Aluminium -- Black, Conductive Finishes 1

[drawoh]

Does anybody know of a black, conductive finish for aluminium? I am looking for something that allows noise shielding of electronics, and absorption of stray light. A matte finish is preferable to a glossy finish.

Thank you.

JHG

 

[IRstuff]

Why does that matter? Aluminum itself is as conductive as any finish you can put on it. You simly need to have portions of the chassis unanodized so that there is a good electrical bond to the metal. There is no need for a conductive finish.

TTFN

 

[drawoh]

IRstuff,

I design a lot of boxes that contain high speed circuits. These require serious shielding, which is achieved by grounding everything, hence, the conductive surface.

Right now, I try to use yellow alodine, which is conductive and which provides some corrosion protection. You cannot always tell if a clear alodine finish has been applied, so I prefer not to use it. Alodine is a chromate conversion coating.

My next choise is black anodize, which our optics people like, but which provides limited shielding due to its lack of conductivity. I don't like the idea of scraping off finish, especially in production. We want continuous, conductive seams.

I am going to look into masked anodize finishes. I have seen artwork silkscreened on a panel by an anodizer.

JHG

 

[TVP]

drawoh,

You should definitely investigate masking in conjunction with anodizing. Any good vendor of anodizing will be capable of selective anodizing or using a suitable mask.

 

[IRstuff]

The finsh is NOT the shield, the aluminum is. You should consult with your EMI guy, if you have one.

Military equipment designers use anodized aluminum chassis with EMI requirements up to 20 kV/m at up to 40 GHz routinely.

TTFN

 

[kenvlach]

IRstuff & TVP are correct as usual.
Some more detail: The usual procedure is to Alodine (Henkel's chromate brand) the entire part, plug threaded holes and apply masking over any additional desired electrical contact areas, strip the remaininng Alodine and anodize, dye and seal the anodize, and unmask.
Larger diameter threaded holes may be anodized if not required for electrical, but this often requires oversized tapping to allow for anodize buildup on all thread faces.

Silk screened or hand brushed artwork is an entirely different animal. The entire part is anodized; then masking is used to allow selective dyeing.

 

[drawoh]

kenvlach,

What do you mean by "larger" tapped holes? I have learned the hard way never again to screw bare or Alodined alumimium parts together. Our maximum sized screws are around 3/8 or 10mm. We make fairly large threaded connections for lens mounts, which we always anodize.

I was under the impression that anodizers caused a net removal of material, with the etching more than making up for the material added by the actual anodize process.

Thanks everybody. This has helped.

JHG

 

[unclesyd]

Give these people a call and ask about the "Ag Process"in anodizing Al. This process will restore some electrical conductivity to the anodized coating.

http://www.metalast.com/main.html

 

[IRstuff]

The material is NOT added, it comes from coverting the aluminum to an oxide

TTFN

 

[kenvlach]

drawoh,
For conventional sulfuric acid anodizing (MIL-A-8625F, Type II) the buildup per surface is about 1/3 the anodize thickness. For hard anodize, buildup is about 1/2 the anodize thickness.
These are only general 'rules of thumb.' The buildup-to-thickness ratio varies with alloy and anodizing parameters: acid concentration, temperature, current density, use of additives, etc. Anodizers usually control thickness to +10% using eddy-current thickness testers and process control; tighter tolerances cost more.

Re anodized threaded holes. Unless it's a deep blind hole or one for which trapping air pockets is unavoidable, anodizing buildup occurs on upper and lower thread faces on both sides of the hole, so the usual approximation is 4x the buildup on the diameter as on a flat surface. Machine shops use larger diameter, 'looser,' taps for holes to be anodized.

Please explain whatever problem is meant by "I have learned the hard way never again to screw bare or Alodined alumimium parts together." Al on Al can produce galling.

Re "anodizers caused a net removal of material, with the etching more than making up for the material added by the actual anodize process."
-- Only if the customer specifies heavy etching and a thin 'flash' of anodize.

Unclesyd, silver impregnation is somewhat expensive.

