Looking fo rgood information on anodizing 1

[hinged]

Good afternoon,
I hope everyone is staying warm.
I'm searching for information on whether feature sizes will decrease or increase after anodizing.
I've heard both, that the feature size will increase and decrease, does this depend on the method of anodizing?
Looking at both Class I (AA-M12C22A44) and Class II (AA-M12C22A31) anodic coatings.
I understand that you build a layer of oxide on the surface of the base aluminum, but has the base aluminum been altered in size before the oxide is created.
All of your thoughts and opinions are welcomed .
Have a great day and thanks for your assistance in advance.

 

[kenvlach]

I seem to haave misplaced my AA Finishing specs., but yes, as the volume of aluminum oxide formed is greater than that of the aluminum consumed, external dimensions will increase, while IDs will decrease.

On conventional sulfuric acid anodizing, Type II per MIl-A-8625F, the anodic coating will be 2/3 'in,' 1/3 'out.' So for an overall 0.0010" anodic oxide, the surface buildup will be roughly 0.0003."

For hard anodize, Type III per MIl-A-8625F, the anodic coating will be 1/2 'in,' 1/2 'out.' So, from the initial metal surface, a 0.002" oxide will have grown 0.001" both inward and outward.

The results for the less dense Type II may not be what you expect from a mass balance, but are due to simultaneous occurring dissolution of the oxide in the sulfuric acid electrolyte, which is warmer than for Type III.

Note: the ratio of buildup to penetration varies somewhat with alloy and even more with operating conditions (electrolyte concentration, temperature, voltage ramp rate during hard anodizing, whether additives are used).

Also note: the buildup on threads is much greater due to the multiplicity of surfaces. Always run a prototype, machine oversize holes, or else plug standard size holes, etc.

Hope this helps. Any further questions, just ask.
Ken

 

[kenvlach]

Some additional information re “has the base aluminum been altered in size before the oxide is created?”

Yes. In the Aluminum Association’s Designation System for Aluminum Finishes (DAF-45), the ‘C22’ in each of your specifications is for a caustic etched, medium matte (satin) finish, in sodium hydroxide solution, 4-6 oz./gal., at 140^oF for 5 min. This definitely removes material prior to anodizing. The etch rate varies with the amount of dissolved aluminum in the etchant, but an estimated etch rate of 5 microns/minute will thus result in a loss of 25 microns or 0.001 inch from the surface. A satin etch is good for removing minor machining lines and scratches, and for hiding fingerprints afterwards.

The Class I and Class II correspond to architectural anodic coatings of 0.7 and 0.4 mils minimum thickness, respectively – not related to MIL-A-8625F nomenclature. With the C22 etching, the parts will end up smaller in external dimensions. For sheet material finished both sides, a rough estimate is that the Class I material will end up 1.5 mils thinner overall than the original stock, and the Class II material will be 1.7 mils thinner.
Please do some before and after micrometer measurements on flat test panels and report the results.

The Designation System for Aluminum Finishes, 2003 is available at

http://www.aluminum.org/Template.cf...e=/Ecommerce/ProductDisplay.cfm&ProductID=251