Pitting seen in AL 6061 T-6

[SandyH]

Hello all..

I am having some parts anodized that will be deployed in saltwater for years. Recently have seen pitting. The pitting does not cover the entire surface of the part. Mainly seen in the ridges and grooves. Any suggestions? Also, can a part show up from the anodizer with no pits and within days they begin to form?

Here are the instructions for the anodizer..

Anodize per MIL-A-8625, Rev F, Type III, Class 1, Duplex Seal (Sodium Dichromate first, nickel acetate second soak)
Build up: 0.00075" per side

Thanks all..
SandyH

 

[kenvlach]

“Recently have seen pitting.” -- What conditions/exposure, change in design, change in material supplier, change of anodizer? Were these parts done successfully in the past?

I suggest examining the anodize for crazing with a 20x magnifying lens. Sealing (actually, the heating) of hard anodize sometimes causes crazing of the anodize (hard anodizing is usually conducted at about 32^oF), resulting in worsened properties. Letting the hard anodized parts first warm to ambient temperature in a flowing rinse and then heating in warm DI prior to sealing helps. If crazing is a problem, seal at 160-165^oF rather than at the near boiling temperatures specified in Para. 3.7.1.2 of MIL-A-8625F.

“Also, can a part show up from the anodizer with no pits and within days they begin to form?” – This is very abnormal. After sealing, parts should be final rinsed in DI water and thoroughly dried. Shelf life should be nearly infinite.
Or, do you mean within days of saltwater immersion?

“ridges and grooves” sounds like threads. Due to the nature of the anodic oxide growth, which both extends outward from the original surface and penetrates inward as aluminum is consumed, sharp edges and inside corners can be problematic. The outer edge of a sharp thread will have nearly no anodize, while the bottom of the thread can have 2 intersecting anodize growth planes. For your build-up, the anodize thickness is about 0.0012” and you need a radius of curvature of about 0.05.” See MIL-A-8626F, Paragraph 6.10.3 and Table III. The spec. can be downloaded at

http://assist2.daps.dla.mil/quicksearch/

Hope this helps. Please give more information & results.
Ken

 

[SandyH]

Hi Ken,

Yes, the parts were anodized properly in the past by the same anodizer. What we are looking at is possibly contaminated aluminum stock. We are having it assayed to determine this. We had other parts anodized in the same bath with different Al stock and they were fine. So.. doesn't that make you wonder about the raw material itself?

Ken.. will keep you apprised of this problem. I'll let you know the outcome of the tests.

BTW the pits were in the anodization upon receipt from the anodizer.. no saltwater immersion needed!

Thanks for responding to my email,
SandyH

 

[kenvlach]

OK, this sounds more like a problem due to inclusions in the alloy. I've never seen this particular problem in 6061, but it is not uncommon with 2024. During anodizing, inclusions at or just below the surface interfere with the growth of the anodize, and if the inclusion diameter is greater than about 0.0001", it can create a pimple-like defect.
Please examine the surface under an optical microscope or a good hand lens; you may find raised areas (like a boil under skin) where the anodize hasn't yet fractured. You may also find some where the anodize has just started to fracture (looks like an incipient volcano). 'Pop' one of these mounds with a sharp point, and you may find the inclusion underneath the fractured oxide. Use an EDS analyzer on an SEM to positively identify. For 6061, it could be Si, Mg₂Si or oxide.

What was the anodizer's explanation? One thing for sure -- blamed the material!!
Ken

 

[SandyH]

Ken.. I looked at the pitting under a microscope and they look like the entrance to a small crystal cave.. white, crystalline structures. Will definitely look for the "boils"..don't recall seeing those though.

Haven't spoken with the anodizer. Since the latest batch had some good and bad parts we are thinking that it is bad material rather than bad anodizing.

Again.. will let you know more when I do!

Thank you so much for your expertise on this subject.

Take care, SandyH

 

[SandyH]

Hi Ken,

I don't know if I will have access to the results of the aluminum alloy testing. Since I am a third party in the anodizing process, just don't know.

But I do know that some more parts were anodized by the same anodizer and the results were good.
The AL 6061 T6 was purchased from a different supplier.

If I find out anything will let you know.

Again, thanks for responding to this thread.

Sandy