1. What is anodizing?
2. How long does it take?
3. What is anodized aluminum good for?
4. How do you color aluminum?
5. What is an alkaline etch?
6. What is bright dip?
7. Can I weld anodized aluminum?
8. Is the anodic coating a good electrical insulator?
9. Can anodizing hide scratches?
10. What is a hardcoat anodizing?
11. Is anodizing environmentally friendly?
12. Can I expect uniform color?
13. Should I request color range samples?
14. What about rack marks?
15. What alloys are recommended for anodizing?
16. What casting alloy is recommended for anodizing?
17. How do I clean anodized aluminum?

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1. What is anodizing?

A coating of aluminum oxide is grown from the aluminum by passing an electrical current through an acid electrolyte bath in which the aluminum is immersed.  The coating thickness and surface characteristics are tightly controlled to meet end product specifications.
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It only takes a few hours to process and pack a part. Most anodizers need anywhere from a few days to a few weeks to plan, process and invoice for projects. Lead times of longer than four weeks for anodizing are rare.
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Because anodizing is such a versatile process there are thousands of different applications. Just to name a few, these include:

• Architectural products like windows and doors
• Appliances
• Automotive
• Lighting
• Food preparation equipment
• Furniture
• Sporting goods
• Marine

4. How do you color aluminum?

There are four ways to color aluminum:

1. Dye: The freshly anodized part is immersed in a liquid solution that contains dissolved dye.  The porous anodic coating absorbs the dye.  The intensity of color is related to the thickness of the anodic film, the dye concentration, immersion time and temperature, among other things.
2. Electrolytic Coloring (a.k.a. "two-step") - After anodizing, the metal is immersed in a bath containing an inorganic metal salt.  Current is applied which deposits the metal salt in the base of the pores.  The resulting color is dependent on the metal used and the processing conditions (the range of colors can be expanded by overdyeing the organic dyes).  Commonly used metals include tin, cobalt, nickel, and copper.
3. Integral Coloring - This so-called one-step process combines anodizing and coloring to simultaneously form and color the oxide cell wall in bronze and black shades while more abrasion resistant than conventional anodizing.
4. Interference Coloring - An additional coloring procedure, recently introduced, involves modification of the pore structure produced in sulfuric acid.  Pore enlargement occurs at the base of the pore.  Metal deposition at this location produces light-fast colors ranging from blue, green and yellow to red.  The colors are caused by optical-interference effects, rather than by light scattering as with the basic electrolytic coloring process.

Southern Aluminum Finishing currently does 1 and 2.
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5. What is an alkaline etch?

Etching aluminum in sodium hydroxide is by far the most common pretreatment for aluminum that is to be anodized.  It imparts a matte finish to the end product.  Alkaline etches are also used to strip off anodic coatings.  However, under no circumstances should an etch-only finish be used in a finish product because there is no way to prevent the rapid onset of corrosion.  An etch-only finish normally is regarded as an unsighly mess by our customers.  It must be anodized immediately upon etching to preserve the uniformly satin appearance imparted by the etch.
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Southern Aluminum Finishing does not do the bright dipping process.  For a source, refer to http://www.anodizing.org/html/members.html#Suppliers.

Bright dipping is a process for increasing the specularity or brightness of aluminum by leveling the microscopic roughness or "peaks and valleys" on the surface of the aluminum. The process can not do macropolishing or smoothing of scratches or pits. This limitation underscores the need for careful handling prior to bright dipping.

Most commercially available bright dip baths consist mainly of phosphoric and nitric acid. Additives are introduced to reduce nitrogen oxide fumes and to enhance the brightening ability of the bath.
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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.
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If you have an application where electrical conductivity is important, you should test an actual production run for this characteristic.  You should also make sure that the anodizer knows this is a design parameter for your application.  

The dielectric strength of an anodic coating should be approximately the same as that of Alumina.  The Handbook of Chemistry and Physics, 43rd Edition, (Published by The Chemical Rubber Publishing Co, Cleveland, OH) gives the dielectric strength of Alumina as between 40 - 160 volts/mil.  In metric units that is 1.6 - 6.4 V/uM
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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.
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This can be a vague term, but usually hardcoat refers to a very thick and hard anodic coating. This kind of anodizing is accomplished with a bath similar to the standard sulfuric process, but with the temperature reduced to about 0?C to slow the dissolution rate. A higher voltage is applied to enable the coating to continue to build after the insulation value of the coating starts slowing down the coating formation.
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Yes. Anodized aluminum is often used on cookware. (However, FDA approval depends on the anodizer and on the process.  Southern Aluminum Finishing is not FDA approved on any of our anodizing processes.)  In contrast to electroplating process, it is not difficult for anodizers to comply with all federal and state environmental regulations.  There are no RCRA heavy metals used with anodizing. Southern Aluminum Finishing complies with all federal, state and municipal environmental regulations.  For more on SAF's compliance, see our news page.
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The answer is relative. Anodizers have made incredible improvements in controlling the color of anodized aluminum. The color consistency is comparable to metallic paints.
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Generally yes. It is important to remember that color is not defined by simply "light" and "dark." Like the controls on a conventional TV set, the hue and chromaticity will have an impact on appearance as well as light-to-dark. Color is at least a three dimensional thing, and cannot be adequately represented on a one dimensional light-to-dark axis. It may be impossible for the color of a flat sheet to perfectly match an extrusion anodized at the same time because the color of the metal may be fundamentally different. Range samples are good for adjusting expectations and for representing the amount of color variation that one can expect. However, if representing the exact color is important, then one must make samples from the exact lot of material you expect to use. When striving for uniformity, it is similar to buying fabric or carpet because you want all the material to come from a single lot.  Please see our letter that we send out with range samples by clicking here.
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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.
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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.  For architectural applications Southern Aluminum Finishing recommends 5005 alloy for sheet and 6063 alloy for extrusion.
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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.
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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.  It is best 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.
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