Pitting corrosion on wing Spar

[maniago]

As is an option for the M2 kit, I had my wings factory built by a contractor in FL.

I'm now working on hanging the wings and plumbing and found some pitting corrosion on the Alodined main and rear spar - it looks like chicken pox of very small salt mounds. It'll be at least 2yrs before the plane is done and painted, so I need to do something that'll stem the tide but not be an issue for paint. (Its not enough of an issue to go back to the contractor, but enough to want to do something about it).

CorrosionX is pretty liquidy and flows out of the seams - not good in my case. I see BoeshieldT-9 is a waxy substance as is LPS3. Anyone have experience with either of these?

I was thinking of wiping it on with a soaked rag on a long pole for now, then commit to the spray thing once its painted, so whatever product I pick, I need to stick with.

Thoughts?

 

[JonJay]

It is a common misunderstanding that Alodine will protect for corrosion. As you are finding out, it may not. You should topcoat within 48 hours with a suitable primer/paint.
This is well documented in AC43-13-1b, Chapter 6. It is required reading as far as I am concerned when dealing with corrosion issues.

If you do not top coat in the specified time period, it is recommended that the surfaced be alodined again before primer/paint. You may not appreciate hearing that.

In the mean time, I would follow AC43, manually remove the corrosion, and then protect the surface with LPS-2, or LPS-3. If you are in a high salinity area, MIL-C-16173 "cosmoline" (use it in a modern form suspended in a solvent that you can brush or spray) is recommended. When you are ready to re-alodine (if that is even possible), use KleenStrip PreAll, or similar degreaser/wax remover to remove the coating.

I am not sure you will be able to totally get rid of the weeping you may get between parts, but PrepAll is the best, low cost, degreaser/wax remover I have found.

Sorry if this isn't the solution you where looking for. When you have a pile of post WWII parts that are poster children for all types of corrosion, you research the topic a bit.

 

[maniago]

Ok, great, thanks for the comprehensive answer. Good to know stuff.

I read Ch6 for AL, though the one I have is 2001 and it only covers phosphoric Mil-C-38334 chemical corrosion removers. I can manually get to some of the spar, but for sure, I wont be able to get to what I cant see or reach manually.

From what I read so far: LPS2 is a heavy lubricant with anti-corrosion properties, LPS3 is a wax based corrosion cover/inhibitor, as is BoeshieldT9; CorrosionX is only a weeping inhibitor like WD-40 on steroids - and it stinks (literally); and ACF says it penetrates and stops active corrosion as well as well as weeps and prevents.

Seems like ACF is a logical direction. Anyone have experience/lessons learned with it?

 

[JonJay]

I don't have any experience with Corrosion X or ACF. Let's hope someone chimes in.

From my experience with the Fiat projects, you want a penetrant of some type. The Fiat's used hydraulic gear and flap actuation, with hydraulic lines running all over the place. Even with some severely corroded ribs, when taken apart, many of the mating surfaces between flanges and skins where covered with a light film of hydraulic oil, still red'ish colored, and they where in perfect shape, literally like new. This is after more than 60 years with bad corrosion to the exposed parts on the same part.
So, my reasoning would be, a penetrant with corrosion protection would stay in the mating surfaces for a long, long time. Painting the exposed surfaces might be another issue with weeping as you pointed out.

 

[rv7charlie]

JonJay,

Can you help me out, with the place in AC43 that says you must paint/overcoat an alodined surface withing 24 hrs?

I searched my soft copy for text around 'alodine' & 24 hours' could only find a 24 hr timeline was a reference to alodining within 24 hrs of cleaning for electrical bonding, in 11.189a, and to 'recoat' with the original finish after removing it for electrical bonding, in 11.189b.

FWIW, an engineer friend who lives near the FL coast ran a test by preparing test tabs of 2024 in many possible states, from bare, to etch/alodined, to epoxy primed, to epoxy primed/painted, etc. He dipped them all in a bucket of ocean water every few days, and left the sample set sitting across the top of the open bucket, outdoors, for months. The etch/alodine sample survived just as well as the most elaborately coated sample.

Charlie

 

[JonJay]

JonJay,

Can you help me out, with the place in AC43 that says you must paint/overcoat an alodined surface withing 24 hrs?

I searched my soft copy for text around 'alodine' & 24 hours' could only find a 24 hr timeline was a reference to alodining within 24 hrs of cleaning for electrical bonding, in 11.189a, and to 'recoat' with the original finish after removing it for electrical bonding, in 11.189b.

FWIW, an engineer friend who lives near the FL coast ran a test by preparing test tabs of 2024 in many possible states, from bare, to etch/alodined, to epoxy primed, to epoxy primed/painted, etc. He dipped them all in a bucket of ocean water every few days, and left the sample set sitting across the top of the open bucket, outdoors, for months. The etch/alodine sample survived just as well as the most elaborately coated sample.

