Anodising statistics
Anodising is a process for producing decorative and protective films on articles made of aluminium and its alloys. The article is made the anode of an electolytic cell with aqueous sulfuric acid as electrolyte where the following overall oxidation reaction occurs.
2Al + 3H 2O → Al2O3 + 6H+ + 6e-
A dense even layer of oxide about 0.08 μm thick is formed rapidly, followed much more slowly with a more porous layer up to 25μm thick.
Anodising is a process for producing decorative and protective films on articles made from alumnium and its alloys. It is essentially a process where a thick film of aluminium oxide is built up on the surface of the aluminium through the use of a direct current electrical supply.
In the majority of anodising plants it is carried out in an electrolyte bath containing sulphuric acid with aluminium sheet cathodes and the work to be anodised attached to the anode
When the current is flowing in the cell the following sequence of events is believed to occur. Sulphuric acid begins to decompose, the hydrogen ions moving to the cathode where they are reduced to hydrogen gas:
2H + + 2e- → H2(g)
Simultaneously, negatively charged anions, i.e. hydroxide, sulphate and maybe oxide ions move to the anode. The electrical charge in the circuit causes positively charged aluminium ions (Al 3+) to be generated in the anode and in turn move toward the cathode. At the anode surface they react with the oxide/hydroxide ions to form aluminium oxide (in the case of the hydroxide ion, hydrogen ions are released into the solution).
Fresh aluminium reacts readily with oxygen to produce aluminium oxide. Once formed the oxide remains firmly bonded to the surface forming an impenetrable layer. Consequently, further reaction ceases. The film is very thin (0.01um), and despite its tenacity it can be removed by abrasion and chemical corrosion. In such instances the aluminium is subject to wear or the surface will mark or become pitted at the site of corrosion.
Anodising produces much thicker coatings (12 - 25um) which, if properly sealed, can extend the life of the surface appreciably. Recent research has shown that pitting of the surface can be reduced by up to 90% with a 12um coating, and by up to 93% with a 25um coating. In the initial stages (i.e. first 60 s) of anodising the oxide layer formed is dense and of even consistency. It provides the greatest resistance to wear and corrosion and consequently is called the barrier layer. The growth of this layer ceases when
the high electrical resistance of the oxide reduces the potential of the applied voltage in the electrolytic cell. The depth of the coating at this stage is about 0.08um. Subsequent growth is very slow and competes with the acid reaction:
Al 2O3 + 6H+ _ 2Al3+(aq) + 3H2O which releases Al 3+ ions into the solution. Note that the H+ can be at high concentration
near the oxide layer due to one of the anode reactions above.
PRE-TREATMENT
Step 1 - Cleaning
Correct and adequate cleaning of the aluminium object prior to anodising is essential if the finished work is to have a uniform and attractive appearance. When aluminium arrives from the rolling, casting or extrusion mills it may be soiled
Step 2 - Etching
Etching is most often achieved by the use of a warm, 10 % (2.5 mol L -1) sodium hydroxide solution. It gives the surface of the metal a light grey satin finish (through diffuse reflection of the incident light).
The rate of etching is dependent on the concentration of the sodium hydroxide solution, the temperature and the concentration of aluminium ions which are released into the solution. When high concentrations of aluminium ions are present the solution loses its effectiveness. Presence of other ions, some of which may be a component in the alloy, can also interfere in the process, causing blemishes to appear on the surface of the work. The problem of ion contamination is overcome by employing etching solutions which
suppress the action of the Al 3+ and other metal ions released. The composition of these solutions are the propriety of the companies that develop them, but generally contain sequestering agents which complex metal ions. Such solutions do not have an infinite capacity to do this but, due to the 'carryover' of solution by the etched work and periodic replacement by fresh etch solution, the etching batch is maintained in an effective condition
POST TREATMENT
After cleaning and anodising the work is coloured and sealed. As all anodised work is sealed, sealing will be considered first, although if colouring is to be done it is carried out prior to sealing.
Sealing
Sealing is the process in which the pores at the surface of the oxide layer are closed off. It is effected by placing the anodised object in boiling water for a 15 - 20 minute period. During that time the water reacts with the aluminium oxide to produce the mineral Boehmite - Al 2O3.H2O or AlO.OH:
Al 2O3 + H2O → 2AlO.OH
Boehmite is a hard, transparent material with a greater volume than the aluminium oxide. As it forms it closes off the openings of the pores. As would be expected, the durability of the anodised surface, especially in regard to chemical corrosion, is greatly influenced by the effectiveness of the sealing. If the duration of the sealing is too short the pores, although constricted, remain open for corrosion agents to be in close proximity to the aluminium surface. Corrosion studies have shown that anodised aluminium which has been sealed for 15 minutes or more has greatly increased resistance to pitting by chemical corrosion agents such as H+ and Cl-.
