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Removing corrosion

00Dan

Well Known Member
I detected some intergranular corrosion on some of the rivets and along the seam on the underside of my flaps recently. A hangar neighbor has a rivet blaster he bought to address a similar situation on his plane that he said I could borrow.

Once the area is stripped down, how long is it safe to leave that aluminum exposed, or does it need to be primed and repainted without undue delay?

I also read about specifications for thickness removed that exist, but I’m unsure how to go about measuring that. Any tips?
 
aluminum starts the corrossion process seconds after removal. The invisible layer of oxidation is what prevents the alum from corroding further and this layer creates an adhesion problem for paint.

The general guideline is to apply primer within 2 hours of scuffing.

Larry
 
If you're actually "seeing" intergranular corrosion with the naked eye, that part should be replaced. Evidenced by puffing of the surface, with significant material involved, lots of white powdery residue that flakes off when disturbed.

ICG is also most common in highly worked or welded high strength materials. Don't know if sheet 2024 and rivets qualifies. Possibly so after the riveting process.

Regardless, as everyone says, do the cleanup and prime soon after the prep is done. A topcoat of paint to seal the boundary between the rivet and skin, hopefully slowing further issues.

Good luck and keep an eye on that!
 
If you're actually "seeing" intergranular corrosion with the naked eye, that part should be replaced. Evidenced by puffing of the surface, with significant material involved, lots of white powdery residue that flakes off when disturbed.

ICG is also most common in highly worked or welded high strength materials. Don't know if sheet 2024 and rivets qualifies. Possibly so after the riveting process.

Regardless, as everyone says, do the cleanup and prime soon after the prep is done. A topcoat of paint to seal the boundary between the rivet and skin, hopefully slowing further issues.

Good luck and keep an eye on that!

Here’s an example picture in case I misidentified.
 

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Hard to say if that's IG corrosion or filiform or what, but it doesn't look good.

I dealt with this as my job for many years and the correct thing to do would be to remove the rivets in that seam, sand/abrade the corrosion away (being careful not to smear metal into the corrosion to hide it), inspect it with high magnification to ensure all corrosion is gone, and measure the remaining thickness (micrometer or ultrasonic thickness gauge). For big jets the maintenance manual often has an allowable corrosion removal limit, often 10%, sometimes as high as 25%, but nothing like that exists for these aircraft that I know of. You can look at the allowable tolerance range for the bare aluminum sheet (which I believe is in the AMS material spec or H35.1) and as long as you're not thinner than the thinnest allowable you should be fine. For that skin it is probably on the order of .0015" however, so not much.

Given the amount of time this rework would take, the low cost of Vans parts, and the high likelihood that the corrosion extends between the sheets and into the rivet area and would be unusable anyway I would strongly consider building a new flap or searching for a replacement.
 
Nothing too bad.

That's not IG corrosion. As that name implies, that reaction occurs within the metal grain boundary from metals that don't like each other; most don't. There's most always a tendency for electron exchange -> corrosion.

So Aluminum and Copper are alloyed together when they should generally be insulated from each other yet 2024 can last forever. Someone pointed out stress being an IG contributor. This is true. A catalyst like water is way worse for said electron exchange. BTW. Look at 7075 sometime. Impressive properties but zinc and aluminum fare worse together.

Yours should clean up. make sure it's all gone before retreating the area. That's another rabbit hole. If you have a rivet spot blaster, you're most of the way there.

I have some 2024 extrusions removed because of IG corrosion. I'll post a pic if I can find them.

If you don't now, going forward: Fog your airframe, Fog your airframe. Fog your airframe. Keep the catalyst away and the internal corrosion stays away as well.
 
Well that’s quite the range of options, from replace the part to it should clean up. After reading the responses here I believe I may have misidentified filiform as intergranular.

I’ll gather the supplies to refinish it; since I will have access to the spot blaster I’ll clean up the area and see how it looks under the paint to determine if further measures are necessary behind clean and repaint.
 
A tangential question relating to treatment: since I’m tackling just small spots on the underside of the wing (i.e. not seen), does anyone have experience with rattle can touch ups? The original paint is Imron but I don’t know the color code, when I had a paint shop scan a panel it came back with a Maserati white PPG code.
 
FreeMasm . . . "If you don't now, going forward: Fog your airframe, Fog your airframe. Fog your airframe."

Could you elaborate a bit more, please? When you say Fog, do you mean spray the entire plane? Which product do you use?

Thanks
 
Yes treat your entire airframe. You'll get some contrary opinions based on people who've never discovered corrosion on their aircraft because of various reasons or never had a problem because of their mastery and selection of the perfect conversion coating/primer/paint system, etc. I owned a Mooney that had several areas where access was painful and not required at annual. The spar caps were also 7075T6 which have a higher tendency to suffer IG corrosion when exposed to catalyst (moisture). It's not unusual for one of those to get "totaled" because of wing corrosion. As there's no insurable event, the owner is often left with an assembly of salvage parts at that point.

There are people who you can pay to do it (they already have the equipment); maintenance shops, hangar neighbors, etc. You can buy equipment from the preservative suppliers or there is a plethora of fogging equipment out there or you can buy from the preservative suppliers. The biggest "dedicated" equipment benefit is probably the very long wands that will reach >1/2 way down the wing.

