Sandpaper is a generic term that can include materials not wise to use. There are various abrasive materials used such as aluminum oxide, garnet, silica and now plastics.
Corrosion..
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Sandpaper is a generic term that can include materials not wise to use. There are various abrasive materials used such as aluminum oxide, garnet, silica and now plastics.
flightmech
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It's been my understanding that intergranular corrosion is pretty much the metal's way of saying "I give up". You can clean it with a green scrubby and treat it with alodine if I'm not mistaken.
jesse
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Why certainly, Mr. Tom! One would simply refer to chapter on corrosion treatment and control in the "Cessna 170 Structural Repair Man...........hey this isn't another of your trick questions is it?! I am guessing there exists no such book!
In any case, never would I feign to have any knowledge approaching yours in this department. Do tell! High speed wire wheel? H2SO4 bath? Steel scouring pad?
PS Thanks, Grant.
Obviously the proper solution would be an air powered angle grinder.
Your first post..... Welcome to POA..
Your idea of intergranular corrosion is correct, IMHO.. The key to this thread is not only cleaning up the affected area, but to analyize it to confirm the structural integrity has not been breached by that corrosion. Whoever signs the logbooks after that process in vouching for its safe operation... That puts the burden squarely on Toms shoulders..
I'd still poke it first to see if it was worth spending the time cleaning or do I just start drilling. Here I am working on your plane in a shop. The shop bills the same whether I'm cleaning corrosion or drilling rivets. If I can take my awl in bad spots and pop right through, do you want me to waste $800+ cleaning up corrosion just to then determine that the metal needs to be replaced? Not all corrosion can be cleaned and treated, a hell of a lot of the time you have to replace metal. Why waste a day and materials cleaning when you can condemn the metal in 5 minutes? If you don't condemn the metal with the awl first, THEN you start cleaning and treating.
flightmech
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Your first post..... Welcome to POA..
Your idea of intergranular corrosion is correct, IMHO.. The key to this thread is not only cleaning up the affected area, but to analyize it to confirm the structural integrity has not been breached by that corrosion. Whoever signs the logbooks after that process in vouching for its safe operation... That puts the burden squarely on Toms shoulders..
Thanks! I went in to "hangar talk" and posted an intro. That's the thing about intergranular corrosion, once you can see it the metal is more than likely to far gone. The next step from there is delamination. You can clean it but you can't stop it. My opinion is if the bulkhead can be replaced it needs to be.
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+1....
Tom-D
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Tom-D
This is not inter granular corrosion.
We must remember this is the rear cabin former in a Cessna 170, it's .025 2024-T3, and you can do more damage with an awl than can be repaired. there are no replacement parts, so if you ruin it the aircraft becomes scrap.
There is no structural repair manual for the 170 series, any repair would be a field approval and a long time turn around for the repair.
the recommended cleaning procedure is to etch with detergent using a stainless steel platers brush to remove the corrosion deposits and see how deep we are before we make a decision as to how to return to service.
we may be able to do the easy return to service simply by treating the area and preserving it.
do the easy stuff first.
why would any one sand a sheet .025" thick ?
Tom-D
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Your first post..... Welcome to POA..
Your idea of intergranular corrosion is correct, IMHO.. The key to this thread is not only cleaning up the affected area, but to analyize it to confirm the structural integrity has not been breached by that corrosion. Whoever signs the logbooks after that process in vouching for its safe operation... That puts the burden squarely on Toms shoulders..
When was the last time you saw inter granular corrosion in a .025" sheet?
I'm not gonna do a bit of damage with my awl. I look for the worst spots, I'll gently push the tip, if it resists it passes and we start cleaning; if it's gonna go through, it goes right through. I could just start cleaning on the bad spots and see what remains but it would take more time and make a mess.You make a good point about repairing it, this same issue condemned a 170 that I brought from FL to TX for a guy to do his first annual, basically that plane ended up condemned. As for the repair of just the bulkhead, what would the deal be if you (theoretically since one may not exist) pulled a good one out of a salvage plane and riveted it into place. How would that go paperwork wise?
Tom-D
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We must remember to not remove any material that is not already corroded. power tools do that.