Colouring
Colouring involves the absorption of a coloured dye into the pores of the oxide coating which becomes fixed after the sealing process has been completed. Dyestuffs which can bond to the oxide or metal ions in the anodised layer have better colour properties than those that do not. Electrolytic colouring is the most important method of colouring anodised aluminium today. It produces attractive finishes of very great colour and heat fastness and is easy to perform. The anodised work is dipped in a tank containing coloured ions
of other metals. Although the constitution of the solutions are patent
Under the influence of alternating current the colouring agents deposit rapidly at the very base of the pores and the take is even over the entire surface. Unlike the process of dye absorption, electrolytic colouring is easy to control and gives uniformity of colour from one run to the next. The success of this technique is evident in the widespread use of bronze coloured aluminium in joinery and house fittings. Approximately 66% of all bronze tinted aluminium is coloured by this technique. To a much lesser extent coloured inorganic compounds can be used to colour the work. Ammonium ferric oxalate is a very common compound used to impart a goldy colour to the metal. Other colours can be imparted by treating the absorbed ferric ammonium oxalate with other compounds: for example,
potassium ferrocyanide solution will react with the ferrioxalate compound to produce a blue colour. The technique used is to dip the work firstly in a solution of the ammonium ferrioxalate followed by dipping the work in the potassium ferrocyanide solution. This double dipping technique can be used with other compounds to produce a variety of colours: e.g. copper sulphate followed by
ammonium sulphide gives green, and lead nitrate followed by potassium chromate gives yellow.
2Al + 3H 2O → Al2O3 + 6H+ + 6e-
A dense even layer of oxide about 0.08 μm thick is formed rapidly, followed much more slowly with a more porous layer up to 25μm thick.
Anodising is a process for producing decorative and protective films on articles made from alumnium and its alloys. It is essentially a process where a thick film of aluminium oxide is built up on the surface of the aluminium through the use of a direct current electrical supply.
In the majority of anodising plants it is carried out in an electrolyte bath containing sulphuric acid with aluminium sheet cathodes and the work to be anodised attached to the anode
When the current is flowing in the cell the following sequence of events is believed to occur. Sulphuric acid begins to decompose, the hydrogen ions moving to the cathode where they are reduced to hydrogen gas:
2H + + 2e- → H2(g)
Simultaneously, negatively charged anions, i.e. hydroxide, sulphate and maybe oxide ions move to the anode. The electrical charge in the circuit causes positively charged aluminium ions (Al 3+) to be generated in the anode and in turn move toward the cathode. At the anode surface they react with the oxide/hydroxide ions to form aluminium oxide (in the case of the hydroxide ion, hydrogen ions are released into the solution).
Fresh aluminium reacts readily with oxygen to produce aluminium oxide. Once formed the oxide remains firmly bonded to the surface forming an impenetrable layer. Consequently, further reaction ceases. The film is very thin (0.01um), and despite its tenacity it can be removed by abrasion and chemical corrosion. In such instances the aluminium is subject to wear or the surface will mark or become pitted at the site of corrosion.
Anodising produces much thicker coatings (12 - 25um) which, if properly sealed, can extend the life of the surface appreciably. Recent research has shown that pitting of the surface can be reduced by up to 90% with a 12um coating, and by up to 93% with a 25um coating. In the initial stages (i.e. first 60 s) of anodising the oxide layer formed is dense and of even consistency. It provides the greatest resistance to wear and corrosion and consequently is called the barrier layer. The growth of this layer ceases when
the high electrical resistance of the oxide reduces the potential of the applied voltage in the electrolytic cell. The depth of the coating at this stage is about 0.08um. Subsequent growth is very slow and competes with the acid reaction:
Al 2O3 + 6H+ _ 2Al3+(aq) + 3H2O which releases Al 3+ ions into the solution. Note that the H+ can be at high concentration
near the oxide layer due to one of the anode reactions above.
PRE-TREATMENT
Step 1 - Cleaning
Correct and adequate cleaning of the aluminium object prior to anodising is essential if the finished work is to have a uniform and attractive appearance. When aluminium arrives from the rolling, casting or extrusion mills it may be soiled
Step 2 - Etching
Etching is most often achieved by the use of a warm, 10 % (2.5 mol L -1) sodium hydroxide solution. It gives the surface of the metal a light grey satin finish (through diffuse reflection of the incident light).
