Chromic Acid Anodizing

As told in an earlier post regarding different anodizing electrolytes, the Chromic Acid Anodizing, CAA, was the first commercial anodizing process patented in 1923 by Bengough and Stuart.

Chromic acid anodizing is mostly used for protection of critical structures with all kinds of joints. The corrosion resistance is excellent relative to the thickness of the coating, which normally lies in the range of 0.08 – 0.2 mil. The oxide film is softer and less porous than those formed by the other processes, and is formed without any significant fatigue loss of the material. The film is easily damaged and the color is light opaque gray. When this film is sealed in a dichromate seal a greenish color appear.

The process is voltage controlled with a ramping in the beginning of the process increasing up to 40V depending on the type specified. Two types are specified in the military specification MIL-A-8625F, Type I and Type IB, whereas the first is conventional coatings produced by a voltage of around 40volts and Type IB uses a voltage of 20 to 22 volts.

Other specifications are AMS 2470 and ASTM B 580 for Chromic Acid Anodizing.

The anodizing process steps for Chromic Acid Anodizing are usually more simple than the ones for the Type II anodizing (SAA = sulfuric acid anodizing). The work should be cleaned by vapour degreasing and if necessary an additionl alkaline cleaning. After a final rinse in clean water the work should be ready for anodizing.

Different customers call out different process parameters which sometimes makes it difficult to handle a variety of customers.

Some of the various customer specifications are:
  • Boeing BAC 5019
  • Cessna CSFS020
  • Eclipse EAC1006A
  • Bombardier MPS160-10

The main use of Chromic Acid Anodizing is due to the fact that residues from the chromic acid trapped in parts that are difficult to rinse does not lead to corrosion. Another important feature is the fact that Type I coatings keep the aluminum materials fatigue strength and the very thin layer makes a minimal dimensional change.

Alloys are not allowed to contain more than 5% copper or 7% silicon, and total alloying element must not exceed 7.5% according to MIL-A-8625F. The alloys should be in one of the following temper before anodizing, T4, T6 or T73.

The electrolyte should consist of 50 - 100 g/L chromic acid and with a temperature of 95 - 105F. The purity of the chromic acid should not be less than 99.5% CrO3. Chloride is the worst contamination for the electrolyte and shouldn´t exceed more than 20 g/L. Chloride present in the electrolyte causes etching of the aluminum.

The hexavalent chromium content, the free chromic acid, decreases during the process and the trivalent chromium and aluminum increase.

Most of the Chromic Acid Anodizing is processed at 40 V, the low voltage is only used for special alloys which are difficult to handle at the higher voltage, as e.g. 2014 and 7075. Ramping is essential is this type of anodizing.

An easy way to remember the ramping time is the following;

For the all the alloys using 40 V, use 40 minutes to increase the voltage to 40V and then anodize for another 30 minutes. For the alloys using 20V, the same pattern can be used, use 20 minutes to get to the 20V and then spend another 30 minutes at that voltage.

The two different voltages used for Type I and Type IB create a slighty difference in the oxide film formed which can be seen in SEM images below of the surfaces.

Type IB, 22Volt

Type I, 40 V

The specifications for the performance of the coatings are a little different than for hard anodizing but the same procedures should be used. The difference is the weight of the formed coating which should be min 200 mg/ft2 and the coating should pass 336 hour in a salt spray test for Type I coatings.

It has to be mentioned that hexavalent (CrVI) compounds, often called hexavalent chromium, exist in several forms. Hexavalent chromium is recognized as a human carcinogen via inhalation. For more information check out, United States Department of Labor or The European Union directive, Directive 2002/95/EC.

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