anodizing aluminum parts

[Mark Goldberg]

Among the big group participating or just monitoring this forum - I am hoping someone has professional expertise in dealing with anodizing aluminum parts for airplanes. I have read and studied a bit on the internet and have general knowledge. But my hope is that someone reading this has actual experience having aerospace parts (not furniture) anodized. There are various different processes. Type I (chromic acid) was the one used for a long time in aerospace but has become harder to find because of nasty environmental stuff. Type II is now more common but it would seem used less on aerospace parts. There are other types as well.

Some advice from someone with experience would be much appreciated. Please give me a call if you can help. Mark 512/626-7886.

[S Lathrop]

I have a lot of experience anodizing parts that I make. I am not all that knowledgeable on the subject. I anodize for corrosion protection. I do not anodize any thing that is highly stressed.

[Mark Goldberg]

There are some ways of anodizing that are not for stressed parts. Especially what is called Hard Anodizing. Other anodizing techniques are ok for stressed parts. Mark

[svyolo]

Are you thinking about anodizing the wings, inside or out?

[Mark Goldberg]

Little more complicated that that John. Mark

[Mark Goldberg]

What I did just find out is that the more common type of anodizing these days - Type II - is what Vans uses on their AL parts. The older way it was done, Type I, is harder to find now. Mark

[S Lathrop]

I use a process called "Bright Dip" for a lot of parts where I am concerned about stress or I think I might have to do a repair at some time down the road. It does not change the color of the material, but it does stop oxidation and corrosion.

[spinningwrench]

Mark, I used to do a lot of anodizing both hard surfacing and cosmetic, not so much anymore, However Pioneer Metal Finishing which has branches throughout the USA is very good at this process and can give you more in depth technical information. Only the good shops can anodize both the 6061 and 2024 series of aluminum, the high levels of copper in one of them makes a difference. For simple shop use I do a fair amount of alodining which is a great prep for a surface coating such as epoxy primer.

web site is pioneermetal.com

[Mark Goldberg]

Thanks Bruce. Unfortunately Pioneer does not have any locations close to me here in TX. But they do have a facility in Queretaro north of Mexico City where a lot of big plane aerospace manufacturing is located. Your suggestion might be helpful. Mark

[spinningwrench]

Some additional thoughts. Hard coating can only be used on the 6061 alloy not 2024. There will be some dimensional changes with the hard coating process. Generally if I hardcoat and have some very fussy hole sizes I leave that for after the final anodizing and then ream them to size. Hard coating generally can only be in two colors, clear or black which turns out a nice brown color.

[svyolo]

Mark I tried to post a screenshot of anodizing at Boeing. Boeing and Airbus both do it. Airbus anodizes wing skins. The screenshot actually explained the anodizing process Boeing uses.

[Mark Goldberg]

Boeing came up with a proprietary process called Boric Sulfuric acid process (BSAA) to replace the old Type I Chromic acid process. I have read that is what they use now. But I was hoping someone on this forum works for Boeing and could confirm that. Not sure what process businesses use down here in Texas but I assume some at least have a variety of options. We are not ready with parts that need anodizing - so I am waiting until that is closer to start actually calling potential facilities that we might use. But since I learned that Vans uses the real common Type II process, I am less worried about finding a good shop.

[JimParker256]

Mark, what parts would "need" anodizing? Enquiring minds...

[Glenn Patterson]

Safari Helicopters used to be built and sold near me. We did a tour of their shop to learn about welding and working with 4130 tubing. They had a brilliant & very skilled organization. I remembered reading about a Safari accident attributed to anodized parts and a early failure of an anodized part that caused a crash. The article may have been in the monthly EEA magazine. Safari stopped anodizing parts following the accident. I did a quick web search and found this.

http://www.redbackaviation.com/anodizing-and-fatigue-life-safari-helicopter/

[fairchild]

For Mark and Glenn and others.... Does the article cited in this Redback piece conclude that we shouldn't alodyne the innerds of our wings ? I THINK I would be more concerned
with the main spars than ribs ----- but none the less--- same problem/question---- just a matter of degree I suppose......
Are we reducing the fitigue life of our ribs by alodyning them ?