The rate of etching is dependent on the concentration of the sodium hydroxide solution, the temperature and the concentration of aluminium ions which are released into the solution. When high concentrations of aluminium ions are present the solution loses its effectiveness. Presence of other ions, some of which may be a component in the alloy, can also interfere in the process, causing blemishes to appear on the surface of the work. The problem of ion contamination is overcome by employing etching solutions which
suppress the action of the Al 3+ and other metal ions released. The composition of these solutions are the propriety of the companies that develop them, but generally contain sequestering agents which complex metal ions. Such solutions do not have an infinite capacity to do this but, due to the 'carryover' of solution by the etched work and periodic replacement by fresh etch solution, the etching batch is maintained in an effective condition
POST TREATMENT
After cleaning and anodising the work is coloured and sealed. As all anodised work is sealed, sealing will be considered first, although if colouring is to be done it is carried out prior to sealing.
Sealing
Sealing is the process in which the pores at the surface of the oxide layer are closed off. It is effected by placing the anodised object in boiling water for a 15 - 20 minute period. During that time the water reacts with the aluminium oxide to produce the mineral Boehmite - Al 2O3.H2O or AlO.OH:
Al 2O3 + H2O → 2AlO.OH
Boehmite is a hard, transparent material with a greater volume than the aluminium oxide. As it forms it closes off the openings of the pores. As would be expected, the durability of the anodised surface, especially in regard to chemical corrosion, is greatly influenced by the effectiveness of the sealing. If the duration of the sealing is too short the pores, although constricted, remain open for corrosion agents to be in close proximity to the aluminium surface. Corrosion studies have shown that anodised aluminium which has been sealed for 15 minutes or more has greatly increased resistance to pitting by chemical corrosion agents such as H+ and Cl-.
Colouring
Colouring involves the absorption of a coloured dye into the pores of the oxide coating which becomes fixed after the sealing process has been completed. Dyestuffs which can bond to the oxide or metal ions in the anodised layer have better colour properties than those that do not. Electrolytic colouring is the most important method of colouring anodised aluminium today. It produces attractive finishes of very great colour and heat fastness and is easy to perform. The anodised work is dipped in a tank containing coloured ions
of other metals. Although the constitution of the solutions are patent
Under the influence of alternating current the colouring agents deposit rapidly at the very base of the pores and the take is even over the entire surface. Unlike the process of dye absorption, electrolytic colouring is easy to control and gives uniformity of colour from one run to the next. The success of this technique is evident in the widespread use of bronze coloured aluminium in joinery and house fittings. Approximately 66% of all bronze tinted aluminium is coloured by this technique. To a much lesser extent coloured inorganic compounds can be used to colour the work. Ammonium ferric oxalate is a very common compound used to impart a goldy colour to the metal. Other colours can be imparted by treating the absorbed ferric ammonium oxalate with other compounds: for example,
potassium ferrocyanide solution will react with the ferrioxalate compound to produce a blue colour. The technique used is to dip the work firstly in a solution of the ammonium ferrioxalate followed by dipping the work in the potassium ferrocyanide solution. This double dipping technique can be used with other compounds to produce a variety of colours: e.g. copper sulphate followed by
ammonium sulphide gives green, and lead nitrate followed by potassium chromate gives yellow.
United Anodisers
BENEFITS OF ANODISED ALUMINIUM FOR ARCHITECTURAL APPLICATIONS
Introduction
Modern materials must permit design creativity but at the same time they must be:
produced responsibly and economically
durable, and
recyclable at the end of their life
Aluminium, with its exceptional recyclability, minimal maintenance and proven lifetime performance, successfully responds to the need for environmentally friendly and sustainable solutions. In building applications, aluminium virtually always requires surface treatment. Yet, the choice of surface treatment of aluminum is as important as the choice of the material itself.
Anodising enhances the natural qualities of aluminium still further; it permits a strongly contemporary finish with incomparable corrosion and abrasion resistance. Unique amongst surface treatments, such as coatings, anodising is totally integrated with the metallic substrate – it is not simply a film applied to cover the surface. The result – pure aluminium – is a perfectly and
repeatedly recyclable material with remarkable performance characteristics.