Which preservative? There's no clear best, IMO. See this thread ~ post 5 for a summary from the way back machine.

https://vansairforce.net/community/showthread.php?t=197941

Want the best creeping/easiest atomizing? Probably ACF50 (witnessed it lift/remove bluing from a firearm). Downside is it will get through the seams/joints and some rivets to your paint. Atomizes so well you can see the fog coming out of access panels, other end of the wing when applying.

Want the one that best met the applicable mil-spec? CorrosionX. Lays down thicker and would probably protect better against the unusual (urine, battery electrolyte, soft drink spill, etc) Downside = it weighs a little more. Seems to collect dirt better. Didn't atomize quite as well or creep as well as the above.

Want to keep the mess down and save your paint job? Dinitrol AV-8 or equivalent.

All of these are very good products though someone will argue that their selection/knowledge is better than anyone else's. Are you already fighting corrosion issues? Where is the aircraft in it's service life? Is the paint still brag worthy? Hangared/outside? Environment? Got access to real fogging equipment? Answer these questions and proceed. Any is far superior than nothing. It's very cheap insurance.

My lord, you might lose a fraction of a knot from the weight; of course, you could go lite on breakfast on flying days and get it back. The main thing is to protect your safety and investment. My opinion of course, for what it's worth.

Stay safe.
 
I sort of hate to give a contrary opinion to a post with a lot of good detail, but fogging is not ALWAYS the best solution for every airplane in every situation. My experience with fogged airplanes on the gulf coast was fine. Do that in an area like where I live now - right on the edge of the great desert - and you will find that the fog will remain the fine grit that blows around all the time, and before you know it, you have a gritty grinding compound all throughout the airplane that is next to impossible to clean up.

This can quickly degenerate into a “primer wars” discussion, but there are numerous ways to keep corrosion in check - primer, fog…or just keep a close eye on things to see if you have a corrosion problem that might be developing. It does not happen overnight in most cases - it develops over time - which means that you have time to react if you see something developing. Then deal with it as appropriate.

There are just so many environments in which airplanes live that one size does not fit all.

Paul
 
Hey Paul.

No one is going to argue with you but most aircraft aren't in desert environments. Some desert sand is as fine as talc but retains its abrasiveness. Cathodic corrosion probably isn't your concern either.

I did ask the enquirer to consider some things including environment. I tried to convey there's no single correct answer though poorly it would seem.

If such debris/contamination (FOD is probably pushing the intent of that definition) is a concern and there's still corrosion potential, I'd use nothing but Dinitrol. It dries to a solid film and isn't sticky. It retains a degree of surface wash-ability. Some slight solvent resistance as well (think short exposure times to mineral spirits, much shorter with xylene). Hopefully the formulation hasn't changed since I starting using it.

Going back to the beginning, the OP showed photos with blistered paint, most certainly surface corrosion, This doesn't halt on it's own.

Cheers Sir.
 
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Got me thinking/remembering

The OP had originally inquired about intergranular corrosion. I won't dive deep but refer back to post 6. This majority of this type of electrolysis is usually taking place inside of the metal versus the surface. I kept some real life examples from aircraft. I've only found one of the pieces I've kept so far. Hopefully the others will turn up, flap hangars IIRC. This part was adjacent to the wheel well of a certified aircraft, technically inside the fuselage . Finding it at such an early stage took a lot of luck. You could feel it more than see it. The was alodined when installed; maybe primed too. It's been too long. two surfaces wire brushed for these pix. The picture details make it far more noticeable than it really is.
 

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The OP had originally inquired about intergranular corrosion. I won't dive deep but refer back to post 6. This majority of this type of electrolysis is usually taking place inside of the metal versus the surface. I kept some real life examples from aircraft. I've only found one of the pieces I've kept so far. Hopefully the others will turn up, flap hangars IIRC. This part was adjacent to the wheel well of a certified aircraft, technically inside the fuselage . Finding it at such an early stage took a lot of luck. You could feel it more than see it. The was alodined when installed; maybe primed too. It's been too long. two surfaces wire brushed for these pix. The picture details make it far more noticeable than it really is.

That’s pretty nasty looking. It’s good to have the picture reference, I clearly got the two types mixed up on my initial post.

You mentioned that treatment after removing the corrosion was a separate can of worms, and I’m a bit confused on that. After blasting off the old paint and any corrosion, can I just hit the area with a self etching primer of my choice followed by touch up paint or does the area require further treatment/prep before primer?
 
That’s pretty nasty looking. It’s good to have the picture reference, I clearly got the two types mixed up on my initial post.

You mentioned that treatment after removing the corrosion was a separate can of worms, and I’m a bit confused on that. After blasting off the old paint and any corrosion, can I just hit the area with a self etching primer of my choice followed by touch up paint or does the area require further treatment/prep before primer?

Don’t think that was me but I’m pretty brain dead these days. If the surface corrosion cleaned up by mechanical means, you’re good to go. There are a million opinions on surface prep, conversion coating, primer, paint, etc. I won’t go there. Go with a method of your choosing. This is touch-up, not finish work. It will not be cosmetically perfect. The very best touch-ups Will reveal themselves in time. Protect your investment and stay vigilant on corrosion. You’ll be fine.
 
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