Anodize Over Chromate? Chromate over Anodize?

[tetwin11]

We have an aluminum part that we want to be protected against corrosion. We also want the durability of an anodized finish. However, some faces of this part need to be conductive, which an anodized finish is not.

Would we be able to use a conductive chromate finish over the whole part, then mask the portion that needs to stay conductive, and anodize the remainder over the chromate?

Or could we mask the conductive surfaces, anodize the rest, then chromate over the whole part?

Any other thoughts or suggestions?

Thanks!

 

[IRstuff]

Supposedly, you can do chromate after anodize:

http://en.wikipedia.org/wiki/MIL-DTL-5541#Intended_Usage,

but the chromate will only work on bare aluminum. Note, also, conductive chromate coatings are noticeable lower in corrosion resistance than non-conductive.

My recollection, though, is that chromate conversion coatings are not really that conductive, i.e., they will most likely fail EMI bonding tests.

TTFN
faq731-376

 

[tbuelna]

tetwin11-

Anodize first, with any electrical bonding surfaces masked-off. Then selectively apply chemical conversion coating (alodine) in accordance with MIL-C-5541, type I, class 3, or a similar commercial process.

Just be aware that this chem film only provides marginal corrosion protection. If you need a better level of corrosion protection for your aluminum part you might want to consider some type of plating.

Hope that helps.
Terry

 

[tetwin11]

IRstuff, thanks for the input. The need for the conductive surface is for EMI purposes, so I'll have to look into the conductivity of chromate.

tbuelna, good tip that the chromate only has marginal corrosion protection. When you say "selectively apply", does that mean using a masking over the other surfaces?

Thanks

 

[Cockroach]

I've also done electroless nickel instead of the chrome. You may want to get a quote on this for comparison of economics and keeping the chrome guys honest, price tends to jump with no options.

Regards,
Cockroach

 

[swall]

tbuela has it right. Mask, anodize, then chromate. Cockroach--are you confusing chromate (Alodine and its ilk) with chrome plating?

 

[MikeHalloran]

I don't think Cockroach is any more confused than is usual for him.

I have seen chrome plated aluminum parts. I think there was a nickel strike under the chrome. It looked pretty nice.



Mike Halloran
Pembroke Pines, FL, USA

 

[tbuelna]

tetwin11-

MIL-C-5541, cl.3 alodine is intended for electrical bonding of faying aluminum surfaces. But it is also common practice to seal the joints after assembly, using primer/paint or sealing compounds, to prevent intrusion of moisture. The alodine is only really effective for preventing corrosion of aluminum electrical bond surfaces during extended periods between manufacturing and final assembly or during inventory.

You mention that this is an EMI shielding application. While I have very little electrical engineering knowledge, the typical aluminum shielded enclosures (such as aircraft avionics boxes) that I have seen commonly use a braided stainless steel mesh EMI gasket at the joints. If you are using a similar type of gasket, then an alodine chem film would not be a good choice for the aluminum electrical bond surfaces. The stainless steel mesh gaskets would quickly wear through the alodine chem film. With stainless steel mesh EMI gaskets, it would be better to use something like electroless nickel plating.

Hope that helps you.
Terry

 

[IRstuff]

I've never seen boxes constructed that way myself; all the boxes we've ever done were straight Al-Al on the joints, and could have possibly passed EMI that way with Iridite on the joints, but our customer's EMI guy used a micro-ohmmeter to test for bonding for AWR, which failed, which required us to sand off the Iridite.

TTFN
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[tetwin11]

IRstuff,
I find that interesting. So with the Iridite on the joints, there was enough of a reduction in conductivity between the parts that it would not suffice for EMI?

Terry,
We are not using a mesh gasket. There is a buna o-ring for moisture protection, though.

We used a RoHS compliant clear chromate/iridite finish on both parts, bolted them together, and they passed our requirements for EMI. I don't know a lot about it, but from what I understand, we needed to pass a very high rating.

 

[IRstuff]

We were never given the chance to prove it or disprove it. The older EMI specs required BOTH bonding AND EMI requirements to be met. New specs only require the desired effect, the EMI protection, to be met, given that there was never any proof that the bonding requirement was necessary and sufficient to meet the EMI requirement.

TTFN
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[tbuelna]

......We used a RoHS compliant clear chromate/iridite finish on both parts, bolted them together, and they passed our requirements for EMI. I don't know a lot about it, but from what I understand, we needed to pass a very high rating.....

tetwin11- Getting an electrical bond interface to pass an acceptance test when new is one thing. The much more difficult thing is ensuring that the bond interface maintains its integrity over time, with exposure to the environment or after dis-assembly/re-assembly during service.

There are also different classes of electrical bonds. My background is design of aircraft mechanical systems, and these systems usually require class "L" electrical bonds (lightning strike) to the airframe. As you can imagine, this class of electrical bond has some pretty demanding requirements. A typical approach used is a weatherproof cable assembly attached with stainless steel locknuts/washers to a stainless steel stud, that has been installed into clean bare metal, and then had all joints thoroughly sealed. After the cable is attached and checked for continuity, it is common practice to encapsulate any exposed metal parts in the attachment using something like RTV silicone. While all this may sound extreme to most people, I just wanted to give you an idea of how serious the issue of electrical bonding is to some industries like aerospace.

If you'd like to read more on the subject, I'd suggest starting with MIL-STD-464. Also, here's a link to a company that performs electroless nickel plating of aluminum electronic enclosures.

Good luck to you.
Terry

 

[tetwin11]

That's some good info, Terry. Something like that really puts this into perspective. It certainly does sound extreme, but for the type of products/devices you're working with, it seems like ti would be necessary. Good links too, thanks!