Aluminium Anodising Types

ANODISING successfully combines science with nature to create one of the world's best metal finishes.

It is an electrochemical process that thickens and toughens the naturally occurring protective oxide. The resulting finish, depending on the process, is the second hardest substance known to man, second only to the diamond. The anodic coating is part of the metal, but has a porous structure which allows secondary infusions, (i.e. organic and inorganic colouring, lubricant aids, etc.)

ANODISING DEFINITIONS AND METHODS
While the chemical anodising process remains the same for all applications, the mechanical methods vary according to the two physical types and shapes of metals used:

Batch Anodising - Involves racking parts and immersing them in a series of treatment tanks. Extrusions, sheets or bent metal parts, castings, cookware, cosmetic cases, flashlight bodies, and machined aluminium parts are just a few of the items that are batch anodized.

Continuous Coil Anodising - Involves continuous unwinding of pre-rolled coils through a series of anodising, etching and cleaning tanks, and then rewinding for shipment and fabrication. This method is used for high volume sheet, foil and less severely formed products such as lighting fixtures, reflectors, louvers, spacer bars for insulated glass, and continuous roofing systems.

 

Appearance options and quality are improved through the use of dyes and special pre-treatment procedures. This makes the aluminium look like pewter, stainless steel, copper, brushed bronze or polished brass and can also be coloured with brilliant blues, greens, reds, and many varieties of metallic gold and silver.

The unique dielectric properties of an anodized finish offer many opportunities for electrical applications.

The surface of the aluminium itself is toughened and hardened to a degree un-matched by any other process or material. The coating is 30 percent thicker than the metal it replaces, since the volume of oxide produced is greater than that of the metal replaced.

The resulting anodic coating is porous, allowing relatively easy colouring and sealing.

Hard Anodising is a term used to describe the production of anodic coatings with film hardness or abrasion as their primary characteristic. They are usually thick by normal anodising standards (greater than 25 microns) and they are produced using special anodising conditions (very low temperature, high current density, special electrolytes). They find application in the engineering industry for components which require a very wear resistant surface such as piston, cylinders and hydraulic gear. They are often left unsealed, but may be impregnated with materials such as waxes or silicone fluids to give particular surface properties.

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.