IRstuff, I prefer the phrasing 'oxidizing the aluminum.' Converting the aluminum to an oxide sounds a bit like alchemy.
Ken

 

[drawoh]

kenvlach,

>> Please explain whatever problem is meant by "I have learned the hard way never again to screw bare or Alodined alumimium parts together."

>> Al on Al can produce galling.

You answered your question. It did produce galling.

Thank you all very much.

JHG

 

[IRstuff]

Ken,

Sure, but it is called a "chemical conversion coating" in the MIL spec, alchemy not withstanding...

Oh, btw... Alodine or Iridite, while conductive, usually do not pass the MIL standard for bonding. Only bare aluminum meets that requirement.



TTFN

 

[kenvlach]

IRstuff,
Semantics don't matter long as people know what is meant & you use a specification; idiom can be a matter of personal preference.

For chromate (chemical) conversion coatings on Al, it is clear that there is an added ingredient in the coating. Although, turning bare Al into gold via Alodining has always appealled to the alchemist in me.

But for anodizing, the added ingredient is oxygen from electrolytic action. "Converting the aluminum to an oxide" suggests
4 Al --> Al₂O₃ + Li + h[ν]

So, I prefer the more descriptive 'electrolytic oxidation' to describe anodize. The military isn't always clear. MIL-HDBK-349, Para. 5.4.4.2, describes anodizing as "converting the surface of the aluminum...to corrosion resistant aluminum oxide." Sounds like 'surface' is a key ingredient?!
But MIL-A-8625F is correct: "electrolytically formed anodic coatings on aluminum"

Re bonding on bare Al: I recall that phosphoric acid anodize (large, open pores for bonding grip, BAC 5514 Phosphoric Acid Anodizing of Aluminum for Structural Bonding, Boeing) is often considered to give the best bond strength. Sulfuric + chromic acid etching improves mechanical grip, and residual chromate may improve corrosion resistance.
But, doesn't bonding onto bare Al often allow corrosion & de-lamination?

The thinner, Class 3 chemfilm can give better adhesion under paint, but the thicker Class 1A is usual for greater corrosion protection. Chromic acid anodize is more expensive but much stronger, used where there may be some abrasion, as with miltary aircraft landing gear.

Have a good weekend.
Ken

 

[IRstuff]

Sure, there's always a trade with that specific problem. However, 25 milliohms is a near-term constraint, while corrosion is a long-term problem.

TTFN

 

[jeff1234]

I think there may be a few misconceptions in the above thread...so here are a few thoughts. I work with aluminum enclosures in the aviation world and have often been bit by aluminum interfaces. In short, bare aluminum or chemical conversion coated films both stink as conductors!!
1)chemical conversion coating is NOT conductive...but it is thin and easily penetrated, hence it is mistakenly assumed to be a conductive coating.
2) bare aluminum quickly forms a non conductive film when exposed to oxygen.
3)If you really need an aluminum part to be conductive, you'd better be penetrating into the surface, or plate the aluminum with tin or nickel or other conductive plating.

 

[drawoh]

jeff1234,

Quite a few years ago, I worked on a Lidar sensor that was mostly black anodized for the sake of the optics. The electronics debugging was getting way out of hand, so we stopped testing and took the whole thing apart. We stripped off the anodize finish, chem. filmed everything and painted the optical surfaces matte black. When we reassembled everything, the voltage drop from end to end, through multiple mechanical interfaces, was close to zero ohms. We were very happy with this.

I was wondering about plating. Can any of this be coloured black without losing conductivity?

JHG

 

[IRstuff]

You've already listed your options:
anodize colored regions, bare or chemfilm elsewhere
black paint colored regions, bare or chemfilm elsewhere

TTFN

 

[jeff1234]

You might check into black chrome, black nickel, and ebonol. Not sure how conductive they are and if they are appropriate for aluminum, they just came to mind.
Also, check out the excellent site

http://www.finishing.com

 

[unclesyd]

I have a sample of Aluminum with a dark Zincate coating and it appears to be fairly conductive. No idea what type or where it came from.

What you may look at is using a Zincate coating on the Aluminum then a black coating for the Zinc.

I would query some of the big conversion coating companies about this.