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Abstract
Processed by sulfuric acid anodizing, several unexpected strip defects exhibited on the anodizing film of AA5252 aluminium alloy sheets. Their formation mechanism was studied in detail. The normal zone and strip zone were compared, with respect to surface brightness, porosity of anodizing film, and microstructures of the corresponding aluminium substrate tested by spectroscopic colorimeter and SEM, and SEM-EBSD. Results showed that the brightness of strip zone was lower than that of normal zone. Additionally, compared with normal zone, the porosity of anodizing film was higher, and cubic grains fewer, which located in the corresponding surface layer of aluminium substrate in strip zone. In order to further clarify the effect of grain orientation on the brightness of anodizing film, single crystal aluminium sheet with (100), (110) or (111) orientation was anodized. Results showed that the anodized (100) oriented specimen, i.e., cubic grain was brighter, whose anodizing film had lower porosity, compared with (110) and (111) oriented ones. Therefore, the formation mechanism of strip defects was proposed as uneven distribution of cubic grains. The zones with fewer cubic orientated grains were darker, and were visible as strip defects after anodizing.
Highlights
Aluminium and its alloys have attractive attention because of their low density, high strength, good corrosion resistance and electrical conductivity etc
The brightness L* of the normal zone and strip zone was 87.93±0.11 and 87.59±0.10 respectively, the deviation △L*=0.34. This indicated that the strip zone was darker than normal zones, and was visual slightly on the appearance of anodized AA5252 aluminium alloy sheets
Surface brightness of anodized aluminium comes from two mechanisms: the reflected light from anodizing film surface and the reflected light from anodizing film interior
Summary
Aluminium and its alloys have attractive attention because of their low density, high strength, good corrosion resistance and electrical conductivity etc. Aluminium alloy materials processed by colour anodizing could present a variety of colours and excellent corrosion protection, they are widely utilized as exterior structural components in 3C devices, including computer, cell phone and consumer electronic. The surface appearance is a critical characteristic for these components. Strip defects often become visible after anodizing, rather than in the rolled condition. It difficult to identify the original causes of the defects. To achieve reproducible and wellcontrolled surface appearance of anodized aluminium alloy is still a challenging issue
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