BATCH AND COIL ANODISING
Batch and coil anodising are accomplished in five carefully controlled, calibrated, quality-tested stages:
1. Cleaning. Alkaline and/or acid cleaners remove grease, and surface dirt.
2. Pre-Treatment. Etching. An appealing matte 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 matte or dull finish.
Brightening. A near mirror finish is created with a concentrated mixture of phosphoric and nitric acids which chemically smoothes the aluminium's surface.
3. 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.
4. Colouring.
Colouring is achieved in one of four ways:
Electrolytic 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 over dyeing the organic dyes). Electrolytic colours can be specified from any AAC member. Commonly used metals include tin, cobalt, nickel, and copper. This process offers colour versatility and the most technically advanced colouring quality.
Integral 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 intensities that cannot be matched by any other paint system in 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 sulphuric 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.