November 7, 2017

Chromic Acid Anodizing Process

The sequence of anodizing of each process can be different, its depend on the type of object to be anodized, and also the objective of anodizing. Chromic acid become prefer for anodizing because it is less corrosive compare with sulfuric acid. Especially for component that have riveted or welded assemblies where this is impossible to remove first before anodizing.

Using this process will result of yellow to dark olive color finished, depending on the anode film thickness. For alloy metal like high contain copper alloy, the sequence of anodizing different with aluminum anodizing. Chromic acid anodizing solution contain from 3 to 10% of CrO3 weight. To prepare this solution always attention to dissolve acid into water little by little don’t dilute water into acid, its can be dangerous. Because hot water contain acid can be splashed into your eyes.

Chromic acid solution for alloy anodizing condition should be:

  • Have pH is between 0.5 and 1.0.
  • The concentration of chlorides (as sodium chloride) is less than 0.02%.
  • The concentration of sulfates (as sulfuric acid) is less than 0.05%.
  • The total chromic acid content, as determined by pH and Baume readings, is less than 10%.

Operation Control

When anodizing is started, the voltage is controlled so that it will increase from 0 to 40 V, don’t directly 40 Volt. This changes is increasing regularly in about 5 to 8 minutes. The voltage is regulated to produce a current density of not less than 0.1 A/dm 2 (1.0 A/ft2), and anodizing is continued for the required time, generally 30 to 40 min. For certain alloy can be fall if run in 40 volt, it should be run in 22 volt. Casting alloys should also be processed at 22 + 2 V, as specified in military specification MIL-A-8625, type 18.

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