Effect of case depths on corrosion properties of hard anodic oxidized A356 aluminum alloy

Abstract

In this work, A356 alloy samples having a dimension of 4x20x20mm were coated by hard anodic oxidation method at eight different thicknesses up to similar to 80 mu m. The sample surfaces and longitudinal section of these surfaces in both coated and uncoated conditions were investigated with a scanning electron microscope (SEM). Microhardness of the coating layers and core regions (base metal) were determined with the microvickers hardness testing method. Corrosion properties of the coated and uncoated samples were investigated by an electrochemical corrosion test set up according to ASTM G5 standard. The test results obtained from the coated and uncoated surfaces were compared to each other. Coated surfaces exhibited fluctuant surface profile. Microcracks were formed on these surfaces. Microhardness of the coating layers increased with increasing coating layer thickness. Energy dispersed X-ray spectroscopy (EDS) analysis of the surfaces showed that oxygen rate on the surfaces increased after the corrosion tests. Corrosion rate of the A356 alloy coated by hard anodic oxidation decreased with increasing coating thickness up to 40-50 mu m, but after this coating thickness, the corrosion rate increased. The results obtained from the investigations were discussed based on the characteristics of the coated surfaces.