Colourization of anodized Aluminium at different anodization times for improved thermal emittance

Abstract

The main aim of this study was to produce a surface good enough to emit radiation received from the sun to produce a cooling effect while protecting the surface from conditions such as corrosion by the principles of anodization and colouring. Eight aluminium sample plates were polished and then brightened in a brightening mixture of phosphoric acid, nitric acid, and copper nitrate solution at 80 °C. The brightened aluminium sample plates were anodized in sulphuric acid at 18 – 20 °C for different anodization times of 5 - 50 minutes. The anodized brightened aluminium plates were sealed and coloured by immersion into lead acetate and potassium permanganate solutions respectively at 18 – 20 °C. The thermal emittances of the brightened anodized and coloured anodic films of the sample plates as well as the thickness of the anodic films were determined by the use of an emissometer and gravimetric method respectively. The colouring of the sulphuric acid anodized aluminium plates produced hard coloured anodic films, with interference colours of darkbrown, golden-brown, yellowish-brown, deep brown and golden-yellow on the samples with high emittance values, depending on the anodizing time. The yellowish-brown coloured anodized aluminium plate obtained at anodization time of 40 minutes was found to be the most favourable, with high emittance value of 0.86 ± 0.01 and film thickness value of 0.97 ± 0.01 μm. This coloured anodic film produced on the aluminium sample plate could be used as thermal control coating in spacecraft, in some electronic gadgets, and in the production of aluminium cooling roofs to produce a cooling effect during hot weather conditions, and also for protective and decorative purposes.