These same exceptional qualities can be found in new product applications in anodising, which now provide new and infinite texture and design possibilities for building exteriors. The modern, aesthetic beauty of anodised aluminium is founded on its many
unique properties. Anodising is a transparent finish integral with the metal, which retains and enhances the inherent beauty of the aluminium; it is totally UV resistant.
A totally homogeneous appearance can be guaranteed, thanks to the nature of the anodising process and the very tight quality controls on both the aluminium and the surface treatment; Colours, textures and patterns can be combined with the anodic film,
enhancing the natural metallic appearance without affecting the total UV resistance.
The "living" quality of its natural metallic sheen, combining texture with colour, guarantees a creative interaction of the surfaces and shapes of the building with the constantly changing light conditions through the day and across the different seasons. This effect can be accentuated by employing textures and colours.
The anodic layer will retain its original beauty and protection against corrosion throughout the life of the building – as proven by independent inspection of buildings of more than 30 years old.
The following information provides owners, developers, architects, system companies, specifiers and contractors, with an in-depth understanding of the proven benefits of anodised aluminium and explains why this is increasingly the material of choice for buildings of the future:
Authentic metallic lustre and feel
The anodic layer is transparent and integrated in the surface of the metal; the natural metallic appearance and feel of the aluminium are fully preserved. The anodised finish will enhance and reinforce the natural beauty of aluminium to create a living surface constantly interacting with the natural or artificial light playing across its surface
Regardless of the substrate material – aluminium, steel, plastic – a painted surface always has the same, flat appearance. In trying to achieve the authentic metallic lustre of anodised aluminium, painted aluminium uses multi-layer systems with metallic pigments with the risk of creation of colour variations, including metameric failure.
Excellent corrosion resistance
Even in its natural state, aluminium does not corrode in the same, destructive, way as iron or steel. Corrosion of aluminium, however, permanently damages the aspect of the metal. The natural oxide layer of aluminium provides a self defence against corrosion.
However, this natural oxide layer of aluminium is thin, irregular and unstructured with only superficial corrosion protection. Anodising creates a thick, perfectly formed and scientifically controlled oxide layer, which ensures a surface of unparalleled corrosion resistance and locks in the pure and natural metallic aspect of the metal. Anodising has been used for external building
applications for over 60 years. With an appropriate anodic layer thickness for external use, anodised aluminium will perform without problem, even in the most severe environments. In particular, anodised finishes are highly durable in city and marine environments, due to their resistance to chlorides and sulphates.
Proven durability + technical continuity
The technology of anodising has been established for almost 100 years. Whilst the process has been continuously refined, especially in terms of quality, the chemical and technological fundamentals remain unchanged. The on-site behaviour of the anodised surfaces produced today is fully predictable, based on the proven, long term, service experience. Independent inspection commissioned by United Anodisers has confirmed that the anodic protection remains undiminished after more than 30 years.
The chemistry of organic coating is much more recent and, further, due to legal and environmental constraints, has been subject to continuing modifications. As a result, the long term, on-site behaviour of these modern coatings is not proven. Indeed, even some landmark buildings completed in the last 10 years with organic coatings are already showing signs of surface deterioration such as adhesion failure, fading, chalking and filiform corrosion
No risk of adhesion failure
Anodising is an electrolytic process, which transforms the surface of the metal into an oxide layer integral with the metal itself. It is not a coating applied to the surface of the metal.
Because it is only a coating, painted aluminium can suffer surface failure over time
There is, therefore, no risk of adhesion failure such as peeling, blistering, cracking, splitting or chipping which may occur with surface coatings, such as painting. There will be no adhesion failure, even on cut edges or joints. Certain guarantees for polyester paint specifically exclude adhesion failure.
No possibility of fading
Natural silver, bronze, Anolok I and Anolok II blue-grey anodised finishes contain no organic elements. No colour fading is possible during the service life of these finishes. Organic coatings are always subject to fading in varying degrees over the lifetime of a building.
No possibility of filiform corrosion
Filiform corrosion is an attack on the hidden inter-layer between the aluminium and the surface finish which results in the propagation of corrosion under the surface finish. It cannot be remedied. It originates from surface defects, surface damage or scratches. With anodising, the oxide (anodic) layer is integral with the aluminium. There is no inter-layer between the metal and this protective oxide layer. As a result, the finish will remain free from filiform corrosion. If the surface is punctured or damaged, the aluminium will simply repair itself through natural oxidation. The absence of an inter-layer zone ensures that there can be no corrosion as found with painted products.