The easiest method is etch with a detergent and agitate with a stainless steel platers brush. The corrosion products will turn black, and the good aluminum will become bright, simply keep working until the deposits are gone and then measure with a scratch gauge to see how deep the pits are and how many you have. Using that info, determine the method of return to service.
The picture shows a yellowish substance, that is headliner glue. not exfoliating metal.
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flightmech
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That's good info Tom. Thanks.
Tom-D
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The corrosion must be removed in any case, why poke a hole in structure for no reason.?
Saves time for me to poke through with an awl rather than go set up to do a cleaning and then rub through with the brush, either way the results are the same, a condemned piece of metal with a hole in it. One took 10 seconds, the other 10 minutes. It's about time efficiency in a shop, cost control which effects the bottom line of both the shop and owner. 10 minutes here and 10 minutes there across the course of a month in a busy shop adds up to $$$.
Tom-D
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Well, if you poke thru with an awl, then what ?
Are you going to separate the fuselage and change the aft cabin former?
where are you going to get one?
To change that bulkhead will cost more in labor and parts and paper work than the aircraft is worth.
got a fuselage fixture to hold the fuselage while you pull the aircraft apart?
We are not worried about time and efficiency, the owner can clean this mess up, on his own time.
plus to complete am inspection with an awl, it requires you make a grid and poke at each cross of the grid lines, that takes much more time than you suggest.
grabbing an awl and start poking is the best way I know of to ruin a good bulkhead.
who is going to set the pressure at which the awl is to be poked to determine failure?
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Right, if I go through the airplane is condemned!!! It's freaking rotten!! Either I have a structure or I don't. If I go through with my awl the metal will NOT meet specifications.
What method of returning to service would you intend to use when you make holes cleaning the corrosion with your brush? I'm not understanding the issue here, either the metal is sound or it is not, the method of determining that soundness is irrelevant and the faster it's found the less it costs everybody. Say I spend the day cleaning it and then condemn it, do I charge the client for corrosion cleaning and treatment?
Tom-D
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Right, if I go through the airplane is condemned!!! It's freaking rotten!! Either I have a structure or I don't. If I go through with my awl the metal will NOT meet specifications.
What method of returning to service would you intend to use when you make holes cleaning the corrosion with your brush? I'm not understanding the issue here, either the metal is sound or it is not, the method of determining that soundness is irrelevant and the faster it's found the less it costs everybody. Say I spend the day cleaning it and then condemn it, do I charge the client for corrosion cleaning and treatment?
To start with I have inspected the back side and there is no corrosion showing, so, there will be no holes unless you poke one.
So when you get lucky and find the right spot, and poke a hole, your going to scrap the whole aircraft ?
When I get it cleaned up, we can determine if a dubber is required and the best method of attachment.
we may simply form a doubler from 2024-0, have it heat treated (T3) and bond it to the existing structure.
Bottom line here is we save as much as we can, and don't destroy anything as we do.
We don't just start poking.
I am holding a piece of .025 aluminum right now..... It has been exposed to a corrosive enviroment and there is pitting that has eaten its way completely through the material... Pitting = granular corrosion. So...... now you started a thread on corrosion... posted pics of it... then stated the buckhead is structural... Went on and on about the proper way to clean the area....... Now, weeks later you continue to comment... I hope like heck in those weeks you have investigated the corrosion /damage, assessed the remedy and course of action/repair. What's your next step ..
..Care to post some close up pics of the corrosion .
I haven't seen it, that's what I said at the beginning, can't tell from pictures with corrosion. If there is no corrosion, I will not go through with my awl, I will not even put a dent in it, a shiny spot at best where it poked through surface corrosion or simple oxidation.
I'm having trouble imagining all this damage you think I'd cause on good metal by tapping it with an awl. If it's corroded through it goes through, if it isn't it doesn't. It's not like I'm testing fabric with a Maule gauge, I'm just taking a quick peek to see if there is metal left at all. Poking a hole through corrosion with an awl is not doing any damage or destruction, that was done already. Anything I can poke through will have to be replaced/reinforced whether I poke through or not, and the cleaning process will create an even larger hole than my awl will.
I can tell you for a fact poking is the primary method of testing all metals for integrity, wood as well.