Charlie

AC 43.13-1B, 6-148.

"If 48 hours or more have elapsed since the conversion coating was first applied and the pimer or final paint system has not yet been applied, then reapply the conversion coating before continuing".

In fairness, this chapter is dedicated to Corrosion, and this section to repair of corrosion. It is possible that this recommendation is coming from that angle. I would refer to the manufacturers recommendations for the paint system you are using for something other than corrosion repair.
I do know when my aircraft was painted, my painter made sure he primed and painted within this specified period.

In regard to your buddies testing, Scott at Van's "unscientific" experiments of hanging samples under the outside stairs at Van's showed surface corrosion on untreated Alclad pieces (there are several posts regarding this testing if your curious and want to search). Also, this entire post is about corrosion found on spars that where alodined, and then not treated further. There is little doubt that Alclad can corrode, even when alodined.

 

[rv7charlie]

Not enough data to draw that conclusion. Work was done in FL, by a 'hired gun'. Did the hired gun properly prep the raw stock before alodining and assembling the spars? I owned an old M-II project for a while, and the spar materials consisted of the untreated pre-formed web and untreated, sheared to width/length 1/8" plate stock used to build up the caps. Perhaps the OP can tell us whether the 1/8" thick cap material is 2024, or 6061.

I'm not surprised that the untreated 2024 (alclad) in Van's test eventually corroded; the untreated sample in my friend's test did, as well (that wasn't the question). But the alodined sample fared as well as the epoxy protected samples. What was the result of Van's test on an alodined sample of 2024 alclad in the same environment?

We must be working from different versions of AC-43.13-1B; Mine's dated 9-8-98; 6-148 in mine deals with magnesium:

>>>
6-148. GENERAL. Magnesium and magnesium alloys are the most chemically active of the metals used in aircraft construction and are the most difficult to protect. However, corrosion on magnesium surfaces is probably the easiest to detect in its early stages. Since magnesium corrosion products occupy several times the volume of the original magnesium metal destroyed, initial signs show a lifting of the paint films and white spots on the magnesium surface. These rapidly develop into snow-like mounds or even white whiskers. The prompt and complete correction of the coating failure is imperative if serious structural damage is to be avoided.
>>>

In any case, isn't it true that admonitions to apply paint or paint-like coatings within a particular time frame after prep is more about adhesion, than corrosion protection? It's not unusual to see the same kind of prime/finish coat time window idea applied to many materials; even painting sheet rock or house siding.

 

[JonJay]

Fair enough. Same version. Check the paragraph just ahead of that in the Aluminum section. My browser might have led me to the wrong page reference. I can check my hard copy at home.
Not trying to make it a "primer war" Just trying to help the OP.

I would be curious what the manufacturers of Alodine have to say. Do they recommend top coating and if so, do they state a time limit?
PPG states that their wash primer has poor corrosion protection properties and recommends a top coat, but it seems to hold up very well in service without. I would suspect Alodine might be the same.

For my projects with known corrosion, I plan on Acid Etch, Alodine, and then epoxy, for any parts that are within tolerance after removal of the corrosion.

 

[rv7charlie]

Here's the 1st link I found to an old (1970's) milspec for alodine:
http://everyspec.com/MIL-SPECS/MIL-SPECS-MIL-C/MIL-C-81706_17793/
(Requires a download of the pdf)

The doc is typical gov. legalese, but if I'm reading it correctly, they are specifying both treated/unpainted and treated/painted samples for testing.

The 1st page of the doc says, 'for maximum protection against corrosion, painted or unpainted'. Table II on pg 8 specifies 'None' in the 'Paint System' column for the corrosion resistance test, if (big if) I'm interpreting the doc correctly.

The test for corrosion appears to be a 5% solution salt spray for several hundred hours.

No doubt finishing off with epoxy primer/topcoat would offer max protection, but as Jerry Pournelle often said, 'Better is the enemy of good enough.' Point being (in this case), if the alodine is 'good enough', the epoxy is just adding weight, unless the product will be in an incredibly hostile environment.

Charlie

 

[az_gila]

....
I would be curious what the manufacturers of Alodine have to say. Do they recommend top coating and if so, do they state a time limit?
.....

I know it's wiki, but their description of the scope of the Mil Spec is pretty good.

The MIL-DTL-5541 specification covers chemical conversion coatings that form protective coatings by chemical reaction with aluminum and aluminum alloy; these coatings are categorized by the following types and classes.
Type I: Addresses compositions containing hexavalent chromium. This film typically appears to be gold or brown in color, but in some cases may be optionally specified as having no color (Having no color is described as "clear").
Type II: Addresses compositions containing no hexavalent chromium. This film typically appears to have no color (Having no color is described as "clear").

