What is Anodising?

Anodising is an electrolyte process which is used to increase the thickness of the oxide coatings for metal, more specifically with Aluminium and its alloys.

Specifications & Alloys

Does anodising affect the mechanical properties of aluminium?

Mechanical properties of aluminium, such as ultimate strength and yield strength, along with percent elongation, reportedly have been tested before and after anodising without exhibiting any difference in the strength of the aluminium. Aluminium is a very soft metal and if not protected can be abraded away, whereas the anodic oxide is much harder.

Anodising services available at Nevilles Precision Engineering.

Why do aluminium parts I send to be black anodised sometimes come back with a purple hue, and how can this be avoided?

The purple hue is an iridescent colour–the result of a very thin film on the surface of the black anodised part. Other iridescent colours can be produced, and can appear even on clear anodised parts, but are most evident on black anodising.

An analogy is a black asphalt road after rain. A droplet of oil causes iridescent rings that are quite apparent on black asphalt but can hardly be seen on a light-coloured concrete road. There are a number of possible causes of the film. One is sealing smut. Hot water sealing blocks the pores of the anodic coating, thus improving its weathering resistance, reducing its adsorptive properties, and sealing in any pigment. However, some sealing product forms on the outer surface of the anodic coating. This is sealing smut. It consists of very small, needle-like crystals of aluminium hydroxide. They act as a thin film that can generate iridescent colours. Anti-smut additives for sealing baths are available, which minimize the problem.

The formation of smut can be favoured by high pH. The bath chemistry should be controlled as recommended by the supplier of the sealing additive. Alternatively, the smut can be wiped off. Before the invention of anti-smut additives, people used to wipe down parts with lanolin in white spirit. This temporarily masks the smut as much as it removes it. Manual removal is time-consuming and can be hard work.

Anodising under conditions that are too aggressive can lead to iridescent effects. In particular, if the bath temperature is too high, the surface region of the porous anodic coating (the part of the coating that was first formed during anodising) can be dissolved in the acid solution to the extent that it is very much more porous. This is called a “soft” coating.

In general, anodising at over 75°F for more than 45 minutes can produce a soft coating. A simple abrasion test to detect a soft coating is described in British Standard 6161: Part 18: 1991. Also, purple iridescence has been seen with certain types of black dyed parts if the film thickness is too low or the dye bath is inadequately controlled. The advice of the supplier of the dye should be followed.

What about rack marks?

Electrical contact must be made to each part that is anodised. The more electrical current required, the bigger the electrical contact must be. The size of the contact therefore depends on the anodising process and the size of the part being anodised.

Can anodising hide scratches?

The usual rule of thumb is that if you can feel a scratch by rubbing your fingernail across the surface, you will be able to see the scratch after anodising. It is always helpful for the finisher to understand the application. It is also good for the finisher and client to agree on a viewing distance. If a part is to be viewed from 10 feet away, like a window or roofing component, then the inspection may be relatively insensitive to scratches. However, if the part is to be viewed from 24″ or closer, then even a scratch which you cannot feel may be unacceptable.

Can I weld anodised aluminium?

Parts can be welded prior to anodising. The use of 5356 welded rod is strongly recommended, though some discolouration will still occur. 4043 is the worst choice because it will turn a smutty black when anodised. Grinding away the weld before anodising will result in decreased mechanical integrity and will not solve the appearance variation problem. It is not a good idea to weld after anodising. Because most welding process require electrical conductivity the anodic coating must be ground away where the weld will be applied.

This normally results in an unsightly mess around the welding area.

Why do weld areas on some parts look different than the rest of the part? How can such anomalies be minimised?

Welds can discolour for a couple of different reasons. First, the metallurgy of the welding wire is different than that of the alloy being welded. Since the finish produced in the anodising process is somewhat dependent on the metallurgy, the metallurgical difference will show up as a shade difference.

Secondly, during the welding process a significant amount of heat is built up around the weld. This heat build-up actually changes the temper of the aluminium immediately surrounding the weld bead. Since a temper change is really a metallurgical change, again this shows up after anodising as a colour difference. These areas are commonly called halos or ghosts. There are a few things that can be done to minimise the colour differences.

