Bending anodized aluminum tube/pipe

A. Anodized coatings are very hard and brittle. According to Properties of Electrodeposited Metals and Alloys [affil link to Amazon or on AbeBooks affil links] by Safranek, this is something that "isn't done".

A. Hi Friend,

I have read of a Florida company that daily builds custom boat towers for large yachts from anodized aluminum pipe & tube materials and daily bends and even WELDS anodized materials including for newly ordered custom yachts, like Hatteras and Viking Yachts which were mentioned in the article. Based on that article, it seems not a problem to bend anodized alum. The article I read was mostly about TIG welding techniques they use to weld anodized aluminum. I don't recall the company's name. Bending anodized alum. seems not a problem.

A. Here is the article (in a .pdf file) on the boat company that welds anodized aluminum in the tower manufacturing business.

A. I have no personal knowledge of aluminum tube bending for railings, but technology keeps changing and improving, and just because some of us are not familiar with something doesn't necessarily mean it's no good. It could be it's a proven technology that I don't happen to know about yet. Maybe the cracks are so fine and well dispersed that blooming corrosion doesn't occur. But from book knowledge it sounds like a very big problem because the coatings are brittle and glass-like and surely crack during bending.

Color matching is difficult. If you send the parts out for re-anodizing they will probably come back darker, lighter, or just generally "off" in one direction or another. If you were able to send them back to the original anodizer they would probably be a lot closer.

A. Anodize is a ceramic. I do not know any ceramic that will not crack or break when slightly bent. I would bet a large Starbucks that you will see corrosion in at least a few bent portions within a year. Paint is an option.

A. Now that you have sent photos I don't think anyone will takes James' bet :-)

From this distance the integrity of the anodized coating appears to be destroyed, and it appears that the right answer is to strip and re-anodize them now that they are formed.

Thanks, Michael. Although the referenced article discusses welding anodized aluminum, I didn't see a claim that they bend it after anodizing as John M asserts.

A. I build Ttops and fishing towers out of anodized aluminum pipe every day. We use pipe that is already anodized when we receive it. The finish on the bent sections seems to hold up fine on our projects, and nearly all of them are exposed frequently to salt water. Hope this helps

That is quite interesting, John. Do you have any theory why you are able to bend your pipes without problems while the anodizing on Alan's pipe appears to be cracked to worthlessness?

A. The coating does crack exactly like the pipe in the picture when we bend our pipe, its never really been an issue as far as corrosion goes. The coating does corrode in the environment that most of our work is subjected to, but it's pretty much spread out evenly and isn't concentrated on the bends in any way.

A. I have been Welding and fabricating anodized aluminum for quite some time. With the proper tooling and bending equipment you can greatly reduce the galling of anodizing in the bent area. Anodizing is a tough coating in general. From the look of the pics above, the bent section looks rough. I think using the right techniques it could be prevented.

! ! I build boat towers, ladder racks, etc. everyday of my life and other products made of anodized aluminum. Yes you most certainly can bend this stuff after it is anodized. You need the proper tooling and experience to work with anodized material. It's not for everyone and separates the entry level fabricator from the professional in my opinion.

Thanks, Larry. Anodized coatings are ceramic and can't be stretched. As Michael says, they are like an eggshell.

As with anything else, experience counts, and I'll trust that you are able to bend the tubing such that you get thousands of microcracks instead of a small number of large ones, and are able to avoid the ugly white discoloration that less skilled fabricators get, and which is illustrated and which you commented on.

I would be very curious to see an ASTM B117 salt spray test on such post-anodizing fabrications to learn how they fare in certified corrosion testing. Maybe the cracks are small enough that the corrosion of the exposed aluminum is not an issue -- I know that anodizing also cracks from expansion of the aluminum when it is strongly heated, yet hard anodized frying pans seem to held up fine. Thanks again!

Regards,

Feel free to stop by my shop and pick up a free sample. I will bend a piece for you and you can test it. I have seen My towers abused by salt water after 10 years look good. anodizing is an electrical process not a ceramic coating.

Anodizing: Aluminum is immersed in a tank containing an electrolyte having a 15% sulfuric acid concentration. Electric current is passed through the electrolyte and the aluminum is made the anode in this electrolytic cell; the tank is the cathode. Voltage applied across the anode and cathode causes negatively charged anions to migrate to the anode where the oxygen in the anions combines with the aluminum to form aluminum oxide (Al2O3).

Thanks, Larry. Most of our readers have spent their whole careers designing, installing, starting up, and operating anodizing and plating lines, and understand how anodizing is done, but we probably should have used the term 'ceramic-like' rather than 'ceramic' in making the point that anodized coatings have no elasticity.

My curiosity isn't sufficient to authorize the cost of a salt spray test with no way to recover the cost, but hopefully a reader may come along who wants to take you up on this. Thanks again!

Regards,

A. My name is Talbert. I used to work with a marine custom fab and tower builder. John is correct. The anodized coating will "craze" or crack at the bend area but, as John said, there is virtually no decrease in corrosion resistance.

A. Hi, Peter. It can be, for example M-16 rifle barrels, but it's much more common that it's not. Please tell us your situation: marine tubing, condominium guardrail, machine components, chemical piping, etc. Thanks.

Regards,

A. Hi, Dodo. Well, I'd suggest buying one or two pipes and trying it, rather than buying thousands of dollars worth :-)

... but I think a 3/4" pipe wrapped around a 7" mandrel would be doable (remember that 3/4" pipe is a little over an inch in O.D.). I think a thinner wall tubing or a lighter schedule pipe would be easier to bend. Good luck.

But if it is possible to bend the tubing first, and then anodize it, that would be much better!
Regards,

A. YOU SHOULD HAVE NO PROBLEM BENDING ANODIZED PIPE, BUT YOU WILL BREAK THROUGH THE ANODIZING.

A. Hi Dodo, try filling dry fine sand in the pipe and bend, it will bend without any cracks. Even if it is of small wall thickness.

A. Yes it can be bent, I watched a tower builder buy all anodized tubing and components then bend weld and build the towers. I bought the materials and built my own. You can bend it but you need a special bender that supports the sides of the tubing, a regular steel tubing bender will make it kink. Then you can heliarc weld everything and paint the welds over with silver paint.

A. Hi Hallvard. Yes, anodizing does create a controlled oxidized film on the aluminum, but that does mean that the "corrosion product" created by anodizing is exactly the same as from natural corrosion. Anodized coatings are thick, uniform, and considered attractive, as a result of the fairly high voltage/current that is applied, followed by controlled sealing, whereas more random corrosion can be rough and quite ugly.

Regards,

A. The term "micro crazing" might not be the right word, but, indeed the pores will be spread and the seal broken open as a function of the alloy, anodic film thickness, and the angle of the bend. Corrosion places will open up. the unaided eye will not see the fracture until after the corrosion sets in.