No chalking
Chalking is the formation of a fine powder on the surface of the paint film during weathering. It can cause significant deterioration in surface appearance, with a reduction in the gloss level, surface lustre and colour. Anodised aluminium does not suffer from this disfiguring condition, even when exposed to difficult exterior environmental conditions, such as marine locations or very
sunny and/or humid climates.
Uniform surface finish for all metal shapes and textures
Anodising is a process which can be carried out prior to or after the transformation of the metal. Since it is an electro-chemical immersion process rather than an applied finish, anodising provides a particularly uniform and regular finish on most metal shapes and textured surfaces, such as perforated sheets, tread plates, linished or brushed metal, mesh material, stucco surfaces etc.
Anodising avoids the risk of localised high coating thickness or orange peel effect, typically associated with organic coatings on such surfaces.
Optimum coverage
Anodising, as an electro-chemical immersion process, achieves a more uniform surface coverage, especially for extruded sections. With spray paint processes and, especially with powder-coating, the non-visible surface is often not coated.
Highly impermeable anodic layer
A properly sealed anodic layer is impermeable. An organic coating is always porous. Furthermore, when severe environmental temperature fluctuations occur, the anodic layer is not subject to damaging physical changes and consequential embrittlement. With an organic coating, these changes may occur when the coating is subjected to temperature cycles above and below its glass transition temperature – the pivot temperature at which the mechanical properties of the organic coating will change from elastic to
brittle.
Abrasion resistance
Aluminium oxide is a very hard compound which is second only to diamond, on the Mohs scale of mineral hardness. Anodised aluminium surfaces offer, therefore, superior scratch and abrasion resistance than coated surfaces.
Anodised surfaces are very hard – 80% of the hardness of diamond
Quality controls
The tightly controlled quality assurance in the anodising process can be followed up on site without damage to the anodised surface.
Environmental benefits of pure anodised aluminium
One of the key advantages of aluminium over other materials is that it is can be recycled repeatedly through simple re-melting with minimal loss on each cycle. In Europe, more than 30% of aluminium consumption is satisfied with recycled aluminium and over 90% of the aluminium used in buildings is recycled at the end of the building’s life. The recycling process requires only 5% of the energy required for the production of primary aluminium – a significant contribution to environmental sustainability.
Anodised aluminium is unique, comprising only pure aluminium, its alloying elements and oxygen. As pure aluminium, it is fully recyclable without intervening chemical processes and emissions. Because of this and the ready market for quality aluminium scrap, the anodised aluminium will have a cash value to offset the cost of demolition at the end of the useful life of the building.
Fully recyclable, anodised aluminium answers all the environmental imperatives of today and tomorrow
Modern architectural anodising plants are capable of responding to the most stringent environmental standards with full chemical recycling and a large recovery of waste water. Anodising does not require environmentally sensitive pre-treatment like chromating.
The anodising process does not produce CO2 or solvent emissions.
Guarantees
Guarantees are often considered by specifiers or building owners to provide reassurance about the long term performance of a surface finish. In recent years, there has been a trend of extending guarantees for longer and longer periods as a way of outbidding competing surface finishes. But, guarantees have often been full of exclusions and reservations. Invoking guarantees, often years after the event, frequently involves costly litigation. Further, even if the guarantee is successfully invoked, replacement of defective parts may be impossible without partial demolition of the building.
Where this is not practical, the owner of the building will be simply left with the problem over the long term; legal recourse for aesthetic damage is notoriously difficult to assess. Surface failure may render a building impossible to sell or otherwise seriously blight its open market value.
Historically, quality claims against leading architectural anodisers have been negligible. In the highly unlikely event of a claim, the anodiser alone bears full responsibility for all properties of the anodic layer. Guarantees are important, but only if they are simple, straightforward and long term – this is only possible when they can be given backed with the full confidence of proven, long-term, on-site performance.
As a result of recent independent surveys of long term on-site performance of anodising - including in polluted and marine environments - a simple, straightforward, unequivocal design life guarantee for anodising is available for the first time, exclusively from United Anodisers.
Guarantees for organic coatings are not backed by the same site exposure history. For any organic coating, the responsibility for any failure is necessarily divided between the paint manufacturer and the paint applicator. The guarantees available from the manufacturers of polyester powder specifically exclude adhesion.
Selection of the architectural anodiser
In order to obtain all the advantages of anodising described above, it is of key importance that both the aluminium and the anodising treatment are perfectly suited to the performance requirements of both buildings and their surrounding environment.
