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Frequently Asked Questions

Does anodizing affect the mechanical properties of aluminum?

Mechanical properties of aluminum, such as ultimate strength and yield strength, along with percent elongation, reportedly have been tested before and after anodizing without exhibiting any difference in the strength of the aluminum. Aluminum is a very soft metal and if not protected can be abraded away. The anodic oxide is much harder that the aluminum and the anodize process is used on parts like pistons so that the parts will last much longer.

We have commercial 380 aluminum alloy, die cast into aluminum wiper arm heads, which are sulfuric acid anodized for paint adhesion (electrodeposited epoxy primer) and for corrosion protection (scribed salt spray). In the finished anodized parts we see mottled brownish areas in the dark uniform gray anodized finish. What are these areas? Die lubricants? Chemical segregation? Microstructure variation?

Alloy 380 is a high-silicon alloy (about 8 percent). The solubility of silicon in aluminum is about 1.5 percent. When the casting cools, silicon precipitates out and forms these gray mottled areas. Silicon does not anodize, so the adhesion around these areas is likely to be poor. To minimize this problem, the surface finisher should pretreat the casting in a nitric acid/ammonium bifluoride solution under well-ventilated conditions. Doing so will effectively dissolve much of the surface silicon and thereby enrich the surface with anodizable aluminum.

We anodize mostly aluminum products for interior use. However, some material is for an exterior application. We are looking for a simple method to test lightfastness. Is there a quick and easy test to get a lightfast rating?

The best test for lightfastness is to expose production color anodized panels outdoors, facing south, positioned on a rack that is tilted 45 degrees, in an area that is not shaded. Usually after a month or so the exposed panels can be compared to sections of the same anodized panels that were retained indoors to get an idea of how well they will perform.

Alternatively, a bench-top accelerated exposure device (approximate cost $10,000) or a full-size carbon-arc or xenon-arc light exposure apparatus (approximate cost $40,000) will yield information in a shorter period of time. The latter provides lightfastness data that can be referenced to military specification MIL-8625F, which calls out 200 hours' exposure. It has been found that 200 hours in this unit correlates to about 80 days' exposure to natural light and weathering, based on an annual worldwide average. Weathering services are also available (e.g., Atlas Weathering Services, 800/255-3738).

Information on light-fastness can usually be obtained from the dye supplier. Customers' needs and product applications should be discussed with the dye supplier before tests are run. Light-fastness of an anodized part depends on many factors--not just the choice of dye. Equally important are the porosity and thickness of the anodic film, the degree of saturation, as well as the type and quality of the seal.

How should I specify my anodized finish?

There are a number of different items that can be specified when ordering anodized aluminum. 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 anodized coating
Color of the anodized coating
Gloss of the finish
Quality of the seal of the anodized coating
Corrosion resistance measured by salt spray testing
Aluminum alloy used and its temper

What alloys are recommended for anodizing?

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

What casting alloy is recommended for anodizing?

Castings are challenging to anodize because they are often porous. The alloy preferred for anodizing castings is 518. C443 is also good, but it is not inherently corrosion resistant. These are also the alloys preferred for painting since paint pretreatment will attack a poor casting similar to anodizing chemicals.

What anodize 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 anodized products are to be used out of doors, anodized film thickness is usually specified at 0.000400" minimum or 0.000700" minimum. Parts used in automotive applications are usually specified at 0.000300". Parts used in interior applications are usually specified between 0.000100" and 0.000350". There are some applications where coating thickness of 0.00001" is used. Hardcoat anodizing, commonly used in high abrasion applications, may range from .001" to .005" in thickness.

What are the commonly used units for anodizing coating thickness?


Inches 0.001"
Mils 1.0 mil
Microns 25.4 µ

0.001" is read as one thousandth of an inch
0.0001 is read as one ten thousandth of an inch
1.0 mil is also read as one thousandth of an inch
0.1 mil is one ten thousandth of an inch
25.4 µ is read as 25.4 microns

0.001" = 1.0 mil = 25.4 µ

For a Class I coating of 0.7 mil the equivalents would be:
0.7 mil = 0.0007" = 17.78 microns

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Does exterior anodized aluminum appear to change color under different light conditions?

Many factors influence the appearance of anodized aluminum architectural components. Many architects and designers find that to be one of the attractive features of anodized aluminum: 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 aluminum from the mill supplier (mill finish surface roughness, longitudinal versus transverse surface roughness), and the process of the anodizer (clean and anodize, etch and anodize, bright dip and anodize, 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 colored, 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 would like to use colored anodized aluminum to replace a few parts on my sailboat. What issues might arise from this? I mostly sail in fresh water but occasionally in moderately salty or brackish waters. How is the corrosion resistance of anodized aluminum?

Anodized aluminum is very suitable for applications involving exposure to sea water and is routinely used for parts such as masts. Corrosion resistance is good; however, the parts should be kept clean. The buildup of dirty surface deposits can provide sites for corrosion, particularly if there is associated acidity. Coloring using dyes may be a problem because many dyes are susceptible to the effects of sunlight and may fade or change color.