AND

Class 1A: Provides maximum protection against corrosion, whether painted or unpainted. This is a relatively thick coating used as a final finish or pre-treatment to paint or powder coating.
Class 3: Provides protection against corrosion where low electrical resistance is required. This is a thin coating providing low contact resistance, and the coating weight is lower as is the corrosion resistance. There may be some advantages using this thinner film in bonding applications.

Spec is here -

http://quicksearch.dla.mil/qsDocDetails.aspx?ident_number=5499

 

[az_gila]

....

....

No doubt finishing off with epoxy primer/topcoat would offer max protection, but as Jerry Pournelle often said, 'Better is the enemy of good enough.' Point being (in this case), if the alodine is 'good enough', the epoxy is just adding weight, unless the product will be in an incredibly hostile environment.

Charlie

I think this paragraph from the newer spec may be key to priming after alodining...

6.7 Abrasion resistance. The abrasion resistance of chemical coatings is relatively low. Coatings are reasonably durable when subjected only to moderate handling, but are readily removed by severe wear or erosion. However, cold forming operations, when performed with care, can generally be performed on treated metals without appreciable damage to the coatings.

 

[49clipper]

49clipper

I don't have any experience with Corrosion X or ACF. Let's hope someone chimes in.

I used ACF 50 on at least 40 airplanes during my mechanic tenure. I highly recommend it. It did seem to stop corrosion and has successfully kept it at bay for many years. I saw their demonstration at OSH decades ago and I tried it with much success for surface corrosion. Do not do it right before you paint or you will sorry. Paint it then treat. It tends to bleed out of the cracks for a couple of years, then stops. I tend to retreat every 3-5 years depending on how severe the corrosion was prior to first treatment. I was very skeptical, but became a believer over the years.
Jim
RV-6

 

[JonJay]

Good stuff guys. As with all of these discussions, no clear "one answer fits all."
On the Fiat's, anodized skins and zinc chromate still wasn't "good enough" for sitting out in the weather as a gate guards near the coast of Italy!
I am not sure anything would have been. You can clearly see where the sacrificial coating "sacrificed" itself to the point it had nothing more to give. Mother nature just kept on chewing away....

For the OP, sounds like you have more research to do. It appears unlikely that corrosion would have set in if the surfaces where properly alodined.

 

[JonJay]

I don't have any experience with Corrosion X or ACF. Let's hope someone chimes in.

I used ACF 50 on at least 40 airplanes during my mechanic tenure. I highly recommend it. It did seem to stop corrosion and has successfully kept it at bay for many years. I saw their demonstration at OSH decades ago and I tried it with much success for surface corrosion. Do not do it right before you paint or you will sorry. Paint it then treat. It tends to bleed out of the cracks for a couple of years, then stops. I tend to retreat every 3-5 years depending on how severe the corrosion was prior to first treatment. I was very skeptical, but became a believer over the years.
Jim
RV-6

Good to know and thank you. I have heard similar reports. I might consider this but I don't think these airplanes will be neglected again to the point that it would be necessary. However, there are some places that will be very difficult to get into to properly prep for treatment and top coat.
This might be the answer. These parts are not corroded, yet.... Perhaps an etch and alodine bath, then this post treatment might just be the trick.

 

[JonJay]

While we are on the topic, does anybody know of or see any issues in etching/alodine a finished part?

 

[maniago]

ACF to the rescue

Yikes. I must have really tickled someones corrosion bone to get this much interest

I appreciate the user experience on ACF and I think thats the right direction for me to catch this in its infancy. I'll have to do it before the plane is done, so I guess I'll paint the body and polish the wings for a while.

 

[maniago]

I used ACF 50 on at least 40 airplanes during my mechanic tenure.
Jim
RV-6

Can you expound a bit on how your applied it - ie wands and tips etc. Can I get away with a regular Home Depot pump sprayer or do I have to buy some kind of expensive gear to get the job done? Any advice is appreciated, thanks.

 

[Pilot Dog Ship]

One issue with the TFD ( Thin-Film Dielectric) compounds such as ACF and Corrosion X is that some folks who have used them for years have noticed the rivets nearest the spars have started "smoking", producing black rings around the heads or black streaks streaming downwind with the airflow.
The apparent cause of these rivets working loose is that the original riveted joint was held tightly together enough that there was no movement between the parts. With the invention of fluids that can flow, creep or use capillary action to work their way into the tightest of joints, they inadvertently act as a lubricant and allow the components to start to move, relative to one another, during high-stress maneuvers.
While some rivets could be bucked again using heavy-hitting guns and bars, most could not be reached without removing the wings, putting them in a wing jig and dismantling.
Seems like sometimes we just can't win; trying to make things better but creating another problem...