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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%.