How is it done?
Anodising is accomplished in five carefully controlled, calibrated, quality-tested stages:
- Cleaning. Alkaline and/or acid cleaners remove grease, and surface dirt.
a) Etching. An appealing matt surface finish is created with hot solutions of sodium hydroxide to remove minor surface imperfections. A thin layer of aluminium is removed to create a matt or dull finish.
b) Brightening. A near mirror finish is created with a concentrated mixture of phosphoric and nitric acids which chemically smooths the aluminium's surface.
- Anodising. The anodic film is built and combined with the metal by passing an electrical current through an acid electrolyte bath in which the aluminium is immersed. The coating thickness and surface characteristics are tightly controlled to meet end product specifications.
- Colouring. Colouring is achieved in one of four ways:
- Electrolyte Colouring (the two step method) - After anodising, the metal is immersed in a bath containing an inorganic metal salt. Current is applied which deposits the metal salt in the base of the pores. The resulting colour is dependent on the metal used and the processing conditions (the range of colours can be expanded by overdyeing with organic dyes). Electrolytic colours can be specified from any AAA member. Commonly used metals include tin and cobalt. This process offers colour versatility and the most technically advanced colouring quality.
- Intergral Colouring - This so-called one-step process combines anodising and colouring to simultaneously form and colour the oxide cell wall in bronze and black shades while more abrasive resistant than conventional anodising. It is the most expensive process since it requires significantly more electrical power.
- Organic Dyeing - The organic dyeing process produces a wide variety of colours. These dyes offer vibrant colours with intensitites that cannot be matched by any other paint system on the market. They can also provide excellent weather-fastness and light-fastness. Many structures built with these finishes have lasted more than 20 years. The colour range can be broadened by over-dyeing the electrolytic colours with the organic dyes for a wider variety of colours and shades. This method is relatively inexpensive and involves the least amount of initial capital of any other colouring process.
- Interference Colouring - An additional colouring procedure, recently in production, involves modification of the pore structure produced in sulfuric acid. Pore enlargement occurs at the base of the pore. Metal deposition at this location produces light-fast colours ranging from blue, green and yellow to red. The colours are caused by optical-interference effects rather than by light scattering as with the basic electrolytic colouring process. Further development will produce a greater variety of colours.
- Sealing. This process closes the pores in the anodic film, giving a surface resistant to scratching, abrasion, crazing and colour degradation.
Quality Control. Throughout the entire anodising process, AAA members monitor the process and quality of the product. The application of electrical power and colour is preprogrammed and verified on all batches and coils.
This quality control ensures uniformity to end product specifications for film thickness, density, abrasion resistance, corrosion resistance, reflectivity, image clarity, insulative properties, adhesion and sealing.