Effects of surface nanocrystallization on the anodic oxidation behavior of Aluminum

Abstract

Gradient microstructures on the surface of metals induced by severe shot peening (SSP) is relatively a new research topic in the field of severe plastic deformation processing. The treated surface is known to show three distinct features: a spectrum of grain size from nanometer to sub-micron to micron regime, a work-hardened surface layer, and a compressive residual stress profile near the surface. The current study is defined to investigate the effects of SSP pre-preprocessing on the anodization behavior of Aluminum. Gradient microstructures were obtained on AA1050 Al substrate using SSP with different coverage levels. SSP was performed with ceramic media to prevent contamination of the treated surface. Then the treated surface was anodized at a constant voltage in a sulfuric acid solution. The current-time responses were recorded to study the reaction kinetics. The average grain size on the surface treated by SSP was evaluated to be 255.6 nm and 181.5 nm for 1000% and 2000% coverage levels, respectively, using X-ray diffraction; while the Vickers microhardness was increased from 10.8 HV for the untreated sample to 19.8 HV and 25.8 HV by SSP with 1000% and 2000% coverage levels, respectively. It was found that the nanocrystalline microstructure of Al substrate enhances the growth rate of anodic oxide. A variable growth rate is detected within the treated layer. The nucleation of anodic oxide nanopores is discussed in terms of underlying oxide formation mechanisms and the extent of surface grain refinement.