Anodising Aluminium

Anodising aluminium

Adding beauty and durability

Bang & Olufsen over the years, has become famous for not only its stylish products but for their 'feel' and looks. Many products produced nowadays are of a hard-wearing aluminium; aluminium which has gone through the process of anodisation in order to harden the aluminium surface considerably and make it highly resistant to physical damage such as scratching. As the products' mirror finish and smooth surface without protrusions are easily cleaned, their beautiful appearance will be the same for many years.

Anodised aluminium has been a Bang & Olufsen hallmark for many years, but as the mirror-finished anodisation process is one of the most demanding metallurgic surface processes, Bang & Olufsen is the only manufacturer of audio and video equipment to use it. Anodising aluminium successfully combines science with nature in order to create one of nature's best metal finishes.

The process is electrochemical which 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 diamond. The anodic coating is part of the metal, but has a porous structure which allows secondary infusions, (i.e. organic and inorganic colouring, lubricity 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. In wider industry, 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 anodised.

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 (see, for example, the range of BeoLab 4000 finishes). The unique dielectric properties of an anodised finish offer many opportunities for electrical applications.

The surface of the aluminium itself is toughened and hardened to a degree unmatched 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.

Anodising is accomplished in five carefully controlled, calibrated, quality-tested stages:

Cleaning:

Alkaline and/or acid cleaners remove grease, and surface dirt

Pre-treatment:

(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 smoothes 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:

(i) 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 over-dyeing 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.

(ii) 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.

(iii) 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 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.

(iv) 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.

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, the process and quality of the product is strictly monitored. The application of electrical power and colour is pre-programmed 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, insulation properties, adhesion and sealing.

The unique anodised finish is the only one in the metals' industry that satisfies each of the factors that must be considered when selecting a high performance aluminium finish.

Features of anodised aluminium:

Durability

Most anodised products have an extremely long life span and offer significant economic advantages through maintenance and operating savings. Anodising is a reacted finish that is integrated with the underlying aluminium for total bonding and unmatched adhesion.

Colour Stability

Exterior anodic coatings provide good stability to ultraviolet rays, do not chip or peel and are easily repeatable.

Ease of Maintenance

Scars and wear from fabrication, handling, installation, frequent surface dirt cleaning and usage are virtually non-existent. Rinsing or mild soap and water cleaning usually will restore the anodised surface to its original appearance.

Aesthetics

Anodising offers a large increasing number of gloss and colour alternatives and minimises or eliminates colour variations. Unlike other finishes, anodising allows the aluminium to maintain its metallic appearance.

Cost

A lower initial finishing cost combines with lower maintenance costs for greater long-term value.

Health and Safety

Anodising is a safe process that is not harmful to human health. An anodised finish is chemically stable, will not decompose; is non-toxic; and is heat-resistant to the melting point of aluminium. Since the anodising process is a reinforcement of a naturally occurring oxide process, it is non-hazardous and produces no harmful or dangerous by-products.

Colour

Anodising provides depth and variety of colour with a brilliant metallic lustre.

Texture Choice

Anodising can provide a variety of textures from mechanically polished to heavily etched giving that extra dimension to a project.

Created: 10th January 2007
Modified: 13th February 2007

Anodising Aluminium