Applications of Anodized Aluminium The properties of anodic oxide coatings on aluminium are unique among metal finishes and it is not surprising therefore that when commercial development blossomed in 1930 onward, anodized aluminium was hailed as a possible replacement for a wide range of materials. In due time many of these hopes proved unfounded but others received permanent acceptance in industry. New uses have been developed since that time, founded on new or improved processes described in this book. Some of the more important uses are described in this chapter, roughly in their chronological order of promotion. Anodizing as an undercoat for organic coatings
This was the first large-scale application and was based on the invention of the chromic acid (Bengough-Stewart) anodizing process. It was adopted as a standard finish for aluminium aircraft components and is still specified in the DEF 151 specification. This combination of an organic finish with an anodic coating ensures maximum life for the paint coupled with an underlying protective coating to provide further protection in the event of any paint failure. Excellent service, even during sea-water immersion, is provided by sulphuric acid coatings, preferably sealed in a dichromate solution and then coated with an appropriate grade of paint. The use of very thin coatings as a base for the subsequent painting and lacquering of continuous strip is also important. Corrosion-resistant coatings
It was soon realized that unpainted chromic acid coatings, 6
Applications of Anodized Aluminium
especially when physically sealed with lanolin, had a high resistance to salt-spray corrosion and the use of this combination was also approved for aircraft components where painting would have interfered with the operational use of the coating. By 1929 the sulphuric acid processes made their appearance with the ability to produce thick hard coatings which could be sealed with lanolin, oil, etc. The first Air Ministry approval of such a coating in the U.K. was granted in 1936. The sulphuric acid coatings have been widely accepted for the treatment of aluminium exposed to marine and industrial atmospheres, and work thus processed before World War II is still in acceptable condition. Today, the use of anodized aluminium for external and indoor building components provides the principal tonnage output for the world's anodizing capacity. This widespread application has depended on the invention of steam sealing and hot-water sealing without which the difficulties of physical sealing may well have inhibited such progress. Typical examples of this class of work are: windows, patio doors, curtain walling, partition systems, double-glazing frames, canopies, grilles, fascias and shop fronts. The corrosion-resisting properties of anodized aluminium have, not unexpectedly, found a wide field of application outside the aircraft and building industries. Many defence items that must survive long storage periods and others that need general protection in a tropical or marine environment are protected by anodizing. These include shell cases, tank armour, rocket components and fuses. Coloured anodizing
Although the ability of anodic coatings to absorb dyestuffs was discovered in 1923 the commercial possibilities were not really exploited until the sulphuric acid process appeared in 1929. A wide range of dyestuffs was then selected and * 'coloured aluminium" as it was called posed a formidable threat to other finished metals and plastics. Some of the applications proved disastrous due to the poor light fastness of most of the available dyes but, on the whole, coloured anodizing secured a permanent and ever-increasing market. Early applications included ash trays, decorative metalware such as fruit bowls, tea trollies and panelling for indoor use. Larger-scale items included escalator metalwork and column casings in banks and stores.
Anodic Oxidation of Aluminium and Its Alloys
The production of coloured designs by multicolour techniques attracted the attention of the label and nameplate industries who still offer this finish today. Apart from the decorative use of colouring, it has also been applied to identify components or different alloys. For example, rivets or rivet wire have been anodized and dyed to avoid an incorrect selection of alloy rivets. Textile bobbins have also been dyed for yarn identification. The, problem of the poor light fastness of organic dyes began to be resolved in about 1937 when metal complex dyes of the "Neolan" class became available and an improved black dye Nigrosine D was developed. Some dyes offered by ICI Dyestuffs Ltd. included Solway Blue BS and Solway Ultra Blue, both of which were successfully used for shop fronts. The use of pigment colours was well established in the early 1930s, but only the cobalt-permanganate bronze colour survived and is still used in some countries. This colour is used for outdoor metal work. In 1939 the brassy-gold colour produced by using ferric ammonium oxalate was developed in Germany but was little used until after World War II, since when it has appeared on many prestige buildings and has provided an economical substitute for gold-leaf-covered letters for shop fascias. Integral colour bronzes appeared in 1959 and account for about 10% of the anodized aluminium used for buildings. Their hardness has also improved the performance of such items as door handles, fingerplates, door plates, shop fronts and entrances. Electrolytic colours can be produced in many shades by using different colouring electrolytes. The majority of this class of work is produced in nickel, cobalt or tin solutions, all of which provide a range of light bronze to black shades having excellent light fastness. Here again, the building industry is the largest outlet. Anti-marking applications
When aluminium alloys, unanodized, were introduced to the metalwork market for such applications as balustrades and grab rails the public complained of the greyish marking of light-coloured clothing or gloves that came into contact with the metal. Anodizing provided a complete answer to this problem and was adopted for example for all the metalwork in buses and coaches. In later years some of these items have been replaced by stainless steel. The successful promotion of aluminium knitting needles has been entirely due to the use of anodizing which prevents marking of the wool or the knitters' fingers.
