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Type I: Chromic Acid Anodizing

Chromic acid anodizing is a popular anodizing system preferred by the government for anodizing aluminum parts that are to be riveted together or that form lap joints. Because the chromic acid does not attack the aluminum, thorough rinsing and neutralization of the acid in lap or riveted joints is not as critical.

There are many chromic acid bath formulations. Many are proprietary and may be licensed.

Generally, two acid concentrations are used; 3% and 10% chromic acid. The higher concentrations are for thicker oxides while the lower concentrations produce a harder and more compact oxide. There is little difference in the operation of the bath as the concentration is increased above 10%.

Unlike sulfuric acid anodizing, the oxide thickness produced is much thinner, seldom approaching 0.5 mil maximum.

The chromic acid bath uses stainless steel cathodes at a cathode to anode ratio of 1:5. The 3 % bath is operated at 104º F while the 10% bath operates at 129º F. The average current density is about 3 amps per square foot (3 asf).

If the parts are to be inorganically colored, the anodizing time should be restricted to less than 30 minutes.

The chromic acid bath is very sensitive to chloride and sulfate contamination. The bath should be operated with a chloride content of less than 0.02% and a sulfate content of less than 0.05%.

Type II: Commercial Anodizing

(Room Temperature Anodizing)

Sulfuric acid anodizing has been defined as “an electrochemical process that intentionally forms a porous anodic oxide on aluminum”. This oxide, an integral part of the metal, is formed when a current is applied to the aluminum parts in a sulfuric acid anodizing bath.

Type II anodizing is by far the most popular form of anodizing aluminum. Everything from high rise buildings to space ships has either been completely anodized or has anodized components as part of their makeup.

Under normal circumstances, the aluminum is anodized using a prescribed set of parameters:

Typically, Type II anodizing is performed in a sulfuric acid bath containing 180-200 grams per Liter of acid and a small amount of dissolved aluminum. The operating temperature is controlled between 68-72º F. Current densities can range from 5-18 amps per square foot (ASF), but commonly are run at 12-15 ASF.

The power supply is a DC rectifier. The aluminum part being anodized is made the anode (or positive pole) in the system. The most efficient cathodes (or negative pole) are 6063t6 aluminum.

Current is applied to the system for a prescribed time, and at the desired current density to achieve the oxide thickness required (oxide thicknesses can range from 0.1 mil to 0.8 mil). The resulting porous oxide can then be colored or sealed.

Type III: Hard Coat Anodizing

(Low Temperature, Hard Anodizing)

Sulfuric acid anodizing has been defined as “an electrochemical process that intentionally forms a porous anodic oxide on aluminum”. This oxide, an integral part of the metal, is formed when a current is applied to the aluminum parts in a sulfuric acid anodizing bath.

Type III (hardcoat) anodizing is an anodizing process that forms an extremely hard, abrasion resistant, porous oxide on aluminum. It is generally referred to as an “engineering hardcoat” due to the fact that the oxide has been found to be suited for applications where the hardness and increased oxide thicknesses are an advantage. Aluminum cookware, military applications and a myriad of other uses have made Type III anodizing very popular.

Type III (Hardcoat) anodizing differs from the typical Type II room temperature anodizing in a number of ways:

The anodizing bath parameters for Type III (hardcoat) anodizing are similar to type II (room temperature) anodizing in that the acid and aluminum concentrations can be virtually the same. The difference becomes apparent when you consider the other operating parameters.

Type III anodizing is performed in a sulfuric acid bath containing 180-200 grams per Liter of acid and a small amount of dissolved aluminum. The operating temperature is controlled between 28-32º F but in some instances an acceptable oxide can be achieved at slightly higher temperatures. Current densities can range from 24-40 amps per square foot (ASF), but commonly are run at 24-30 ASF.

The power supply is a DC rectifier. The aluminum part being anodized is made the anode (or positive pole) in the system. The most efficient cathodes (or negative pole) are 6063t6 aluminum.

Current is applied to the system for a prescribed time, and at the desired current density to achieve the oxide thickness required (oxide thicknesses can range from 0.7 mil to 3.0 mils). The resulting porous oxide can then be colored or sealed but limitations on final color will determined by the oxide produced and color used.

Titanium Anodizing

Anodizing of Titanium can be accomplished in many different anodizing solutions from alkaline to acid and almost everything in between. Some of these baths are designed to achieve a specific result and finish. Much of this technology is proprietary and information on it is very difficult or impossible to obtain.

A general type of Titanium anodizing from a sulfuric acid anodizing bath has the following parameters.

1. 155 to 200 g/l sulfuric acid
2. 65-75 degrees Fahrenheit temperature
3. 0-75 volts
4. 5-15 amps sq ft.
5. 15-20 minutes
6. Glycol additive (optional)

Pretreatment steps

1. As in all metal finishing a through cleaning must be accomplished, preferably in a strong alkaline cleaner, at 140-160 degrees Fahrenheit for a water break free surface. Electro-cleaning may be desirable but not required. 5-10 minutes.
2. Rinse.
3. Acid etch/activate in 20-50% nitric acid 1-3 minutes. For a more even surface the addition of fluoride is advisable.
4. Rinse.
5. Anodize at 10 ASF and 10-75 volts depending on final color desired. Color will vary with voltage and time from golden brown at 10 Volts / 20 min to blue at 75 Volts / 12 min.
6. Rinse.

In addition to the above instructions, you can also look into SAE specification AMS 2488B for another type of Titanium anodizing.

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