First, excellent results may be produced by using welding wire alloy 5356. This alloy reportedly produces the best colour or shade match when used to weld 6xxx series alloys. The second problem, concerning halos or ghosts, is a little more difficult to solve. Try using as little heat as possible to accomplish the job. This can be something of an art and is dependent on the individual doing the work.

Another possibility is to put the aluminium that is being welded in contact with a chill block that will draw the heat away from the working area. One other note: Some people grind the weld bead smooth and mechanically finish the weld area in an attempt to avoid the colour difference. This practice, however, will not help hide the discolouration.

At Nevilles, we offer Anodising Aluminium services with a variety of finishes.

How should I specify my anodised finish?

There are a number of different items that can be specified when ordering anodised aluminium. What you specify depends upon your product requirements. Some of the most common items that are specified are as follows:

  • Thickness or weight per area of the anodised coating
  • Colour of the anodised coating
  • Gloss of the finish
  • Quality of the seal of the anodised coating
  • Corrosion resistance measured by salt spray testing
  • Aluminium alloy used and its temper

What alloys are recommended for anodising?

Most aluminium alloys will build aluminium oxide in an anodising tank, so the answer to this question depends on the anodising process and the desired result. Copper containing 2000 series are generally the most difficult to anodise and 5000 or 6000 series are the easiest.

What anodise coating thickness should I specify for my product?

You can specify a range of coating thickness from 0.00001″ to 0.005″ based on what your product is to be used for and how you want it to look. In general, thicker coatings are used for products to be used outside or in corrosive environments, and thinner coatings are used for parts to be used in interior applications.

When anodised products are to be used out of doors, anodised film thickness is usually specified at 0.0004″ minimum or 0.000700″ minimum.

Does exterior anodised aluminium appear to change colour under different light conditions?

Many factors influence the appearance of anodised aluminium architectural components. Many architects and designers find that to be one of the attractive features of anodised aluminium: their buildings assume a slightly different character depending upon the weather, the time of day, the season of the year, or the angle of observation.

Differences in appearance will also be influenced by the alloy, anodic film thickness, surface texture of the aluminium from the mill supplier (mill finish surface roughness, longitudinal versus transverse surface roughness), and the process of the anodises (clean and anodise, etch and anodise, bright dip and anodise, and combinations).

Critical to all this is the angle of observation, especially as it relates to the primary source of illumination. If the product is coloured, another level of complexity is added, since the shade and hue must also be controlled. Furthermore, if the material is being used as a light reflector, very specific photometric specifications are usually required.

I understand that the colours on anodised aluminium may derive from metallic salts or from organic dyes. How light-fast are these colourants in an indoor application?

Generally speaking, organic dyes are well suited to indoor applications and have enjoyed such use for over 50 years.

Their light-fastness, however, depends on the dye chosen, the amount of dye that is actually contained by the coating, and the conditions of seal. Two-step electrolytic colouring uses metallic salt solutions to produce bronze or black colours that are sufficiently light-fast for exterior, as well as interior, applications.

Colours produced using organic dyes are often susceptible to change due to the effects of ultraviolet (UV) rays from sunlight. However, they should be satisfactory in indoor applications where they are not exposed to sunlight.

Some organic dyed anodised aluminium can show significant colour change within one year of outdoor exposure. If we assume that is 10 hours exposure to sunlight per day, then indoor finishes may be able to withstand one hour of sunlight per day for 10 years.

Anodising is useful for thickening the oxide coatings of Aluminium.

Can anodised aluminium be cleaned with a detergent that has a pH of 10, or would that result in scaling or other surface problems?

Factors such as concentration of the detergent solution, duration of exposure, and temperature will influence the results.

If the anodised aluminium is cleaned at room temperature and promptly rinsed with clean water, then there should be no problem. If cleaned at an elevated temperature or with prolonged exposure without rinsing, then the cleaning solution would start to attack the anodic oxide and etch the metal.

Mild soap is generally preferable to detergent for routine maintenance cleaning.

We have Anodising services available at Nevilles Engineering.

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