Causes for anodize failure

[swertel]

I'm looking for ideas to troubleshoot what we believe is an anodizing problem.

We are black anodizing .0002 MIN thick in accordance with MIL-STD-171, 7.2.2; which is your typical sulfuric acid anodic film per MIL-A-8625, Type II, Class 2 (dyed), black.

The base material is 7075-T6 per ASTM B221 that is impact formed and then post machined.

We have been having some problem with the anodize flaking off. Actually, it just gets little spots all over back to what appears to be the base material. The spots are so small that we can't section it to get an analysis to determine if just the color is missing on the spots or if the anodize is really coming off (wasn't "bonded" into the material).

We had a problem with this anodizer, but they have since improved their process and their baths are cleaner and more importantly, don't get too hot. We also use this same vendor to black anodize per the same specification another part, but it is 3003-O sheet per ASTM B209 that is stamped to shape. That part has no issues. The part with the issue does pass tape tests and salt fog tests. We have even done a more agressive test to verify the anodic coating.

For the next phase of production we cast a material into the cavity. The cast material is obviously heated, and the aluminum parts are pre-heated to improve flow. I'm afraid I don't remember the exact temperatures of each, but I think under 190F (88C). The spots don't show up until after the casting process.

I'm looking for ideas to the cause of the spots or additional tests I can perform. Any additional verifications I can perform after anodizing, before the casting process, are also welcome.

--Scott

http://wertel.eng.pro

 

[TVP]

Scott,

I am confused by the last 2 paragraphs. I understand the current process to be as follows:

Al blank/billet
impact extrusion/forging
machining
anodizing

Can you describe what is changing?

 

[swertel]

The last two paragraphs describe the next steps in the production of the final product. This component that has the anodize problem gets assembled to the stamped component that doesn't have the problem (along with additional parts). The cavity formed by these two parts at assembly is then loaded with a molten wax-like material.

In order to keep the wax flowing, the parts are pre-heated so the wax doesn't cool and solidify when it comes in contact with the AL surface. I don't remember the temperature of the molten wax - it's a part of their proprietary process so I'm not privy to the details.

Basically, the part comes out of anodize and passes all requirements. It gets assembled without any problems. It gets shipped off to the loader who is most likely doing a receiving inspection and they are not seeing the problem. They commense with their loading procedure and after clean-up have spotted parts.

I'm pretty sure the anodize is fine, but their loading process, cleaning or otherwise, is attacking the anodize. Without knowing the details of their process, I can't verify and they are sharing since it is proprietary. They are blaming a bad anodize because if the anodize was good, it would survive the loading process.

So I'll repose a question:
What chemicals will attack an anodize? High heat may be an issue.
What other tests are there to verify the anodize besides a tape test or salt fog? If we can't find a solution to the problem, we would at least like to test for it prior to having to absorb the expense of scraping the loaded parts.

--Scott

http://wertel.eng.pro

 

[CoryPad]

High pH solutions, including common cleaners/detergents, can attack anodized aluminium.

http://www.anodizing.org/html/faq_cont2.html#Cleaning

Regards,

Cory

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[TVP]

Thanks for the clarification. I recommend evaluating the parts that have spots/defects using optical microscopy and maybe electron microscopy to better understand what has happened to the coating/surface. If the spots occur randomly on the part surface, then I would evaluate the thickness of the anodized layer on "good" and "bad" parts across the entire surface. I think it would be useful to destructively evaluate "good" and "bad" parts, meaning cross-section through various areas and observe the anodized structure and thickness. Perhaps the parts are exhibiting excessive variation in thickness or the presence of defects. You may very well be correct that the parts meet the necessary specification requirements, and it is simply something in the final use phase that is attacking the anodic layer.