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Tom-D
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My next step is to wait until the owner says it is cleaned up.I am holding a piece of .025 aluminum right now..... It has been exposed to a corrosive enviroment and there is pitting that has eaten its way completely through the material... Pitting = granular corrosion. So...... now you started a thread on corrosion... posted pics of it... then stated the buckhead is structural... Went on and on about the proper way to clean the area....... Now, weeks later you continue to comment... I hope like heck in those weeks you have investigated the corrosion /damage, assessed the remedy and course of action/repair. What's your next step ..
Care to post some close up pics of the corrosion .
Then I'll go look and see if it really is, and make the decision as to what to do next.
I'd suggest you read up on corrosion and its types, AC 43,13 chapter 6 para 6-17 to start, and if you can find one a USN training manual called the AMS 3&2 rating manual.
Inter-granular corrosion very rarely occurs in 2024- and is mostly found in thick forgings that are improperly heat treated.
In all my days as an aircraft mechanic both in the Navy and out, I have never heard of a direct attack type of corrosion being called inter granular even when the al-clad was exfoliating.
Tom-D
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So, you'd just start poking with no criteria as to how hard to push the awl, or a criteria for the inspection.
and you would be unable to tell the owner it failed IAW this criteria.
IOWs when you poked the hole and got sued for the damage you tell the judge what? And when the judge says you just bought the aircraft, then what?
Every inspection must have some approved criteria, just like the Maule tester it has a green band a yellow band and a red band. your awl has what?
You simply don't start poking holes in folks aircraft with out some thing to back you up.
I'd be really really careful about using your practice on some bodies aircraft.
I have great criteria how hard i will push, it comes from judgement and experience. The criteria is "does the awl fall through the material or is it resisted against?"
I would not tell the owner it failed, I would tell the owner of the odds of it failing after I spend a thousand of his dollars or he spends a couple days of his time cleaning it and let them decide if they want to spend money & time or scrap the airplane. I try to spend other peoples money and time as carefully as possible.
Dan Thomas
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The old Champs had a problem with corroded steel wing strut tubing. Champion's SB on the matter demanded that the mechanic use a prick punch and tap it with a hammer in a grid pattern on the bottom of the strut at the lower end to see if it went through. If not, it was OK. The tiny pricks caused by the punch did not appreciably affect the strength of the strut, apparently, similar to poking around with an awl.
I once did a lot of work stripping a pair of 150 wings and prepping them for paint. On the paint stand, we noticed a little blister on the leading edge of the left wing. Poked at it with the point of a pocket knife, and it went right through. Poked around some more and it went through in lots of places, most of them with no external blisters. All this to say that it might look good on one side, but that's no guarantee of salvageability. The corrosion involved a large area, caused by birds that had found an entry in the aileron bay and had nested in the leading edge next to the landing light cutout where there was sunlight handy.
Dan
I once did a lot of work stripping a pair of 150 wings and prepping them for paint. On the paint stand, we noticed a little blister on the leading edge of the left wing. Poked at it with the point of a pocket knife, and it went right through. Poked around some more and it went through in lots of places, most of them with no external blisters. All this to say that it might look good on one side, but that's no guarantee of salvageability. The corrosion involved a large area, caused by birds that had found an entry in the aileron bay and had nested in the leading edge next to the landing light cutout where there was sunlight handy.
Dan
kyleb
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That sounds like the Piper strut AD. I'm not aware of anything official (AD or Service Bulletin) regarding Aeroncas which sounds like that. But 60+ year old struts are something you really want to take a careful look at. (In reality, you want to take a good look at virtually everything in a 60+ year old airplane.)
Dan Thomas
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Some info on the examination here:
http://www.lightaircraftassociation.co.uk/2011/Magazine/June/safety spot.pdf
Dan
Tom-D
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There are a lot of different causes of corrosion birds being one of the worst. How you find it and what you do about it is a case by case bases.
DouglasBader
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I've seen a lot of intergranular corrosion in 7075T6. it's a real problem in the Sabreliners.
I've seen some parts of P2V's that have completely disintegrated, some of it due to intergranular.
Tom-D
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Neither of those two aircraft are made from .032" 2024-t3 sheet.
