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rhytrr (Mechanical) (OP)
10 Apr 08 21:58
I just read a thread on the issues of applying alodine over hard anodization of 7075 Al Alloy.  My issue is just the reverse, applying a hard anodize finish over alodine (chem film).  Material is a cast Al Alloy A356 T6.  Would like to hear anyone's thoughts on the following spec...

"10. Finish: Bead blast all surfaces specified with glass or polybead meeting MIL-P-85891 (A) screen size 20.  
11. Finish: E512 Per MIL-F14072*(F-21, Method IIIB)
12. Finish: Anodize Flat Black per MIL-A-8625 Type III Class 2.  (Hard Anodization) Mask where indicated."

Would this work, problems?

Thanks,
Scott
 
IRstuff (Aerospace)
10 Apr 08 23:55
I don't get it.  These two processes appear to be incompatible with each other.  Anodization is an oxidation process, which should theoretically supplant any alodining process.  The end result would seem to be a bastardized processing that provides none of the full benefits of either process.

TTFN

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rhytrr (Mechanical) (OP)
11 Apr 08 1:37
Well, the part is masked after alodining, such that only the high wear and IR sensitive surfaces will get the hard anodize.  The rest of the surfaces will only receive the alodining.  The thought here was that if you only bead blast and anodize where indicated, the rest of the surfaces would be bare aluminum.  We wanted to at least protect those without any anodization build up.
israelkk (Aerospace)
11 Apr 08 11:30
I think that it should be in the opposite order. Mask and hard anodize then use alodine (concersion coating) all over the part. As far as I recall alodine will do no harm to the hard anodize. In fact alodine is used as a touch up coating where anodize is missing on the electrodes contact points.  
CoryPad (Materials)
11 Apr 08 13:20
Yes, israelkk's suggestion is better.

Regards,

Cory

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rhytrr (Mechanical) (OP)
12 Apr 08 13:23
Thanks, guys.  From a protection perspective, I agree with your order, anodize 1st and alodine over.  My customer's concern is that the alodine over hard anodize will alter the IR reflectance of the hard anodize surface.  We bead blast to remove any real specular surfaces, then hard anodize flat black.  I have seen this, it is an impressive flat black matte finish.  Would the alodine over it add a sheen, or restore any of the specularity?  Customer also wants to switch (at least for prototypes) to 60601-T6 billet stock, instead of the cast stock listed above.  From an anodizing process, I think that this will be better.  Your thoughts?

Regards,
Scott
coatingengineer (Chemical)
13 Apr 08 21:39

Hi, Scott.

Switching to 6061 will ensure a more uniform finish.
Colorants are pH sensitive and a little 'touchy' using 7050 or 7075 alloy.  

By all means continue bead blasting for your flat, matte finish.  And after MIL-A-8625, add your 'Alodine'.

Keep in mind 'Alodine' (Chemfilm) can be applied locally.  Immersion isn't required.  You'll just have to see what makes the most sense, optically AND economically.

If you need more information, see:
<a href="http://www.industrialcoatingsworld.com/Wear-Resistant-Coatings/Anodizing-.html".Anodizing</a>

Good luck!
       

William Gunnar
http://www.IndustrialCoatingsWorld.com
 

rhytrr (Mechanical) (OP)
17 Apr 08 12:43
Thanks, William.  I may be able to figure out a way of having only the flange alodined last.  Still would need to mask it, even if it wasn't dipped.  But the coating combination (hard anodize body, alodine flange) would be ideal.  Thanks for the website link, very informative.

Thanks to all for contributing to this most informative (at least to me) discussion!

Scott
coatingengineer (Chemical)
18 Apr 08 22:56

Hi, Scott.

I'm glad to help, too.

And thanks for your feedback on my web site.  It's appreciated!  

Have a great weekend.

William Gunnar
http://www.IndustrialCoatingsWorld.com
 

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