Been some confusion here and other places. Finally found another old part I was looking for; so, here's some "real life" pix of IG corrosion taken from a Mooney. Wheel well-wing attach bracket and flap hangar IIRC. Many times very hard to detect until the part integrity is severely compromised. Prime, paint (and maintain both); fog. Just sayin'
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Intergranular corrosion, some real life pix
- Thread starter Freemasm
- Start date
fixnflyguy
Well Known Member
Bad stuff
I deal with IG corrosion often in the heavy jet overhaul business. It is predominant in the 7000 series alloys (7075-T6 is the worst) and most MFGs have switched to 7050 which is less prone. Keel beam longerons, seat tracks, wing spar caps and other robust parts that where extruded are symptomatic to it because of grain orientation, stress loads and other factors. In the RV world, we will seldom see it, and the 6061 extrusions used by VANs are near impervious to it relatively speaking. Good picture from Freemasm of what it looks like..I can't share pictures from day job, and it may scare you from flying on a big jet ever again !
I deal with IG corrosion often in the heavy jet overhaul business. It is predominant in the 7000 series alloys (7075-T6 is the worst) and most MFGs have switched to 7050 which is less prone. Keel beam longerons, seat tracks, wing spar caps and other robust parts that where extruded are symptomatic to it because of grain orientation, stress loads and other factors. In the RV world, we will seldom see it, and the 6061 extrusions used by VANs are near impervious to it relatively speaking. Good picture from Freemasm of what it looks like..I can't share pictures from day job, and it may scare you from flying on a big jet ever again !
Quite the opposite, Sir. (@abuura)
I started to include a wiki link when I made the original post. What I saw in a quick search centered around steels and got to a level that really wasn’t too helpful from a practicality standpoint. Forgive me but it’s early and there’s no one in the office yet so I’ll get my geek on for a few. I’ll try and put into terms for a wider audience. It will probably offend some people who’re skilled in the art but hopefully they remember said audience.
In general, different metals don’t like each other. That degree can be relatively benign (co-existing for decades) to extreme examples where measurable corrosion can be detected within days. There are rare exceptions but one of the materials will be a cathode and the other an anode (like battery terminals) and they start exchanging electrons. This electrolysis (now a "banded" word to describe such but a good way to conceptualize it. Guessing disagreement between the chemistry and materials types) is the basis for this corrosion. Intergranular corrosion, like you'd guess from the name, is happening over/within grain boundaries inside the metal. In simple terms, it’s corroding from the inside.
2000 series aluminum’s main alloying element is copper. They are fairly compatible within reason but in practice, you'd insulate them from each other (see attached galvanic table shamelessly copied from the web). I believe the parts in the original post are 2024 but I can't say for certain. Those materials are listed in the OEM part drawings, not the Illustrated Parts List. Obviously machined from a tee and an F channel; one of the reasons for said guess. 7000 series utilizes zinc as an alloying element, materials that should be compatible according to the galvanic table. It's not that simple. 7075 is a pretty impressive alloy in many regards. It’s a strong as some mild steels but I have to question its application in aircraft (see fixnflyguys post). Stress exacerbates IG corrosion potential (micro movement in crystalline structure? Material imperfections? Don’t know the mechanism) but it’s most always applied in high stress areas due to its strength/unit weight. Mooney makes a very structurally sound airframe. I miss mine. The spar caps are 7075T6. When/if one gets IG corrosion on the caps, it’s very common that the airframe is economically totaled. The aircraft owner becomes an owner of tightly integrated spare parts. Seen it a lot. Those wheel wells are nasty as the tires throw a lot of “stuff” up there where the spar is exposed. Similarly, one spilled soft drink especially in the rear seat area can kick-off some ugly spar chemistry.
So what to do? Keep away the contaminants (catalyst) that promote the reaction; water being the most common. Prime, if not prime and paint the parts before assembly. Faying surfaces are great places for contaminates to hide. Maintain the paint for more than cosmetic reasons. Some will point out Vans materials selections, ease of repair, affordability of parts as reasons for not going the extra mile. We all don't live in dry environments. I’m a big proponent of fogging airframes with preservative. There are some very good products out there. It’s some of the cheapest insurance anyone can purchase for their investment. A flying, airworthy aircraft has a lot more value than a bunch of spar parts that formerly flew in close formation.
Hopefully this helped. Maybe this will kick-off some experts to weigh in. Cheers boys.
I started to include a wiki link when I made the original post. What I saw in a quick search centered around steels and got to a level that really wasn’t too helpful from a practicality standpoint. Forgive me but it’s early and there’s no one in the office yet so I’ll get my geek on for a few. I’ll try and put into terms for a wider audience. It will probably offend some people who’re skilled in the art but hopefully they remember said audience.