United Anodisers is the world leader amongst an elite group of anodisers specialising in architectural applications, with the capability to ensure a homogeneous, high quality architectural finish with a lifetime performance.
For further information and advice on anodised aluminium applications for architectural projects, please contact United Anodisers
Introduction
Modern materials must permit design creativity but at the same time they must be:
produced responsibly and economically
durable, and
recyclable at the end of their life
Aluminium, with its exceptional recyclability, minimal maintenance and proven lifetime performance, successfully responds to the need for environmentally friendly and sustainable solutions. In building applications, aluminium virtually always requires surface treatment. Yet, the choice of surface treatment of aluminum is as important as the choice of the material itself.
Anodising enhances the natural qualities of aluminium still further; it permits a strongly contemporary finish with incomparable corrosion and abrasion resistance. Unique amongst surface treatments, such as coatings, anodising is totally integrated with the metallic substrate – it is not simply a film applied to cover the surface. The result – pure aluminium – is a perfectly and
repeatedly recyclable material with remarkable performance characteristics.
These same exceptional qualities can be found in new product applications in anodising, which now provide new and infinite texture and design possibilities for building exteriors. The modern, aesthetic beauty of anodised aluminium is founded on its many
unique properties. Anodising is a transparent finish integral with the metal, which retains and enhances the inherent beauty of the aluminium; it is totally UV resistant.
A totally homogeneous appearance can be guaranteed, thanks to the nature of the anodising process and the very tight quality controls on both the aluminium and the surface treatment; Colours, textures and patterns can be combined with the anodic film,
enhancing the natural metallic appearance without affecting the total UV resistance.
The "living" quality of its natural metallic sheen, combining texture with colour, guarantees a creative interaction of the surfaces and shapes of the building with the constantly changing light conditions through the day and across the different seasons. This effect can be accentuated by employing textures and colours.
The anodic layer will retain its original beauty and protection against corrosion throughout the life of the building – as proven by independent inspection of buildings of more than 30 years old.
The following information provides owners, developers, architects, system companies, specifiers and contractors, with an in-depth understanding of the proven benefits of anodised aluminium and explains why this is increasingly the material of choice for buildings of the future:
Authentic metallic lustre and feel
The anodic layer is transparent and integrated in the surface of the metal; the natural metallic appearance and feel of the aluminium are fully preserved. The anodised finish will enhance and reinforce the natural beauty of aluminium to create a living surface constantly interacting with the natural or artificial light playing across its surface
Regardless of the substrate material – aluminium, steel, plastic – a painted surface always has the same, flat appearance. In trying to achieve the authentic metallic lustre of anodised aluminium, painted aluminium uses multi-layer systems with metallic pigments with the risk of creation of colour variations, including metameric failure.
Excellent corrosion resistance
Even in its natural state, aluminium does not corrode in the same, destructive, way as iron or steel. Corrosion of aluminium, however, permanently damages the aspect of the metal. The natural oxide layer of aluminium provides a self defence against corrosion.
However, this natural oxide layer of aluminium is thin, irregular and unstructured with only superficial corrosion protection. Anodising creates a thick, perfectly formed and scientifically controlled oxide layer, which ensures a surface of unparalleled corrosion resistance and locks in the pure and natural metallic aspect of the metal. Anodising has been used for external building
applications for over 60 years. With an appropriate anodic layer thickness for external use, anodised aluminium will perform without problem, even in the most severe environments. In particular, anodised finishes are highly durable in city and marine environments, due to their resistance to chlorides and sulphates.
Proven durability + technical continuity
The technology of anodising has been established for almost 100 years. Whilst the process has been continuously refined, especially in terms of quality, the chemical and technological fundamentals remain unchanged. The on-site behaviour of the anodised surfaces produced today is fully predictable, based on the proven, long term, service experience. Independent inspection commissioned by United Anodisers has confirmed that the anodic protection remains undiminished after more than 30 years.
The chemistry of organic coating is much more recent and, further, due to legal and environmental constraints, has been subject to continuing modifications. As a result, the long term, on-site behaviour of these modern coatings is not proven. Indeed, even some landmark buildings completed in the last 10 years with organic coatings are already showing signs of surface deterioration such as adhesion failure, fading, chalking and filiform corrosion
No risk of adhesion failure
Anodising is an electrolytic process, which transforms the surface of the metal into an oxide layer integral with the metal itself. It is not a coating applied to the surface of the metal.
