Corrosion on lift strut end fittings

[Battson]

I was walking around inspecting my plane last night, and I noticed the paint on my top strut attach fitting is flaking off. This is being driven my surface corrosion under the paint, which is the real problem.

I wonder if this might be cathodic corrosion, because of the steel attach bracket. Some of the paint was scratched off the mating surfaces, when the parts were assembled. We live near the sea, and do get salt in the air, plus flying in the rain which creates an electrolytic solution. That idea makes me worried.

Despite being under the fairing, those fittings are still getting wet at times.

Clearly I could repaint that area, but that probably isn't going to solve the problem. I have greased it for now to protect it, until I can solve it properly.

Who has some ideas about how I should address that issue??

[SC Patrol 253]

Have you considered getting the piece anodized?

[Battson]

Interesting suggestion.

It was already painted, and that hasn't protected it because of the tight clearances scraping off the paint. The fit is so tight, I doubt anodization would help, as you would need to resize things to allow for the extra thickness. Not something I am interested in.

[Chewie]

Maybe that steel conversion stuff? Stewart's sells some for rust that has already started. I'm sure other vendors offer similar. Get one of the tech support guys, they will be able to steer you. Take pictures!

[Battson]

I am not familiar with that sort of product - it sounds good - can you provide any more information??

[Chewie]

Here is what I found, but I've never used it:

http://www.stewartsystems.aero/products/95/steel-conversion-coating-quart

[S Lathrop]

As I think about this problem, what you are looking at has bigger implications.

There is always going to be some movement between the steel bars and the aluminum parts they attach to. The bars are bolted to the spar and only maybe the paint on both surfaces will protect the mating surfaces but there will be some amount of fretting. The bracket that attaches the lifting strut and the steel bars is a slip fit and there is little or no clamping of the bracket to the steel bars. And this for sure does move.

You may be onto something with lubrication. Something that will prevent the surfaces from fretting as the move against each other.

[Battson]

I think any movement would be very, very microscopic.

The whole assembly is a press-fit. The two parts have to be hammered together with a rubber mallet, and then the bolt has to be hammered through. That in of itself has probably created the problem, as that is certainly where the paint got removed and how the surface corrosion has started from. Dimensionally, I think the assembly is quite stable, and I am not convinced it would fret.

I feel certain it's the addition of water to wash the plane which has led to the corrosion. Similar joints elsewhere in the place, such as the spar attachment points, are corrosion free because they are dry 100% of the time. The nobility difference between high carbon steel and alloyed aluminium isn't high enough to corrode without the presence of water. This corrosion has just shown itself after 430 hours.

[patrickh99]

You might want to try something like ACF-50 or Boeshield T-9

[lsa140]

I agree, one of the penetrating anti-corrosion lubricants applied as needed to keep a light coating on the area might work well. (after the surface corrosion is dealt with)

[Battson]

I agree - having spoken to a number of aircraft maintenance people about this, particularly the guys working around marine environments, the correct thing do to would have been:-

Put an aviation-grade anti-corrosion coating product between the dissimilar metals before they were assembled.

^ This is worth anyone building taking note of, that task would be easily done during the build...

Having done an extensive review of the plane during the weekend, I note that this problem exists anywhere dissimilar metals are in contact without a barrier coating AND any water gets into the join.

[Battson]

The only other note - to close this thread out:

Once you get the corrosion on a critical part like this, it's not enough just to put in some anti-corrosion product and let it go.

First, I have to treat and stop the corrosion that has already started. Then I have to put in the anti-corrosion product (I am going to use Duralac Jointing Compound - the nasty yellow stuff). Then I have to re-paint it.

So - I encourage builders to take these steps during the build. The retro-fit is a lot more work overall.

[Bcone1381]

Would it be an improvement for future builders to make the Strut End Fittings from 4130N Steel instead of the 2024-T3 Aluminum thus moving the dis-similar metal joint to the wing strut instead of the attach fittings? Would the ability to protect the surfaces with coatings be easier there?

[Battson]

Would it be an improvement for future builders to make the Strut End Fittings from 4130N Steel instead of the 2024-T3 Aluminum thus moving the dis-similar metal joint to the wing strut instead of the attach fittings? Would the ability to protect the surfaces with coatings be easier there?

As it stands, the bolted dissimilar metal joint is highly visible, easy to monitor, and you have easy access to apply any preventive or corrective treatments to it.

Making that piece from steel would be very heavy, and move the dissimilar metal joint to the inside of the lift strut. Yes the fit is better, but it's hard to paint inside there at all. It would be hard to monitor any corrosion developing inside the lift strut. You couldn't get in there to treat corrosion if it happened.

So my opinion is that I should have taken reasonably obvious steps to prevent corrosion - I don't think the design is flawed.

The slogan for Duralac is basically "use me any time you pick up a drill" - which is a nice ethos. Obviously not applicable to rivets where alloy meets alloy...

[lsa140]

Thanks for bringing this up, and talking through it on the forum. I was wondering about this as I glanced over the plans for the first time and took note of all the steel parts incorporated into the wing.

[Bradrock]

Duralac on bolts and between the surfaces on the strut attach bars? Fortunately I haven't closed up my wings yet.

[Battson]

Not a bad idea, certainly wouldn't hurt, and could help a lot.
I have had no issues in there, everything is painted and an easy fit - so the paint was never damaged during assembly. Also, it stays almost 100% dry in there. Without any water to act as an electrolyte, steel and aluminium form a very poor electrical cell.

[whee]

Anybody has a source for Duralac in the US? Also, is it messy like ACF50 or CorrosionX. Seems like putting Duralac everywhere dissimilar metals are in contact is a good idea especially where I'm not planning to paint my wings or metal sections of the fuselage.

[whee]

Thanks. I doubt my local boat shops will have anything. They are basically just dealers and don't know anything about building and only stock a few parts...it's pretty lame. I'll check with my jet boat friends and see what I can find.

[Battson]

Oh of course - I forgot to consider whereabouts you live... not much salt water around those parts!
A good Chandlery would usually have a range of products.

[patrickh99]

Try this:

http://www.fisheriessupply.com/maintenance/lubricants-corrosion-inhibitors-and-threadlockers/corrosion-inhibitors

They don't have listed the newer Duralac green, which is supposed to work better.
I have not used them, so YMMV.

Found some boat builders recommend Noalox, in the electrical dept. at your local big box store.

Tef-gel was also mentioned, but that appears to be more of an anti-seize.

[Pinejuice]

For discussion sake, maybe an alternative to Duralac (especially for those with already completed aircraft) would be to find a spot for a bolt on magnesium, zinc or other active metal anode? Keeping an eye on the anode and replacing it every so often? I would imagine that it could provide great protection to any area that receives the same exposure as it (to saltwater or whatever corrosive environment).

[Zzz]

It seems like in the engineering phase, had this been a known concern, there would have been some allowance for a neutral material spacer washer, like Delrin or some high load HDPE type material. Thanks for the heads up that it's an issue.