Intergranular corrosion does occur, but only in thicker forgings and always starts from with in the grain boundaries never on the surface.
7075- is a zink alloy and when the heat treatment temps are exceeded the zink will melt within the grain boundaries of the forging or thicker sheet, then it will act as two different metals in contact. and start to corrode which will produce corrosion deposits causing a swelling or blister visible from the surface.
2024- is a copper based alloy, and does not have these properties and seldom if ever will develop inter granular corrosion simply because the melting temps of copper being much higher and easier to control in the bake ovens.
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Let'sgoflying!
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Tom my turn for a trick question.
You tell the owner about the corrosion but don't have a chance to explain what needs to be done (or maybe he ignores your instruction).
You find out he sands the H out of it with an inappropriate material, gets all the corrosion out but it passes all tests including thickness.
Now what.
Not really a trick question, but what about imbedded particles, how do you deal with that. Scrap the airplane? Scrap the part? A cleaning method for imbedded impurities? Continue with a 'watch program'?
You tell the owner about the corrosion but don't have a chance to explain what needs to be done (or maybe he ignores your instruction).
You find out he sands the H out of it with an inappropriate material, gets all the corrosion out but it passes all tests including thickness.
Now what.
Not really a trick question, but what about imbedded particles, how do you deal with that. Scrap the airplane? Scrap the part? A cleaning method for imbedded impurities? Continue with a 'watch program'?
I thought I remembered something about intergranular corrosion only being in forgings.
In reality, sheet metals, whether it be steel, aluminum or other materials is actually a forging to some extent.. Rolling metals using high pressure is a form of forging it... With a real high powered microscope you can see the granular structure of tin foil..
Skylane81E
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I think the real question is what regulatory leg would I have to stand on to tell him that it is unairworthy?
Tom-D
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That's a real possibility
If the cabin former ends up being too thin for any reason we are into making a major repair, most likely by manufacturing a doubler and bonding it into place with a good structural adhesive, simply because the material must be thick enough or rivets won't have any material to grab.
as for foreign material embedded into structure, I'd etch it until it shows clean. then do the bond.
My 48 170 had the entire rear cabin former replace, along with 22 other fuselage skins, including the whole horizontal stab, rudder and elevators skins, due to the aircraft being comprised of 4 other aircraft made into one and ending up bent 3/4" out of alignment, and the wrong elevator installed. 6 months of work. estimated 30,000 in labor and materials.
So we now I go slow, make the best / cheapest repair we can. That's why I am very careful to be sure the owner has a good idea of what they are doing, before I turn them loose to clean up the corrosion.
Tom-D
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It occurs most often in forgings that have been rolled or any other method of elongating the grain boundaries.
Tom-D
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very easy..
" your aircraft is not considered safe for flight "
you the pilot have now been informed, you can't comply with 91.7
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Tom-D
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In reality, sheet metals, whether it be steel, aluminum or other materials is actually a forging to some extent.. Rolling metals using high pressure is a form of forging it... With a real high powered microscope you can see the granular structure of tin foil..
This is true, but only the ZINK alloys are plagued with intergrainular corrosion due the low melting point of ZINK.
In reality, sheet metals, whether it be steel, aluminum or other materials is actually a forging to some extent.. Rolling metals using high pressure is a form of forging it... With a real high powered microscope you can see the granular structure of tin foil..
Yeah, kinda, rolling mills and extrusion use pressure, but it is not near the pressure that the forgings of aerospace are made from using these machines:http://boingboing.net/2012/02/13/machines.html
DouglasBader
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That's the basic characteristic of intergranular corrosion, although I've seen it occur in some fairly thin parts, generally in 7075. What appears initially to be filiform corrosion under a coating quickly becomes something else, as one begins to remove material. I've found it on a number of panels on Sabreliners.
Tom-D
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Yes, that's usually filiform corrosion or direct chemical attack, it is caused by microbe's pizz or electrolyte attacking the alclad, and will quickly get between the alclad and the basic alloy, causing the alclad to exfoliate, But it is not considered intergrainular, that actually starts between the grain boundaries.
Each of the types of corrosion is named by its cause.
read AC 43,13 chapter 6
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