Casting Aluminium Silicon (AlSi) alloys are key materials for the production of automotive components due to their excellent mechanical performance. Nevertheless, the corrosion resistance of AlSi is typically not sufficient to meet the requirements of the automotive market and several corrosion protection strategies (e.g. painting or anodizing) are needed [1]. At this regard, anodizing can greatly enhance the corrosion resistance of AlSi by growing a thick and dielectric Aluminium oxide layer on the surface of the treated component [2].

Despite the widespread use of anodized AlSi alloys, the correlation between anodizing and fatigue performance is only barely investigated in the literature. It is important to mention that addressing this point seems crucial since the surface modification of a material can strongly influence the nucleation of fatigue cracks.

At this regard, this work aims at unveiling the influence of anodizing on the fatigue behaviour of a high-quality cast AlSi_xMg_y-T6 (where x=8%_wt and y=0.15%_wt) alloy. Tensile and fatigue tests are carried out on both pristine and anodized specimens in order to highlight differences in the quasistatic and fatigue responses. In addition, surface roughness measurements, fracture surfaces analyses and SEM inspections are performed in order to investigate the effect of the oxide layer on the fatigue behaviour of AlSi_xMg_y-T6 specimens. Finally, an in-depth metallographic analysis is carried out in order to elucidate the: 1) microstructure of AlSixMgy specimens; and 2) morphology of the oxide layers.

It is demonstrated that, after anodizing, specimens show a reduction of about 20% and 40% in elongation at rupture and fatigue life at low stress levels, respectively. In addition, surface fracture analysis reveals that the presence of the anodic layer modifies the nucleation and early growth mechanism of fatigue cracks. Taken altogether, this work: a) proves the detrimental effect of anodizing on the fatigue resistance of high-quality casting AlSi alloys; and b) will allow to evaluate the effect of anodizing during the design of future fatigue-resistant AlSi components.

[1] Bandiera, M., Abello, M., A., Pavesi, A., Trombetta, V., Valota, G., Bonfanti, A., Mancini, A., Bertasi, F., “Interplay Between Microstructure and Anodizability in Aluminum Alloy Brake Calipers,” SAE Technical Paper (2022), No. 2022-01-1187.

[2] Wielage, B., Alisch, G., Lampke, T., Nickel, D., "Anodizing–a key for surface treatment of aluminium," Key Eng. Mater. 384 (2008), pp. 263-281.