Aluminum is "anodized" for corrosion resistance, improved appearance, hardness & wear resistance, and for a number of specialized reasons like better glue-ability (adhesive bonding) for structural use, improved paint adhesion, and enhanced capacitance in electronic applications.
The aluminum is anodized by immersing it in an acid (the electrolyte) and applying DC (direct current) electricity, with the workpiece anodic (positively charged). The electricity splits some of the water in the acid into positively charged hydrogen and negatively charged oxygen, with the oxygen being attracted to the positively charged aluminum surface, where it reacts with the aluminum to form aluminum hydroxide and related compounds (the hydrogen is attracted to the negatively charged electrode called the cathode, where it forms hydrogen gas bubbles and dissipates into the air).
While the acid helps to conduct the electricity, it also attacks the anodized film that is being formed and tends to dissolve it -- but this is not a bad thing. Rather, the simultaneous formation and dissolution of the anodized film creates a coating that has porosity in the form of microscopic "drill holes" extending from the surface almost to the raw aluminum that can be very important to the utility of the anodized coating. Because of this attack on the anodized coating by the acid, varying the strength and concentration of acid in the electrolyte, and its operating temperature, will result in anodized coatings with very different properties.Although a variety of acids are used for specialized applications (phosphoric acid for adhesive bonding and boric and tartaric acid for electrolytic capacitor formation, for example) the most common acids used for aluminum anodizing are sulfuric or chromic. U.S.MIL-A-8625 is perhaps the most commonly referenced specification, and describes 3 general types of anodizing:
Type I - Chromic acid anodizing (actually there are also types IB (low voltage process) and 1C (chrome-free process) in recent issues of the specification)
Type II - Conventional "room temperature" sulfuric acid anodizing (plus a newer type IIB "thin sulfuric")
Type III - Hardcoat anodizing, done in sulfuric acid at temperatures close to the freezing point of water.
Chromic Acid Anodizing, i.e., Type 1
Chromic acid anodizing is often done on aerospace components. It offers good corrosion resistance while being quite thin (usually less than .0001"), which is extremely important in limiting its impact on reducing the fatigue strength of components. Also, the chromic acid is not corrosive like sulfuric acid is, making it safer to use on critical components where entrapped acid in a seam or spot weld could be dangerously corrosive.
But the world is very concerned with the environmental impact of chromic acid (hexavalent chrome), so there have been great efforts for several decades towards reducing the use of chromic acid anodizing. Phosphoric acid anodizing is one process that has been substituted for chromic acid, and Boeing has been promoting a proprietary boric acid & sulfuric acid electrolyte which is also capable of producing good quality thin anodized coatings. Consult the latest version of Mil-A-8625 for a discussion of type IC "non-chromate" coatings and type IIB "thin sulfuric" coatings. But it goes without saying that the OEM specifies the process, and the applicator must adhere to the specification, not attempt substitutions.
Conventional Sulfuric Acid Anodizing, i.e., Type 2
The great majority of aluminum anodizing is done with room temperature (20 °C 68 °F) sulfuric acid (approximately 15% by weight, 10% by volume) at 10-20 volts. Depending on for how long the work is processed, and the specific aluminum alloy being anodized, this produces an anodizing thickness from about .0001" to .0008" thickness. Thicknesses greater than this are difficult to achieve in conventional sulfuric acid anodizing for two reasons: first, as the coating gets thicker, the dissolution by the acid can limit additional build; second, the coating is non-conductive, with its resistance being roughly proportional to the thickness of the coating. As the coating gets thicker, the conventional 10-20 volts applied will no longer drive sufficient current to cause further buildup.
The anodized film is somewhat, but not completely, transparent/translucent so the thin end of the range is used for aluminum mirrors and reflectors. The thicker end of the range is used where greater corrosion resistance is required.
Conventional sulfuric acid anodized coatings are often dyed to attractive decorative colors. Although "white" is problematic, virtually every other color of the rainbow is readily achieved. The pores ("microscopic drill holes") in the coating absorb the dye, and then the coating is subsequently "sealed", locking the dye in. The degree of saturation of the color will depend on how thick the coating is and consequently how much dye it can absorb. While light pastel colors can be obtained with an anodizing thickness of perhaps .0002", to get a saturated dark black color may require a thickness of .0005" or more.
Most of the dyes used in anodizing are organic dyes, with various degree of color-fastness; but metallic (inorganic) dyes are often used in architectural applications where continuous resistance to strong sunlight is required. These inorganic dyes tend to produce colors ranging from champagne to bronze, which is why those colors are so popular on aluminum roofs, awnings, and building features. Anodizing for architectural work also tends to be quite thick (about .0007") for corrosion resistance.
Aluminum alloys may contain copper, silicon, zinc, magnesium, and other alloying ingredients, but only the aluminum portion can be converted to aluminum oxide. So alloys that are closer to pure aluminum will offer a clearer look when thin coatings are applied, and can generally accept a thicker anodized layer with less unsightly discoloration. All or almost all alloys can be anodized to some extent or other, but the 1xxx, 5xxx, and 6xxx aluminums are better choices than 2xxx or 7xxx when the option is available ... and it can be difficult or impossible to get pleasing aesthetics on anodized castings.
