the changes in the oxide layer in terms of thickness and morphology. Scratch and wear tests were also performed to characterize the surface mechanical properties. The results show how the thickness of the anodic layer can be increased by decreasing the anodizing temperature of the electrolytic bath otherwise increasing the concentration of the sulfuric acid. However, the combination of a low anodizing temperature and a high concentration of free sulfuric acid has no beneficial effect on the formation of the anodic film. In this condition, the dissolution phenomena of the oxide layer inside the electrolyte are promoted and a thinner low-performance anodic layer is formed. Finally, the wear and scratch resistance are maximized after anodizing at −4.5 °C in an electrolyte with a concentration of sulfuric acid equal to 168 g/L.