| dc.description.abstract | This thesis aimed at investigating alumina oxide film formed on aluminum, which is the most popular light metal applying to many common purposes. The study can be divided into three main parts. First, the observation of thermally-formed aluminum oxide on pure aluminum was conducted. The formation of pores and microbubbles derived by hydrogen in aluminum was brought out. Second, the anodic oxide films formed on aluminum were observed. In the part of this study, we measured and recorded the potential and processing time during anodizing treatment performed in different substrate. There are some critical points on the differentiated curves (ΔV/Δt) obtained from the measured data which most likely correspond to micropore initiation and pitting formation. Some more subjects about anodic oxide film were presented. Third, Optical properties of anodic aluminum oxide films on Al1050 and Al5052 alloys were investigated. The individual briefs are as following:
PartⅠ: Pure aluminum (99.999%) cubes were polished by abrasive papers then heated in furnace at 873K for 25 hr to grow oxide on the polished surfaces and coded as Al/oxide. The thermally-formed oxide was investigated with SEM and G.I.A in order to reveal the structure of oxide. These Al/oxide samples were stacked with pure Al cube and Al-7mass%Si cube respectively then heated in furnace at 1023K for 1200 s in an Ar+H2 atmospheric gas. The sandwich samples were sectioned and polished after the heated sample cooled to room temperature. The morphologies of interface (or junction of the sandwich samples) were recorded photographically. Based on the recorded cavities shown at the interface, we measured both the radii of curvatures and contact angles of cavities. When the Al/oxide stacking with pure Al sandwich samples heated in Ar plus H2 gas, cavities were readily shown at the interface; very few cavities has been observed when samples heated in Ar gas. Air-pocket initiated at the microchannels by hydrogen diffusion then grew and coalesced at the interface. The air-pockets remained at the interface of heated Al/oxide stacking with pure Al sandwich sample and entrapped as cavities after samples solidified. Microbubbles detached from the airpocket forming micropores trapped in matrix of the Al/oxide stacking with Al-7mass% Si cube sample.
PartⅡ: The formation behavior of anodic oxide film formed on aluminum was observed. In this part the relation of the anodic potential versus processing time was recorded for the anodizing of an Al1050 aluminum sheet in a sulfuric acid solution. The electrochemical behavior of aluminum during the anodizing treatment can be characterized by measuring the anodic potential-time curves. The nanostructure of anodic films was observed with transmission electron microscopy (TEM) and high resolution Field-Emission Scanning electron microscopy (SEM). The differentiated curves (ΔV/Δt) could be partitioned into four steps indicating the processing conditions during the growth of the anodic film: barrier layer formation, nanopore initiation and growth, pore widening, and quasi-steady state growth of the anodic film. Pitting during the pore widening stage could be characterized by looking at the different types of valleys in the differentiated curve. The effects of the anodic parameters, such as the acid concentration, bath temperature, current density and impurities, on pitting formation were investigated and some conclusions offered.
PartⅢ: A spectrophotometer was used to measure the surface reflectance of AAO films, which formed from the different processing conditions on Al1050 and Al5052 alloys. The factors that affected the reflectance of AAO film were discussed. After cleaning and desmutting Al1050 and Al5052 alloys sheets were anodized in a 15% w/w sulfuric acid solution at 1.2-1.8 A*dm-2 and 283~303K. The potential-time curves were recorded during the anodizing process. The nano-structure of the AAO films changed with the anodic current density and the bath temperature. The area fraction of nano-pores on the anodic film increased with increasing bath temperature and with decreasing anodic current density. A spectrophotometer was used to measure the surface reflectance of AAO films, which formed from the different processing conditions. The factors that affected the reflectance of AAO films are discussed. | en_US |