I understand that the colors on anodized aluminum may derive from metallic salts or from organic dyes. How lightfast are these colorants 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 coloring uses metallic salt solutions to produce bronze or black colors that are sufficiently light-fast for exterior, as well as interior, applications. Colors 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 anodized aluminum can show significant color 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.

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

The purple hue is an iridescent color--the result of a very thin film on the surface of the black anodized part. Other iridescent colors can be produced, and can appear even on clear anodized parts, but are most evident on black anodizing. 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-colored 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 aluminum hydroxide. They act as a thin film that can generate iridescent colors. Anti-smut additives for sealing baths are available, which minimize the problem. The formation of smut can be favored 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.

Anodizing 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 anodizing) 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, anodizing 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 could have caused discoloration (cloudiness) of anodized aluminum windows? How can this be resolved?

This discoloration could be weathering bloom, chalking, or chemical attack.

Weathering bloom may occur when sealing is inadequate, and the surface of the anodized coating has become etched during weathering. Acidic pollutants in the environment promote this effect.

Chalking may occur when the anodizing quality is not high enough. For instance, anodizing at too high a temperature can result in "soft" coating, which, when exposed to the atmosphere reacts a bit like mud, cracking and swelling with variations in humidity. Sometimes this leads to the development of iridescent colors, particularly on dark bronze or black anodized parts. Eventually, the mud-cracked layer spalls off, and then the degradation of the underlying layers commences in a similar manner.

It may be possible to restore the original appearance by an abrasive cleaning using a pumice powder or a fine grade of synthetic abrasive (e.g., "Scotchbrite"). Coarser abrasives such as sandpaper or steel mesh should be avoided, along with acid or alkaline cleaners, any of which would damage the anodized coating.

It is worth noting that the degradation of anodized coatings can be reduced substantially by regular washing with water containing a suitable wetting agent or a mild soap solution followed by thorough rinsing.

Chemical attack occurs when acid or alkaline materials come in contact with the anodized finish. The most common occurrences are when mortar or muriatic acid (used by masonry trades) are allowed to dwell for a time on a window or other anodized aluminum building component. Once the finish is visually affected, irreversible damage has occurred and the discolored item must be re-anodized or replaced.

What tests are used to evaluate an anodized finish?

There are many different tests used to evaluate anodized finishes. Some of the most commonly used tests are as follows:

American Society for Testing and Materials (ASTM).

B136-84 (2008)e1 – Standard Method for Measurement of Stain Resistance of Anodic Coatings on Aluminum
B137-95 (2004) – Standard Test Method for Measurement of Coating Mass Per Unit Area on Anodically Coated Aluminum
B244-09 – Standard Test Method for Measurement of Thickness of Anodic Coatings on Aluminum and Other Nonconductive Coatings on Nonmagnetic Basis Metals with Eddy-Current Instruments
B680-80 (2004) – Standard Test Method for Seal Quality of Anodic Coatings on Aluminum by Acid Dissolution
B117-07a – Standard Practice for Operating Salt Spray (Fog) Apparatus

These are just a few examples; many other standard test methods may be used to test anodized aluminum.

What about rack marks?

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

Can anodizing 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 anodizing. 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 anodized aluminum?

Parts can be welded prior to anodizing. The use of 5356 welded rod is strongly recommended, though some discoloration will still occur. 4043 is the worst choice because it will turn a smutty black when anodized. Grinding away the weld before anodizing will result in decreased mechanical integrity and will not solve the appearance variation problem.

It is not a good idea to weld after anodizing. 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 minimized?

Welds can discolor 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 anodizing 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 buildup actually changes the temper of the aluminum immediately surrounding the weld bead. Since a temper change is really a metallurgical change, again this shows up after anodizing as a color difference. These areas are commonly called halos or ghosts.

There are a few things that can be done to minimize the color differences. First, excellent results may be produced by using welding wire alloy 5356. This alloy reportedly produces the best color 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 aluminum 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 color difference. This practice, however, will not help hide the discoloration.

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Can anodized aluminum 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 anodized aluminum 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.

How do I clean anodized aluminum?

Cleaning anodized aluminum is easy with the right technique. Because anodizing is so hard, you want to use an abrasive cleaning technique with a gentle soap. Do not use harsh acidic or alkaline cleaners because they may destroy the finish. Use solvents with care as they may stain the finish. Regardless of the technique, be sure to try a test area first. One recommended technique is to use an abrasive cleaning sponge with mild dish washing liquid. Always try a test small area first to prevent a widespread problem. (For more detailed advice, obtain a copy of Care of Aluminum from The Aluminum Association. )

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Disclaimer: The information and recommendations provided are believed to be accurate and reliable. It should not be assumed, however, that all responsive information has been provided, or that additional information may not be relevant under certain circumstances or conditions. AAC assumes no responsibility or liability for the use or misuse of any information, materials, processes or techniques described, and it makes no warranty, guarantee, or representation whatsoever as to the absolute validity or sufficiency of any information provided. AAC does not "approve" or "endorse" any specific products, methods, or sources of information. This information should not be referenced in any way which would imply such approval or endorsement.