Because it is only a coating, painted aluminium can suffer surface failure over time
There is, therefore, no risk of adhesion failure such as peeling, blistering, cracking, splitting or chipping which may occur with surface coatings, such as painting. There will be no adhesion failure, even on cut edges or joints. Certain guarantees for polyester paint specifically exclude adhesion failure.
No possibility of fading
Natural silver, bronze, Anolok I and Anolok II blue-grey anodised finishes contain no organic elements. No colour fading is possible during the service life of these finishes. Organic coatings are always subject to fading in varying degrees over the lifetime of a building.
No possibility of filiform corrosion
Filiform corrosion is an attack on the hidden inter-layer between the aluminium and the surface finish which results in the propagation of corrosion under the surface finish. It cannot be remedied. It originates from surface defects, surface damage or scratches. With anodising, the oxide (anodic) layer is integral with the aluminium. There is no inter-layer between the metal and this protective oxide layer. As a result, the finish will remain free from filiform corrosion. If the surface is punctured or damaged, the aluminium will simply repair itself through natural oxidation. The absence of an inter-layer zone ensures that there can be no corrosion as found with painted products.
No chalking
Chalking is the formation of a fine powder on the surface of the paint film during weathering. It can cause significant deterioration in surface appearance, with a reduction in the gloss level, surface lustre and colour. Anodised aluminium does not suffer from this disfiguring condition, even when exposed to difficult exterior environmental conditions, such as marine locations or very
sunny and/or humid climates.
Uniform surface finish for all metal shapes and textures
Anodising is a process which can be carried out prior to or after the transformation of the metal. Since it is an electro-chemical immersion process rather than an applied finish, anodising provides a particularly uniform and regular finish on most metal shapes and textured surfaces, such as perforated sheets, tread plates, linished or brushed metal, mesh material, stucco surfaces etc.
Anodising avoids the risk of localised high coating thickness or orange peel effect, typically associated with organic coatings on such surfaces.
Optimum coverage
Anodising, as an electro-chemical immersion process, achieves a more uniform surface coverage, especially for extruded sections. With spray paint processes and, especially with powder-coating, the non-visible surface is often not coated.
Highly impermeable anodic layer
A properly sealed anodic layer is impermeable. An organic coating is always porous. Furthermore, when severe environmental temperature fluctuations occur, the anodic layer is not subject to damaging physical changes and consequential embrittlement. With an organic coating, these changes may occur when the coating is subjected to temperature cycles above and below its glass transition temperature – the pivot temperature at which the mechanical properties of the organic coating will change from elastic to
brittle.
Abrasion resistance
Aluminium oxide is a very hard compound which is second only to diamond, on the Mohs scale of mineral hardness. Anodised aluminium surfaces offer, therefore, superior scratch and abrasion resistance than coated surfaces.
Anodised surfaces are very hard – 80% of the hardness of diamond
Quality controls
The tightly controlled quality assurance in the anodising process can be followed up on site without damage to the anodised surface.
Environmental benefits of pure anodised aluminium
One of the key advantages of aluminium over other materials is that it is can be recycled repeatedly through simple re-melting with minimal loss on each cycle. In Europe, more than 30% of aluminium consumption is satisfied with recycled aluminium and over 90% of the aluminium used in buildings is recycled at the end of the building’s life. The recycling process requires only 5% of the energy required for the production of primary aluminium – a significant contribution to environmental sustainability.
Anodised aluminium is unique, comprising only pure aluminium, its alloying elements and oxygen. As pure aluminium, it is fully recyclable without intervening chemical processes and emissions. Because of this and the ready market for quality aluminium scrap, the anodised aluminium will have a cash value to offset the cost of demolition at the end of the useful life of the building.
Fully recyclable, anodised aluminium answers all the environmental imperatives of today and tomorrow
Modern architectural anodising plants are capable of responding to the most stringent environmental standards with full chemical recycling and a large recovery of waste water. Anodising does not require environmentally sensitive pre-treatment like chromating.
The anodising process does not produce CO2 or solvent emissions.
Guarantees
Guarantees are often considered by specifiers or building owners to provide reassurance about the long term performance of a surface finish. In recent years, there has been a trend of extending guarantees for longer and longer periods as a way of outbidding competing surface finishes. But, guarantees have often been full of exclusions and reservations. Invoking guarantees, often years after the event, frequently involves costly litigation. Further, even if the guarantee is successfully invoked, replacement of defective parts may be impossible without partial demolition of the building.
