Anodizing vs. Priming Parts

[mdredmond]

I've got free access to anodizing services through an uncle. Is there any reason I shouldn't anodize rather than prime parts? Examples: stab attach brackets, ribs, etc...

I know anodizing has some effect on service life - I saw Van's comments and calculations re the anodized spar (though I don't understand how a .0015 thick anodized layer could cut service life in half!). It seems like if it's good enough for the spar it would be okay for other parts that flex even less. It offers the best corrosion protection and it looks cool too...

Any experts out there?

TIA!

-mdr

[Captain_John]

Matt, I am far from being an egg spurt... but I can tell you what I have learned.

Any effective corrosion measure places a protective barrier between the substrate and the environment. This can be done in many ways.

Anodizing (as I understand it) is a "conversion" method. It electrochemically converts the top surface of the substrate so that it will not easily continue the oxidation process. This is what leads to corrosion.

Priming is the same concept, except it is an application process. We apply the coating to the substrate to form an outer shell which inhibits the oxidation and corrosion process.

Cosmoline does the same thing for artillery, but it is slimy and heavy. This is not recommended for planes!

Oh, and in answer to your question... I would do it! The price is right!

Hope this helps!

CJ

[jcoloccia]

Anodizing is an excellent way to prevent corrosion. I have anodized aluminum parts that are nearly 20 years old, have been through **** and still look as good as the day I bought them. Sweet.

Some of the down side (and the effect on service life) is that anodizing is extremely brittle. An anodized part may have it's fatigue life reduced. Essentially, once the thin anodized layer develops cracks, you've formed a stress riser and the rest of the part will soon follow. Also, anodizing can change the dimensions of your holes, and things. In addition, it's not trivial to anodize 2024 aluminum because of the chemistry involved (it's easy to end up with a good looking part that will not have significant corrosion protection). There's a lot more to it than this...there are lots of advantages, disadvantages, tradeoffs, different types of anodizing, specifying the layer thickness etc etc etc etc.

This is definately a question for Vans. I'm sure he's been asked before and probably has an answer right off the top of his head. I'm gonna guess he's going to recommend you don't do it as it's more trouble than it's worth. I'd be very interested to hear what the master has to say and maybe educate myself a bit more about this.

[mlw450802]

Originally Posted by mdredmond

I've got free access to anodizing services through an uncle. Is there any reason I shouldn't anodize rather than prime parts? Examples: stab attach brackets, ribs, etc...

I know anodizing has some effect on service life - I saw Van's comments and calculations re the anodized spar (though I don't understand how a .0015 thick anodized layer could cut service life in half!). It seems like if it's good enough for the spar it would be okay for other parts that flex even less. It offers the best corrosion protection and it looks cool too...

Any experts out there?

TIA!

-mdr

Matt,
The total fatigue life of a dynamically loaded structure is made up of two components. The first is the crack initiation life and then, once the crack has started, the crack growth life.
For aluminum, the greatest proportion of its life is in the crack initiation stage. A component doesn't have much of its total life remaining after the crack has started.
One of the reasons the anodized spar has reduced projected life is that the anodized layer is sooo brittle that cracks more easily start in that surface. In fact, if you shine a a bright light at the surface in the vicinity of each of the rivets set in the spar, you will see evidence of tiny microcracks radiating from each rivet head.
This is the reason for the admonition to deburr and smooth all edges on the structural pieces. We want to make sure that our planes spend their lives 'initiating' a crack, not growing one that already exists.
-mike

[fixnflyguy]

Well now you have opened the proverbial "can of worms"!
I will put in my two cents worth on this subject. I have spent the last 25 years of my life working in the airline heavy jet bussiness, and specialize in structures/corrosion protection. Here is a condensed crash course of the basics that apply to RV's and other similar metal aircraft.First, there are two basic catagories of metal used: Extrusion as is used for stringers,brackets,ect. and then there is Sheet stock for skins, ribs, and frames. Both categories utilize an alloy composition for strength..2024 for most skin, and 6061 for most extrusion. The skin material is almost allways refered to as ALCLAD..a simple trade name for an applied coating of PURE aluminum on each side of the sheet. The extrusions are NON-CLAD, therefore are the actual alloy exposed to the elements.So, having said that,pure aluminum is almost impervious to corrosion,therefore,a plane can be polished.Imagine ALCLAD as a piece of plywood..its all good until water hits the edge. Extrusions,having no built in corrosion protection need to be completely protected if you expect to keep corrosion away.Anodizing is a good process for any extrusion, but not really nessesary on sheet.The best protection for your skins, make sure the edges, and all the holes get primer in them before assembly...very tedious, but it will pay of later...Airlines and big manufactures have spent mega-bucks doing research, and you wont find a single aluminum part that isn't green,sheet or extrusion, unless its on a seat or galley where people dont want to see green.... just my opinion.

Bill Englert.

[mdredmond]

Thanks, guys, for your replies.

I'm going to talk to Van's about this, for sure. But if I understand this correctly, an extruded part (I wouldn't anodize skins) that is very smooth, deburred, holes drilled before anodizing, etc. should be able to be anodized -or- alodined and be okay?

Bill: You say 'green' - do you mean alodined (I've never used it - is it green)?

TIA!

-mdr

[PJSeipel]

He probably means primed. Standard aircraft grade primer is usually green.

PJ

[fixnflyguy]

What I mean by "green" is the usuall color of chromate based primer... These primers can be enamel based (rattle can usually), or 2 part epoxy or urethane.The 2 part primers are almost indestructable by fluid or abrasion. Alodine is a chemical conversion used to prepare the surface for a final primer/finish. Alodine (an acid etch)usually leaves the metal a light gold color, and is nuetralized with water after application. Anodizing on the other hand is a chemical/electrical conversion requiring specialized facilities to apply...So by "green", I am refering to a primed finish...there are some great primers in other colors also, such as clear,yellow,white and grey. Green is just the old school normal color for aircraft aluminum primer. So many builders spend incredible amounts of time debating this issue...bottom line keep some primer around all the time..its easy to touch up when a part needs reworked,ect.For aircraft primary structure, I view anodizing as a parallel process to alodining (a prep step before paint)...there again,just my opinion.

Bill E.

[RV8RIVETER]

Also, one downside to anodizing is that the aluminum oxide layer grown on the surface to protect against corrosion also inhibits conductivity. Therefore you would not want to anodize the rivet holes after you drilled and sized them.

[mdredmond]

Originally Posted by RV8RIVETER

Also, one downside to anodizing is that the aluminum oxide layer grown on the surface to protect against corrosion also inhibits conductivity. Therefore you would not want to anodize the rivet holes after you drilled and sized them.

Why is that? Just curious.