Adhesive Property of Aluminum Oxide Formed By High Temperature Anodizing

Abstract

Aluminum and its alloys have been widely used various situations because of their light weight, specific strength, heat conductivity. Surface treatment can impart corrosion protection, hardness, coating film adhesion and others to aluminum. Anodizing is one of the most important surface finishing processes of aluminum. In recent years, technology to use multimaterials has been attracting attention to achieve the production of energy-efficient automobiles. In such activities, adhesive bonding is increasingly used in automobile manufacturing. However, the reliability and strength of the joint surface is very important to use adhesive bonding for vehicle such as car, aircraft and motorcycle. Herein, we demonstrate the fabrication of high adhesive aluminum surface via high temperature anodizing process. High-purity aluminum specimens (99.99 wt%, 300 µm thick) were ultrasonically degreased in acetone for 10 min. These specimens were immersed in a oxalic acid solution (0.3 M), and then were anodized at a constant current density using a direct-current power supply. After anodizing, the specimens were quickly removed from the electrolyte solution and then washed with distilled water. The surface nanomorphology of the anodized specimens was examined by field emission scanning electron microscopy (FE-SEM). The adhesion property of anodized specimen was measured by tape peel test using 90 degree peel test machine. Figure 1 shows the surface appearance of the specimens covered with a) typical anodic oxide formed by low-temperature anodizing and b) anodic oxide formed by high-temperature anodizing. Anodic oxide formed by high-temperature anodizing had an irregular shape numerous pores due to the chemical dissolution of anodic oxide. These photographs were taken after tape peel test. On the typical anodic oxide, the tape was easily peeled off and no adhesive remained on the surface. On the other hand, the tape adhesive remained on the irregular anodic oxide covered surface fabricated by high temperature anodizing and the strong tape peel strength was measured. High-temperature anodizing causes the intense chemical dissolution. Thus, the anodic oxide which had wide-pore outer layer and intricate inner layer was formed on the aluminum surface by high-temperature anodizing. Adhesive layer of tape might be able to penetrate such anodic oxide. Therefore, the strong peel strength by the anchoring effect was measured on the surface. Although the adhesive mechanism has not been clear in detail, this surface treatment technique has a potential for the fabrication of composite material which is consist of aluminum and another material． Figure 1