[Mark Goldberg]

Yesterday I had a conversation with the tech rep at a big company who does anodizing. Maybe the biggest company in the Houston area. We talked about the processes they use, and then I asked about the price of anodizing parts for us. His response was a price that is probably 10X the cost of what we do at the factory when we prep and paint/primer aluminum.

As they say in Mexico (lots of sayings) - "the soup was more expensive than the meatballs". So I appreciate all the help some of you have given. But it looks like Anodizing is not in our future after all. And when you read the literature, it talks about anodizing being especially corrosion resistant for 10 years. The life of the good epoxy primer we use is much longer.

As far as anodizing creating problems with the metal - it is a little hard to believe since the process has been used for so long in aerospace applications. But that is neither here nor there for us since we will not be doing it. If this project we are investigating turns out to be something we actually do, I will inform you about it. Until then, we are just checking out some ideas. Mark

[fairchild]

I found this on the web---- I think I will alodyne the ribs but NOT the spar parts. My feeling is that the ribs will be under more indirect stress as compared to the spars. But maybe I shouldnt
alodyne any of it....... I AM using water based epoxy over the alodyne now. As far as I know there is NO fatigue penalty for an epoxy coating- as it is not harder than the material underneath.

I believe I have read that the cessna bird dogs- since they were a military product-- were chromate primed throughout--- and even now the owners claim that the oregional planes dont
have any corrosion problems. And- I would hope that current chromates are maybe better than to 1960's solvent based chromates. (any opinions here? )
But I dont know -- since many "eco-friendly" products aernt as good as the older counterparts in performance.

here is the link ;;;;;

https://www.finishing.com/153/54.shtml

There is another one at the same place but I havnt read it yet.....

Tim

[BTAZ]

Alodine and Anodizing are completely different processes.

Alodine is a conversion coat; the Aluminum is etched to remove the outer layer of aluminum oxide and then it is "reacted" to convert the newly formed outer layer to something with better corrosion resistance and that is also better bonding to a primer. The Iridescent gold color we are used to is from Hexavalent Chromium. This is on the list of "banned" metals in EU countries(look up RoHS, same rule that removed lead from solder in electronics) so is now only used in the US and for "Military grade" exceptions in Europe.

"Clear chem film" took the place of the chromium based conversion coat process. IIRC, it is Phosphate based. Note, as the name implies, the surface looks unchanged.

Anodizing is an electrolytic process that increases the thickness of the protective outer layer of aluminum oxide. It also can change the property of the Aluminum itself and is what caused the issue for the helicopter in the article previously linked.

Alodining parts at home(eve large ones) is pretty straightforward and I have never heard of the process causing issues further down the road.

Anodizing even small parts at home take a bit more effort and I have heard of cracking issues as a result of the process.

[fairchild]

Yes I suppose the processes are completely different and -- I guess yield a product different in degree--- but what they appear to have in common is that they are both
much harder (and more inert) than the base---- but as they are mechanically "one" with the base--- a crack can easily form in the much harder coating --- (maybe with
relatively little cause because of it much greater hardness) and because the surfaces are attached--- the crack in the coating ... when it reaches the surface of the base material--- it generates the beginnings of a crack there where there wouldnt have otherwise been one. I guess it would be the equivalent of a sharp scratch on your part.
That would seem to kind of open up an opportunity for the crack to slowly spread - IF- the stress is high enough there.

It appears that the problem grows rapidly as the coating thickness increases. Sounds like thats why the much thinner alodyne fares much better than regular anodising.
But that last article seems to indicate that even alodyne can reduce fatigue life by as much as 10 %. I am thinking that if I am epoxy priming over top of it anyway---
I might not ge gaining much corrosion protection and loosing 10% fatigue life.

Im thinking on the main spars--- I may go to a traditional expoxy-chromate-solvent system instead of the water based. I think the water based will be fine everywhere else--
I have had some variation in coverage with the water based. I think the od school stuff will be harder and tougher and I can apply it more even.

The other reason I have been alodining -- the material I got had some scratches from where I bought it (in shearing) so I had to work them out-- most went through the clad-
so I felt obligated to alodine those. I will NOT order from there again. Next i will give spruce a try. Maybe get perfect material.

T

[Zook]

Excuse my ignorance. I just sold a 1966 172 and as far as I could see the inside of the wings where bare aluminum? We didn’t have any corousion as far as I know. What is the advantage or doing any treatment at all or just priming them ?