Applications of Anodized Aluminium
Processes for chemical or electrobrightening aluminium did not appear until 1934. However, sulphuric acid anodizing was used to protect the surfaces of floodlights. The initial loss of total reflectivity was acceptable because this lower figure was maintained for years of service whereas plain aluminium continuously corroded and declined in performance. The anodized surface was easier to clean than corroded plain aluminium. After the invention of the "Brytal" and "Alzak" electrobrightening processes the use of anodized aluminium especially based on highpurity sheet became an important material for the lighting industry. The advent of chemical brightening has facilitated the production of reflecting coatings on lower-quality sheet which is used for street lighting reflectors, illuminated ceiling grids and such specialized items as reflectors for operating theatre lights, aircraft lamps, airfield flare paths and miners' lamps. In World War II bright anodized flat sheets were used as shock-proof driving mirrors in tanks and many searchlight reflectors were similarly finished. Heot reflection
Anodizing has long been used for aluminium electric-fire reflectors. The "Dimplex" unit with its heated silica tube is found in many homes. The easily cleaned surface withstands the humidity of bathrooms and is an efficient reflector of radiant heat provided that the anodic coating is restricted to about 1 /¿m. In recent years the heat-radiating properties of thick coatings have been applied in the manufacture of heat sinks for electronic equipment. The coatings are often dyed black to increase their heat emissivity. In addition to the natural colour reflectors some use has been made of coatings dyed in pale colours, particularly copper which enhances the appearance. Reflectors for infra-red stoving ovens for paint have also been made in bright anodized aluminium where the reflectivity is only surpassed by gold. Wear resistance and
Before hydration sealing became available the physical sealing with oils, waxes, etc., led to the rise of sealing with lubricating oils of coatings used for engineering purposes where an oil-wettable surface
Anodic Oxidation of Aluminium and Its Alloys
was an advantage. The principal application in this field was on anodized aluminium pistons for petrol and diesel engines. Sealing with graphite suspensions was also adopted. It was claimed that the coating had improved "running-in" properties and that the wear of the ring grooves in service was reduced. This type of sealed coating is still used on some diesel pistons and on air compressors, using "hard anodizing" to form the coating. Hard anodic coatings are being very successfully exploited by the engineering industry for surfaces that have to withstand lightly loaded rubbing contact and where good corrosion resistance is necessary. Examples are cigarette manufacturing machines, hydraulic cylinders, coin-operated machine slides, textile spinning guides and ciné-camera components. The coefficient of friction of anodized aluminium is appreciably lowered to about 0.1 by sealing with a suitable PTFE-resin mixture. This chemically inert sealant also provides additional corrosion resistance. An interesting application of hard anodizing is for the treatment of the rollers that carry film stock during its manufacture. The wear and corrosion resistance are satisfactory and the complete anodized roller is cheaper than the stainless-steel rollers originally specified for this work. As a general comment, the anodic oxide coating is much harder than the basis aluminium metal so that the general resistance to wear and marking is greatly increased by anodizing. Electrical insulation
Although the anodic oxide coating is a good electrical insulator the danger of local breakdown due to minor defects has militated against any extensive use of anodized wire. However, continuously anodized aluminium strip has for many years been adopted for winding electromagnets and some types of transformers where weight saving is important. The oxide coating is much more resistant to heat than organic insulating media and is therefore selected for hightemperature environments.