anodized and dyed aluminum

Process and benefits

Aluminum anodizing is an electrochemical means of converting aluminum hydroxide (a passive state of aluminum) to aluminum oxide (a reactive state of aluminum). Anodizing changes the physical and chemical characteristics of the aluminum surface by creating an anodic film that is highly porous in nature, thus allowing the aluminum to be decoratively colored by absorbing dye into the anodic film. The anodizing process also renders the surface harder and more abrasion resistant, insulates the aluminum surface against the passage of electricity, and helps improve upon the natural corrosion resistance of aluminum.

Alloy Selection

In the design of pieces to be finished by anodizing and dyeing, the selection of the aluminum alloy should receive careful consideration. The amount and type of alloying constituents has a pronounced effect upon the porosity, clarity, and hardness of the anodic film and thus, upon the appearance of the finished aluminum part. 1000 series high purity alloys are particularly well suited for "bright finishing" applications since they produce exceptionally clear anodic oxides. Other desirable alloys include 5000 and 6000 series which contain magnesium and magnesium-silicide respectively. Special formulations of these, such as 5657 and 6463, are outstanding in brightness retention. 1100, 5052, and 6061 alloys are also desirable and are readily available as surplus. The 2000 and 7000 heat treatable series are not ideal as the anodic films on the alloys almost always exhibit minute discontinuities and are subject to corrosion pitting.

Anodizing Procedure

Procedure Supplies and Equipment
Degrease aluminum parts to be anodized.

aluminum
protective clothing (including rubber gloves, safety glasses, apron, towel)

2. Suspend parts on aluminum welding rods. Fit should be secure and tight. aluminum welding rods
3. Caustic soda etch 1 minutes.
4. Rinse.
5. Nitric acid etch 2 minutes.
6. Rinse.
 
7. Anodize in a sulfuric acid bath at 12 volts for 15 minutes to 1 hour. Longer anodizing times produce larger pores -- the aluminum will accept dye more freely resulting in intense color.
8. Rinse.
 
9. Neutralize 10 minutes in warm tap water, then 10 minutes in hot tap water, then 5 minutes in cool tap water clean beaker
10. Dye. For maximum saturation, dye bath should be heated to 100-120°F
11. Rinse.

thermometer (measuring 150°F)
coffee warmer
aluminum dyes
resists

12. Seal for 10 minutes in boiling solution.
13. Rinse.
14. Pat dry with soft clean cloth.
15. Scrub lightly if needed to remove film.
Dish soap and toothbrush or 0000 steel wool
Dyeing
Dyeing of the anodized aluminum part is a relatively simple process. Dyeing is performed before sealing as the pores of the aluminum anodic film are open and receptive to dye. The anhydrous form of the aluminum oxide film links with dyes more easily, which the monohydrate form that the oxide becomes after sealing will not do. After neutralizing the piece, it should be rinsed and then immediately placed into the dye. Do not allow the piece to dry out after neutralizing. The dye solutions consist of from 0.1 to 10 grams/liter of the proper dyestuff dissolved in water (mix according to manufacturer's instructions). Most dyes require approximately 2 grams/liter of water. Dye baths may be heated with an immersion heater or on hot plates (100-120°F/37°-48°C) to produce darker shades. Dye baths used cold will produce pale tints. Generally, aluminum parts are suspended in dye baths for 5-10 minutes and checked frequently. Once dyed, the aluminum piece is rinsed well and ready for sealing.
Dyeing Variations
Multicolor selective dyeing of an anodized surface is possible. The object is first completely, or partially anodized. A selected area is protected with a masking resist that is capable of remaining intact on the aluminum surface and withstanding the dye bath temperature. Adjoining color dyeing is possible when a resist is applied to a selected area of the anodized aluminum part, the exposed area is dyed. A second application of a resist can then be applied to protect the already dyed area, areas previously protected by the resist are then exposed to a second dye bath. Over-dyeing of a color with a second color, covering the first completely or partially, is also possible.
Sealing
Once the pores of an aluminum anodic film are sealed (the final step in a normal production process), the film acts to protect the base metal and minimizes its corrosion, streaking, and discoloration by shielding it from atmospheric contact. Proper sealing of the dyed anodic film is essential in eliminating film porosity and in making the surface resistant to further dyeing, fading, staining, and corrosion. The sealing process converts the amorphous aluminum oxide anodized film to aluminum monohydrate which has a greater molecular volume. this increase in volume causes the pore walls to swell and close so that the dye is locked in. The appearance of the dye film is not changed in the process. Commercial sealers, hydrolizable metal salts incorporating nickel acetate (and sometimes cobalt), are available and should be mixed according to the manufacturer's instructions. The sealing salt is brought to boiling and the dyed part is immersed for approximately 10 minutes. Once the piece is sealed, it is rinsed thoroughly in water and allowed to air dry or is pat dried with a soft clean cloth. Often, there is a cloudy film left on the surface by the sealing salt. This can easily be removed by lightly scrubbing the piece with a toothbrush and soap, or with 0000 steel wool and soap.

Safety

  1. Fumes are generated during the caustic soda etch, anodizing, dyeing, and sealing processes. The work area must be well ventilated.
  2. Additionally, as with any electrical situation, normal precautions must be observed. Never touch the anode (aluminum part) and cathode (lead sheet/bars) or connecting rods and wires while the rectifier is on. The current generated is extremely dangerous.
  3. Rubber gloves should be worn to insulate against electric shock and also for protection from acid and caustic soda baths.
  4. Rubberized lab coats or lab aprons will provide protection from splashed acids and chemicals.
* The text of this handout is based in part on that given by Billie Theide to her ARTCR 160 class at UIUC.