Scratch behavior of aluminum anodized in oxalic acid: Effect of anodizing potential

Commercial purity aluminum sheets were anodized at 10-30 V in 10% oxalic acid solution at room temperature. The anodized specimens were tested for its resistance against scratch damage using a microscratch adhesion tester operated in the progressive mode. The specimens were studied also for crystal structure, chemical composition, surface morphology, surface topography, microhardness and electrical resistivity by X-ray diffractometry, energy dispersive X-ray spectroscopy, field emission scanning electron microscopy, stylus based surface profilometer, Vickers microhardness tester and four point probe method, respectively. Microscratch test results showed improved adhesion of the anodic aluminum oxide coating with the untransformed bulk, as the anodizing potential decreased. During scratch test, the specimens showed formation of cohesive cracks at a load of 1 N. Energy dispersive X-ray analysis showed that the concentration of oxygen increased marginally on increasing the anodizing potential. Field emission scanning electron microscopy showed comparatively less porous microstructure of the anodized specimens for anodizing conducted at potential 20 V or lower. The results of profilometry showed formation of smooth coating surface at potentials lower than 25 V. Microhardness test showed increased hardness of the anodized aluminum with increasing anodizing potential. The electrical resistivity of anodized aluminum was in the range of 106-109 Ω·cm. X-ray diffraction measurements indicated amorphous structure of the so obtained aluminum oxide coating.