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Anodising Facts & Tips

Fletcher Aluminium in Auckland offers anodising film thicknesses of between 12 and 25 microns for extruded aluminium shapes. These thicknesses cover the standards required for domestic and commercial architecture. Corrosion resistance increases with film thickness.

Before deciding on an anodised finish for your extruded aluminium product, you need to know about factors that affect final appearance and long term durability. This section explains some of these factors, along with the production standards we adhere to.

We recommend you talk to our experienced Custom Solutions team for more detailed advice and recommendations about how to achieve the outcome you want.

Anodic film is substantially transparent

Unlike painting, parent metal surface remains visible through the coating. So the surface of the extruded aluminium surface should be of the best possible quality before anodising.

Anodising does not remove all defects

The cleaning and etching process does not remove defects such as deep die lines, surface abrasions and corrosion. In fact, these defects are often exaggerated. So it's important to control damage risks during production, storage, handling and transport of extruded aluminium.

Because of this factor, if you want us to anodise material from your own supply, we may not always be able to guarantee good appearance. However, we will make recommendations and do everything we can to achieve the best possible finish for you.

Film thickness affects appearance

As anodic film becomes thicker, the film also becomes duller and less transparent. For a good match in appearance, particularly with coloured material, it's best to avoid mixed film thickness.

Heat joining methods affect colour

Weld filler metal and the heat affected zones of weldments anodise to a different colour than the parent metal. This fact must be taken into account when anodising welded structures.

Temper (hardness) and metal types affect colour

T4 and T5 tempers anodise to a different shade, as do sheet metal and extruded metal. The sheet alloys 5005 and especially 5205 provide the best match with 6060 T5 (the preferred extruded aluminium alloy for anodising). Cast material is almost impossible to match with wrought metal (sheet or extrusion).

Cleaning products and environmental factors may cause corrosion

The anodic film on aluminium is one of the most corrosion resistant coatings available, but it is not indestructible.

Strong acid or alkaline material will seriously corrode the coating. In service, it’s essential to avoid:

• Contact with brick or glass cleaners (acid) or alkaline cleaners
• Contact with wet building materials such as plaster or cement and unprotected concrete (alkaline)
• Paint splashes – as attempts to remove them using paint stripper will cause corrosion
• Aggressive scouring type cleaners

We recommend you treat all commercial cleaning products with suspicion and do patch tests before using them extensively on anodised aluminium.

Run off from dissimilar metal building components such as copper guttering or downpipes can also cause serious corrosion.

Dissimilar metal fixings should be properly insulated from anodised aluminium, particularly in a severe environment. Use only good quality, proven sealants.

Reasonably resistant to abrasion

Anodised film is about 4-5 times harder than the aluminium alloy substrate. The greatest hardness is found at the surface of the film, which is capable of marking glass and steel. The aluminium alloy's substrate is not altered by anodic treatment.

Anodic film is therefore quite resistant to abrasion. In fact, specific abrasion tests have found that anodised aluminium is more abrasion resistant than hardened glass. Rubbing type abrasion, particularly with hard scouring material, is more damaging than blast type abrasion – for example, waterblasting.

Bending may cause crazing and reduced corrosion resistance

Anodic films cannot be permanently deformed (bent out of shape) without crazing, which also reduces the film's corrosion resistance. We recommend you avoid bending or forming anodised extruded sections, except for sections with very low thickness film.

Films with a thickness of less than 6 microns may be formed for articles such as windscreen trim (FORD specification). But above this figure, flexibility reduces very quickly. You must always use rounded tools and good lubrication.

ISO and Telarc accreditation

Our anodising plant operates in accordance with Fletcher Aluminium’s ISO 9001-accredited quality management system. Fletcher Aluminium also has ISO 14001 environmental management system accreditation.

The plant's quality and environmental management systems are registered with Telarc – New Zealand’s largest certifier of quality, environmental, and health and safety management systems.

Window Association of New Zealand (WANZ) Standards

Fletcher Aluminium’s anodised finishes comply with the Window Association of New Zealand Voluntary Specification: WANZ SFA 3503-03, along with most other international specifications.