Anodising Aluminium! 1

[var10]

Hi,

We often get our parts alodined and painted as most other aircraft parts manufacturers. This process has got us thinking about the time spent on them. It would be easier if we probably got them hard anodised?

I know there are differences between the two in regards to conduction, grounding etc (Still waiting for answer from electronics dep). Furthermore our parts don't have critical effects on the structure of the aircraft. This way I could save both cost and time.

This is my opinion as a junior engineer. Please correct me if I am wrong.

So is anodising common in aircraft parts?

 

[var10]

Just received a test batch. I got them made to MIL-A-8625F Type lll Class 2 Hard black anodised which replaces Alodine MIL-DTL-5541F and PPG defence primer and top coats.

 

[rb1957]

we'll anodise parts we're adding to the exterior of the plane; unusual in my experience to do it on the interior ... maybe in a particulalry corrosive area (like near the galleys).

wait for Will Taylor to give you an authorative answer.

Quando Omni Flunkus Moritati

 

[3DDave]

Anodize can be good. It can fail under alkali and acidic conditions. If it is dyed to get a color, the color choices are limited and the colors can fade, especially in sunshine. I've seen black go to purple and then clear over a period of a few years outside.

The main advantage is that anodize is much harder than paint, and will resist scratches. The thickness is more easily controlled and typically uniform. Unlike paint it won't develop runs. It's a single process that is difficult to contaminate, unlike paint where a good oil spot on the alodine will ruin the paint. It tends not to flake or peel or chip, which paint might. It isn't thin at sharp exterior corners and doesn't excessively fill holes. You can't use putty to fix flaws in anodize, but anodize doesn't hide flaws. It's not easy to touch-up anodize, but it can be laser marked.

Color match is a problem in anodize as each alloy and each temper will take dye differently. It is a good paint base. It is more reliably non-conductive than paint, but don't depend on it.

 

[WKTaylor]

You have NOT defined what Your parts are: non-structural; structural, fatigue [damage-tolerant] critical, mechanical [cylinders, actuators, etc.

Also what is Your surface finish: rough, smooth, "as machined", shot/bead blasted or peened [ferrous shot, glass or ceramic bead], etc.

Are these parts to be left bare or primed/painted, etc?

what alloys/tempers are You intending to anodize? WHY?

What is Your operational environment?... etc.

The answer to these questions will frame my next response... otherwise the answer will get quit convoluted and expansive, very fast.



Regards, Wil Taylor

Trust - But Verify!

We believe to be true what we prefer to be true.

For those who believe, no proof is required; for those who cannot believe, no proof is possible.

Unfortunately, in science what You 'believe' is irrelevant – "Orion"

 

[ABV97]

Most of the time, we have been using etch and alodine and after that epoxy primer and top coating (for external antenna doublers). Looks like it's faster and more economical than anodise, and its covered as per the A/C SRM.

 

[MikeHalloran]

Speaking of dye, one reason it's recommended is that anodized aluminum's surface is not monolithic, but sort of micro-cracked, and it will corrode in the cracks if it's not coated with some kind of sealer.

The dye helps you keep track of which parts have actually been sealed. I've been bitten by that already; specified 'clear anodize' on some custom stampings. They came in nice and shiny, but when I went to use them, after a few months of storage in benign conditions, they were covered with white corrosion products, because no sealer had been applied. Subsequent orders were specified with black anodize, even though no color was required for their function.

Speaking of micro-cracks, I'd be real careful about using anodize on parts that are subject to fatigue loading.



Mike Halloran
Pembroke Pines, FL, USA

 

[3DDave]

Mike,

Nice tip on the fatigue and corrosion angle.

As far as I know sealing anodized aluminum consists of immersion in boiling water** to cause a reaction that causes the pores to swell shut. The advantage to the dye is that you can tell by handling and having the dye smudge off or not if it's been sealed, but they can still (somehow) manage to skip that step. I've heard of the same failure with respect to alodine/iridite/conversion coat. It's not yellow because it has to be, but so the parts that haven't been alodined can be separated from those that are. Clear alodine is available, but discriminated against.

**Further reading says something about sub-boiling temp nickel-acetate solution. Scratch me from any list of potential anodize suppliers.

Also read for fatigue life concerns

http://www.eng-tips.com/viewthread.cfm?qid=181591

http://www.eng-tips.com/viewthread.cfm?qid=198812

http://www.eng-tips.com/viewthread.cfm?qid=137386

 

[var10]

Thanks for your tips. Will Taylor - Like I said they are not structural(is an enclosure for pcb's). Finish is left at as machined. I am not sure what do you mean by "Are these parts to be left bare or primed/painted, etc? " as my question was to find an alternate to priming/painting? The alodining/anodising is mainly to prevent corrosion & then we have the product aesthetics. Our part is located in the rear. It is not subjected to direct sunlight.

 

[drawoh]

var10,

Chemical film finishes are conductive, and act as part of a Faraday cage, protecting electronics from EMI/RFI. If you have printed circuit boards, I strongly recommend chemical film (RoHS compliant).

If you are not designing primary aircraft structure, I recommend aluminium 6061-T6 for machining, and aluminium 5052-H32 for sheet metal. The high strength, aircraft grades are less corrosion resistant, and require more protection. Generally, we don't paint electronics enclosures.

The down side of chemical film is that it is not tough, and that it has the same galling characteristics as uncoated aluminium. These should not be an issue with an electronics housing.

--
JHG

 

[KENAT]

So unless things have changed in the last few months, last I heard there wasn't actually a gold ROHS compliant chem film product yet.

Posting guidelines faq731-376

http://eng-tips.com/market.cfm?

(probably not aimed specifically at you)
What is Engineering anyway: faq1088-1484​

 

[tbuelna]

As drawoh noted, there are some alodine processes that produce a conductive surface, which is important for components that must be electrically bonded. But alodine chem films do not really provide suitable corrosion protection by themselves for most applications. Alodine is not very durable and can easily be damaged, but it is also very easy to touch up.

Alodine has long been used as a surface prep for primer and paint on aluminum substrates, mostly because primers have a hard time adhering to bare aluminum. If prime and paint are required, the process can be quite labor intensive with components that are painted prior to installation. The faying surfaces of flanges and individual bolt/nut faces must usually be masked by hand. var10 mentioned that this application was an enclosure for pcb's, but the only electronics housings I can recall being primed/painted were those used in dirty/wet/corrosive environments. However, I have seen electronic enclosures/housings that used a combination of anodize and alodine surface coatings. The (black) anodize was used on all of the exterior surfaces, but all of the housing's flange faying surfaces and interior surfaces were masked during anodize and then coated with a conductive type of alodine chem film to allow electrical bonding.

Lastly, as 3DDave noted if a penetrant inspection is required on the component, the primer/paint coatings must be removed.