In general, different metals don’t like each other. That degree can be relatively benign (co-existing for decades) to extreme examples where measurable corrosion can be detected within days. There are rare exceptions but one of the materials will be a cathode and the other an anode (like battery terminals) and they start exchanging electrons. This electrolysis (now a "banded" word to describe such but a good way to conceptualize it. Guessing disagreement between the chemistry and materials types) is the basis for this corrosion. Intergranular corrosion, like you'd guess from the name, is happening over/within grain boundaries inside the metal. In simple terms, it’s corroding from the inside.
2000 series aluminum’s main alloying element is copper. They are fairly compatible within reason but in practice, you'd insulate them from each other (see attached galvanic table shamelessly copied from the web). I believe the parts in the original post are 2024 but I can't say for certain. Those materials are listed in the OEM part drawings, not the Illustrated Parts List. Obviously machined from a tee and an F channel; one of the reasons for said guess. 7000 series utilizes zinc as an alloying element, materials that should be compatible according to the galvanic table. It's not that simple. 7075 is a pretty impressive alloy in many regards. It’s a strong as some mild steels but I have to question its application in aircraft (see fixnflyguys post). Stress exacerbates IG corrosion potential (micro movement in crystalline structure? Material imperfections? Don’t know the mechanism) but it’s most always applied in high stress areas due to its strength/unit weight. Mooney makes a very structurally sound airframe. I miss mine. The spar caps are 7075T6. When/if one gets IG corrosion on the caps, it’s very common that the airframe is economically totaled. The aircraft owner becomes an owner of tightly integrated spare parts. Seen it a lot. Those wheel wells are nasty as the tires throw a lot of “stuff” up there where the spar is exposed. Similarly, one spilled soft drink especially in the rear seat area can kick-off some ugly spar chemistry.
So what to do? Keep away the contaminants (catalyst) that promote the reaction; water being the most common. Prime, if not prime and paint the parts before assembly. Faying surfaces are great places for contaminates to hide. Maintain the paint for more than cosmetic reasons. Some will point out Vans materials selections, ease of repair, affordability of parts as reasons for not going the extra mile. We all don't live in dry environments. I’m a big proponent of fogging airframes with preservative. There are some very good products out there. It’s some of the cheapest insurance anyone can purchase for their investment. A flying, airworthy aircraft has a lot more value than a bunch of spar parts that formerly flew in close formation.
Hopefully this helped. Maybe this will kick-off some experts to weigh in. Cheers boys.
Attachments
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Corrosion inhibitor
I want to second this:
While prime, and optionally paint, is most often provided as the go-to corrosion prevention, as others pointed out, it prevents visual inspection of the component.
Also, if any imperfections in the application occurred, or the primer is scratched during or after installation, corrosion may occur hidden from view. Another alternative is provide a sacrificial anode for the ongoing galvanic activity that is occurring.
How many of you, myself included, have primed dissimilar metals in preparation for riveting together, and then while assembling realized that an alignment tool was needed, or that one hole that needed reaming, or the rivet was a little snug - so you just pushed hard and shoved that little sucker in there? In all of those cases, you very likely abraded the primer coat and bare metal now exists within the structure, hidden from view, making a premium location for corrosion to occur. This is where inhibitors come in that can find there way into the most minute gaps around rivets, between close fitting parts, etc...
My preference is CorrosionX and then just spray out the entire airframe, (fine mist...don't overdo it) every other year (maybe annually if in a high moisture/salt air environment), however, there are several other corrosion inhibitors in the market, such LPS-3, or CRC's Heavy Duty Corrosion Inhibitor that are easy to apply with either aerosol can or brush. The best option is to apply regularly after painting, but can be applied to ongoing corrosion areas to react with and inhibit further corrosion.
I want to second this:
While prime, and optionally paint, is most often provided as the go-to corrosion prevention, as others pointed out, it prevents visual inspection of the component.
Also, if any imperfections in the application occurred, or the primer is scratched during or after installation, corrosion may occur hidden from view. Another alternative is provide a sacrificial anode for the ongoing galvanic activity that is occurring.
How many of you, myself included, have primed dissimilar metals in preparation for riveting together, and then while assembling realized that an alignment tool was needed, or that one hole that needed reaming, or the rivet was a little snug - so you just pushed hard and shoved that little sucker in there? In all of those cases, you very likely abraded the primer coat and bare metal now exists within the structure, hidden from view, making a premium location for corrosion to occur. This is where inhibitors come in that can find there way into the most minute gaps around rivets, between close fitting parts, etc...
My preference is CorrosionX and then just spray out the entire airframe, (fine mist...don't overdo it) every other year (maybe annually if in a high moisture/salt air environment), however, there are several other corrosion inhibitors in the market, such LPS-3, or CRC's Heavy Duty Corrosion Inhibitor that are easy to apply with either aerosol can or brush. The best option is to apply regularly after painting, but can be applied to ongoing corrosion areas to react with and inhibit further corrosion.
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