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Anodizing

• Anodizing v Powder Coating
• European Standards v British Standards v AASC standards
• Qualanod v Acceptance Testing
• Architectural Advisory Service Centre (AASC)

Anodizing v Powder Coating

Advantages of anodizing over powder coating

• Proven durability - Anodizing has been in use for over 60 years whereas powder coating has been in use for around 30 years. Longer guarantees (25 years) are more readily available for anodizing than powder coating.
• Integral finish by dip process - No adhesion problems and all surfaces are coated.
• Fade-free - Better long-term colour retention.
• Resistant to chlorides and sulphates - More durable in city and marine environments.
• Extremely hard surface - Less prone to mechanical damage. Neglected surfaces can also be restored by abrasive cleaning.
• Closer control of film thickness - Sharp corners and details can be better retained.
• Non-destructive testing - Allows testing of installed metalwork.

Advantages of powder coating over anodizing

• Wider colour range - More choice, although colour range for anodizing is increasing.
• Consistency of colour-matching - Fewer visual appearance problems although new anodizing technology is improving colour-matching.
• Wider choice of metals - Anodizing is specific to aluminium whereas powder coating can be used over steel. Powder coating is also more tolerant of poorer quality aluminium.
• Different gloss levels are available - More visual choice.
• Easier to touch-up - Site remedial work is possible.
• Fabrications can be coated - Not advisable with anodizing unless welding is carried out to very high standards.
• Cheaper and more widely available - This is primarily a commercial consideration due to an over-capacity in the powder coating market.

Relative costs

• Natural (silver) anodizing is approximately the same cost as non-metallic powder coating.
• Bronze anodizing is approximately twice the cost of non-metallic powder coating.
• Grey anodizing is approximately three times the cost of non-metallic powder coating.
• The relative prices are sometimes further distorted - to make powder coating seem even cheaper - when there is over-capacity in the powder coating market.
• ... but remember that the cost of the finish will be a very small percentage of the total cost of the system into which the coated metal is incorporated.

European Standards v British Standards v AASC standards

Many major architectural practices base their anodizing specifications on advice provided by the AASC and are frequently asked to justify these specifications against both BS 3987 and DIN 17611 (the most common European Standard).

Comparison of anodising standards (film thickness)

• DIN 17611 : 20 microns average with a local minimum of 16 microns. 25 microns average may be agreed for aggressive environments.
• BS 3987 : 25 microns average with a local minimum of 20 microns.
• AASC : Not less than 25 microns and not more than 35 microns.

European Standards v British Standards - The issues

• It is generally accepted that the UK climate is, for a variety of reasons which are not fully understood, more aggressive than the typical climate in continental Europe, and any proposed change of specification from BS 3987 to DIN 17611 should be resisted.
• The best quality DIN Standard requires an average thickness of 20 microns, whereas the BS requires an average thickness of 25 microns. This is a significant difference and it is generally accepted that a 25 micron finish is essential for the British environment.
• The DIN Standard has no requirement for abrasion resistance testing and this is also considered essential for the British environment.
• These issues are compounded when using anodizers in continental Europe because, although technically possible, it is not a straightforward process to re-tune a plant operating to DIN standards to produce anodising to British Standards. The most significant requirement is for the additional cooling which will allow production of a 25 micron thickness without softening of the coating.
• In principle, if you are offered a cost saving by the use of a continental European anodizer, then beware. If, however, the anodizer clearly understands the requirements of BS 3987, and can demonstrate evidence of previous compliance with an independent testing regime to BS 3987, then you will probably be OK. There are a small number of continental anodizers who are quite capable of complying with either BS 3987 or the further-enhanced AASC standard.

British Standards v AASC Standards - The issues

• The AASC view, based on many years of experience, is that a minimum film thickness of 25 microns provides significantly enhanced durability when compared with the BS requirement for 25 microns average, and can often be achieved for no additional cost.

European anodising companies

The following European companies are known to have produced anodizing to comply with BS 3987:

• AGRU Oberflaechentechnik GmbH - Austria - http://www.agru.net/
• Burox AG - Switzerland.
• BWB Group - Switzerland - http://www.bwb.ch/
• Josef Gartner GmbH - Germany - http://www.josef-gartner.com/
• HD Wahl GmbH - Germany - http://www.hdwahl.de/

Are tin electrolytes acceptable?

• The AASC specification for bronze and black anodizing prohibits the use of tin electrolytes although these are widely used in Europe - the two main UK anodizers use cobalt electrolytes. This prohibition has been regularly questioned by cladding contractors who wish to source their anodized sections from European suppliers.
• The reasoning behind this prohibition is that tin electrolytes generally produce a surface with a lower abrasion resistance than cobalt electrolytes and this is particularly critical for dark colours.
• It is, however, possible to comply with the abrasion resistance requirements of BS 3987 using tin electrolytes for natural anodising and a draconian ban on the use of tin may be both unnecessary and contractually unacceptable.

Qualanod v Acceptance Testing

• Qualanod is a European industry-run quality control system.
• Although membership of Qualanod is evidence of a commitment to quality control, it is not a substitute for independent AcceptanceTesting.

Architectural Advisory Service Centre (AASC)

• The AASC is an industry-sponsored organisation offering authoritative advice on all aspects of architectural metal finishing.
• The AASC has provided assistance to many major architectural practices and their clients, advising on the suitability of various finishes and providing assistance in specification-writing and testing programmes.
• The AASC website is at http://www.aasc.org.uk/

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