Hardcoat Anodizing, i.e., Type 3
If the temperature of the sulfuric acid is reduced to about 0 °C / 32 °F, the activity of the acid, and the dissolution it causes, is significantly decreased. This allows the anodizing to build to a heavier thickness. If the anodizing voltage is also increased to the range of 48-90 volts (depending on the alloy in question), very hard and highly wear-resistant coatings of about .002" thickness are possible. This is called "hardcoating" or "hard anodizing" and is used on thousands of machine parts and automobile components.
Because of the high thickness, and the discoloration caused by the alloying ingredients in the aluminum substrate, most hardcoat coatings tend toward gray to charcoal in color. Hardcoated parts are rarely dyed, and the colors tend to be uneven and not particularly attractive, but they can be dyed when necessary (for example to distinguish between live and dummy military rounds). Dyeing and sealing can somewhat soften the hardcoat layer, which is another reason it is usually avoided.
There are some alloys which are difficult or impossible to hard anodize in sulfuric acid. In fact, Mil A-8625 notes that aluminum with greater than 5% copper or 8% silicon cannot be hardcoated except in special situations. Proprietary "additives", which may be oxalic and glycolic acid among other materials, can help to make hardcoating of these alloys possible. These additives also may allow operation of the bath at higher temperatures, perhaps about halfway between traditional Type 2 anodizing temperatures and hardcoat (Type 3) temperatures. For this reason the slang term "Type 2-1/2" is often used to describe these proprietary anodizing baths.
Typical Anodizing Sequence
The anodizing process is not completed in a single anodizing tank per se, but includes pretreatment steps before anodizing and post-treatments after it. One typical sequence, and perhaps the most common, would be:
In this sequence, the cleaning tank would be a non-etch alkali cleaner for removing soils. Because neither really strong caustics nor electrocleaning can be used on aluminum, and because solvent degreasing is decreasing in popularity for environmental reasons, ultrasonic agitation is commonly employed in the cleaning tanks.
The etch process would most commonly be caustic soda, but could be an acid etch like ammonium bifluoride. Etching is usually done to a smooth and very lightly textured finish (similar to a very light glass beading) for a nice appearance and to minimize the fingerprinting issues of a shinier surface. Etching dissolves aluminum, leaving the other alloying materials behind, so it may be minimized or skipped for some alloys.
As an alternate to etching, aluminum for mirrors or reflectors is bright dipped in a very strong nitric-phosphoric bath to impart a mirror finish. Bright dipping is definitely not "just another tank". It is one of the real "nasties" in surface finishing, and you should see facilities that do bright dipping to understand the ventilation and secondary containments issues before attempting to specify it or employ it.
The desmut step tries to dissolve the gray-black alloying ingredients such as silicon, copper, zinc, and magnesium on the surface of the parts. This step is sometimes called "de-oxidizing", which is a widely accepted misnomer. The constituents in the desmut step will depend upon the alloying ingredients which must be removed. For example, HF or another acid with fluoride salts will be required if the alloys contains silicon. Chromic acid has traditionally been used in the desmut tank but that material is a special target of environmental initiatives. Nitric acid may be necessary to dissolve copper.
It is fairly common to attempt to "neutralize" the sulphuric acid via a sodium bicarbonate dip or a dip in dilute nitric acid (nitric acid doesn't really neutralize the acidity of sulphuric acid, but it does have the ability to help drive it out).
Dyes are usually heated and may be organic dyes fairly akin to fabric dyes or, particularly for architectural work, they may be inorganic metallic salts, often applied with the aid of A.C. electricity, giving rise to the name "two-step" anodizing. It is also possible to combine the processes, applying an inorganic dye and then "overdyeing" with an organic dye.
Sealing is the step where the top of the honeycomb-like anodizing pores are swelled to lock the dye in and lock dirt out. Sealing is a science of its own, with older approaches like steam or boiling D.I. water being used, as well as newer mid temperature processes like nickel acetate, and low temperature seals like nickel fluoride. For military work, chromic acid sealing may still be specified for best corrosion resistance and/or color matching.
The last step in the process is often a D.I. water rinse to minimize staining issues.
Testing of Anodized Coatings
Typical tests for anodized coatings include coating weight, seal quality, corrosion resistance, light fastness, and paint adhesion. Hardcoatings are additionally often tested for thickness and abrasion resistance. Again Mil-A-8625 is a good starting point toward developing test requirements.
Rework of Anodized Coatings
Anodized coatings may be removed with caustic soda, i.e., in an alkaline etch tank. But, as should be apparent, this solution will also attack the aluminum substrate -- so timing to remove the article as soon as the anodized coating is gone can be critical. To remove anodize coatings without attacking the aluminum, chromic-phosphoric acid solutions can be used.
A crucial issue that often goes unappreciated by designers is that an aluminum article from which the anodized coating has been removed will be smaller than it was before coating, even if the chromic-phosphoric acid stripper is used. That is because aluminum from the substrate was consumed in building the anodized coating. A rule of thumb is 50-50 penetration vs. build: about one thousandth of an inch of aluminum is consumed in building a coating thickness of two thousandths.