Where this is not practical, the owner of the building will be simply left with the problem over the long term; legal recourse for aesthetic damage is notoriously difficult to assess. Surface failure may render a building impossible to sell or otherwise seriously blight its open market value.
Historically, quality claims against leading architectural anodisers have been negligible. In the highly unlikely event of a claim, the anodiser alone bears full responsibility for all properties of the anodic layer. Guarantees are important, but only if they are simple, straightforward and long term – this is only possible when they can be given backed with the full confidence of proven, long-term, on-site performance.
As a result of recent independent surveys of long term on-site performance of anodising - including in polluted and marine environments - a simple, straightforward, unequivocal design life guarantee for anodising is available for the first time, exclusively from United Anodisers.
Guarantees for organic coatings are not backed by the same site exposure history. For any organic coating, the responsibility for any failure is necessarily divided between the paint manufacturer and the paint applicator. The guarantees available from the manufacturers of polyester powder specifically exclude adhesion.
Selection of the architectural anodiser
In order to obtain all the advantages of anodising described above, it is of key importance that both the aluminium and the anodising treatment are perfectly suited to the performance requirements of both buildings and their surrounding environment.
United Anodisers is the world leader amongst an elite group of anodisers specialising in architectural applications, with the capability to ensure a homogeneous, high quality architectural finish with a lifetime performance.
For further information and advice on anodised aluminium applications for architectural projects, please contact United Anodisers
Standards reference
BS EN 12373-1:2001
Sulphuric anodising; Film thickness is specified by referring to a specific class;
Class 5 Min av thk 5 μm
Class 10 Min av thk 10 μm
Class 15 Min av thk 15 μm
Class 20 Min av thk 20 μm
Class 25 Min av thk 25 μm
Surface preparation before anodising is designated by the symbols E0 to E8. Sealing either Hot water or nickel sealing as specified.
BS EN 2101:1991
Chromic acid anodising;
Alloy category 1 Min film thickness 2.5 μm
Alloy category 2A Min film thickness 1.5 μm
Alloy category 2B Min film thickness 1.0 μm
Sealing; Type A = Unsealed, Type B = Hot water sealed, Category 2 alloys shall preferable be dichromate sealed
BS EN 2284:1991
Sulphuric acid anodising;
Class A – Unsealed anodising
Class B – Sealed anodising
Thickness class 1 – 12 to 25 μm
Thickness class 2 – 6 to 12 μm
Sealing as specified; Dyed aluminium – hot water seal, Undyed aluminium – hot water seal or dichromate seal.
BS EN 2536:1995
Hard anodising;
Category 1 alloys < 1 % Cu : 30 μm to 120 μm film thickness
Category 2 alloys 1% to 5% Cu : 30 μm to 60 μm film thickness.
Note: Restrict thickness on splines & threads to 25 μm. Sealing is specified either hot water or dichromate seal
BS 5599:1993(2002)
Hard anodising;
Unless otherwise specified on the drawing or order thickness shall be 40 to 60 μm, sealing shall be as specified.
Sulphuric anodising; Film thickness is specified by referring to a specific class;
Class 5 Min av thk 5 μm
Class 10 Min av thk 10 μm
Class 15 Min av thk 15 μm
Class 20 Min av thk 20 μm
Class 25 Min av thk 25 μm
Surface preparation before anodising is designated by the symbols E0 to E8. Sealing either Hot water or nickel sealing as specified.
BS EN 2101:1991
Chromic acid anodising;
Alloy category 1 Min film thickness 2.5 μm
Alloy category 2A Min film thickness 1.5 μm
Alloy category 2B Min film thickness 1.0 μm
Sealing; Type A = Unsealed, Type B = Hot water sealed, Category 2 alloys shall preferable be dichromate sealed
BS EN 2284:1991
Sulphuric acid anodising;
Class A – Unsealed anodising
Class B – Sealed anodising
Thickness class 1 – 12 to 25 μm
Thickness class 2 – 6 to 12 μm
Sealing as specified; Dyed aluminium – hot water seal, Undyed aluminium – hot water seal or dichromate seal.
BS EN 2536:1995
Hard anodising;
Category 1 alloys < 1 % Cu : 30 μm to 120 μm film thickness
Category 2 alloys 1% to 5% Cu : 30 μm to 60 μm film thickness.
Note: Restrict thickness on splines & threads to 25 μm. Sealing is specified either hot water or dichromate seal
BS 5599:1993(2002)
Hard anodising;
Unless otherwise specified on the drawing or order thickness shall be 40 to 60 μm, sealing shall be as specified.