Pulsed Current Effect on the Hard Anodizing of an AlSi10Mg Aluminum Alloy Obtained via Additive Manufacturing

Abstract

The hard anodizing treatments of cast Al-Si alloys are notoriously difficult. Indeed, their microstructural features hinder the growth of a uniform, compact, and defect-free anodic oxide. In this paper, AlSi10Mg samples, produced via Gravity Casting (GC) and Additive Manufacturing, i.e., Laser Powder Bed Fusion (L-PBF), were hard anodized in a sulfuric acid bath, in order to verify how the particular microstructure obtained via L-PBF affects the thickness, hardness, compactness, and defectiveness of the anodic oxide. Moreover, for the first time, Pulsed Direct Current (PDC) procedures were used to perform the hard anodizing treatments on additively manufactured AlSi10Mg alloy. Several combinations of temperature and electrical parameters, i.e., current density, frequency, and Duty Cycle, were tested. The anodized samples were characterized through optical microscopy analysis, Scanning Electron Microscopy (SEM) analysis, and accelerated corrosion tests, i.e., Potentiodynamic Polarization (POL) and Electrochemical Impedance Spectroscopy (EIS) measurements. The PDC procedures allowed improvement of the compromise between evenness, compactness, and defectiveness. Among the attempted PDC procedures, a specific combination of electrical parameters and temperature allowed the best results to be obtained, i.e., the highest hardness and the lowest volumetric expansion values without compromising the oxide quality rating and the corrosion resistance behavior. However, none of the attempted PCD strategies allowed the hardness values obtained on samples produced via GC to be reached.