Intergranular corrosion removal/retreatment

Sleepingsquirrel

Pre-takeoff checklist
Joined
Mar 6, 2011
Messages
429
Display Name

Display name:
Sleepingsquirrel
I have seen in most publications that IGC is inherent in some forms of aluminium and replacement of the part is recommended.
I'm asking the best way to clean the surface and retreat to prevent further corrosion until I'm prepared for replacement. Any ideas?
 
I have seen in most publications that IGC is inherent in some forms of aluminium and replacement of the part is recommended.
I'm asking the best way to clean the surface and retreat to prevent further corrosion until I'm prepared for replacement. Any ideas?

There is no way to stop IGC by treating the top surface. It happens between the grain layers and will continue until it destroys the piece.
 
Will keeping humidity or moisture from the surface help in any way?
What kicks the process off?
 
As an example, we'll use 2024-T3 alclad. This is what most aluminum aircraft are built with.

2024 is code that designates the aluminum base, with a copper alloy. Unfortunately, this has moderate corrosion resistance at best. Moderate is not enough, so a thin layer of pure aluminum is coated onto the sheet of metal. The aluminum surface oxidizes into aluminum oxide, which unlike rust (iron oxide), forms a uniform barrier and makes it highly resistant to corrosion, as long as the integrity of the aluminum cladding is maintained.

What kicks it off, is when we drill into the metal, or rivets get loose and start smoking, galvanic corrosion occurs, etc. Anything that breaches the protective coating or interacts with the metal surface in undesirable ways. For example, water gets in between a rivet and the metal, and it starts interacting with the exposed copper and aluminum, starting corrosion. Water can also get in a lap joint, and start galvanic corrosion (this is why you DO NOT USE A PRESSURE WASHER ON A PLANE!). This process takes many years though, but once it picks up speed, it grows quick.

This is why we paint and treat the aircraft metal. Things like using Alodine, or periodically treating the airframe with something like Corrosion X. Some parts are anodized to force the oxide layer to grow as additional measure. And making sure things stay lubricated. We can't get it all, and eventually the parts will fall victim to the elements and nature's will, but we can make sure they last a very long time.
 
Last edited:
As an example, we'll use 2024-T3 alclad. This is what most aluminum aircraft are built with.

2024 is code that designates the aluminum base, with a copper alloy. Unfortunately, this has moderate corrosion resistance at best. Moderate is not enough, so a thin layer of pure aluminum is coated onto the sheet of metal. The aluminum surface oxidizes into aluminum oxide, which unlike rust (iron oxide), forms a uniform barrier and makes it highly resistant to corrosion, as long as the integrity of the aluminum cladding is maintained.

What kicks it off, is when we drill into the metal, or rivets get loose and start smoking, galvanic corrosion occurs, etc. Anything that breaches the protective coating or interacts with the metal. Water gets in there, and it starts interacting with the copper and aluminum, starting corrosion. This process takes many years though, but once it picks up speed, it grows quick.

This is why we paint and treat the aircraft metal. Things like using Alodine, or periodically treating the airframe with something like Corrosion X. And making sure things stay lubricated. We can't get it all, and eventually the parts will fall victim to the elements and nature's will, but we can make sure they last a very long time.


Great explanation....:yes::thumbsup:
 
Great explanation....:yes::thumbsup:


I'm still making edits. I tend to type it out and submit, then look it over, and go OOOH I can add this too!

EDIT: I think I'm satisfied. Plus the guy above us linked chapter 6 of 43.13, which, as he/she said, is the bible on corrosion.
 
Last edited:
Intergranular corrosion is internal between crystal barriers and has to do with a material defect caused by a manufacturing error. The part needs to be replaced, you can't stop it with paint.
 
Intergranular corrosion is internal between crystal barriers and has to do with a material defect caused by a manufacturing error. The part needs to be replaced, you can't stop it with paint.

Spot on, right out of the book
 
6-17. INTERGRANULAR CORROSION.
Inter-granular corrosion is an attack on the grain boundaries of a metal. A highly magni- fied cross section of any commercial alloy shows the granular structure of the metal. It consists of quantities of individual grains, and each of these tiny grains has a clearly defined boundary which chemically differs from the metal within the grain. The grain boundary and the grain center can react with each other as anode and cathode when in con- tact with an electrolyte. (See figure 6-9.) Rapid selective corrosion of the grain bounda- ries can occur. High-strength aluminum alloys such as 2014 and 7075 are more susceptible to inter-granular corrosion if they have been im- properly heat-treated and then exposed to a corrosive environment.
In common language a infinite number of galvanic cells acting as a line of corrosion between the layers of metal.
 
Last edited:
As an example, we'll use 2024-T3 alclad. This is what most aluminum aircraft are built with.

2024 is code that designates the aluminum base, with a copper alloy. Unfortunately, this has moderate corrosion resistance at best. Moderate is not enough, so a thin layer of pure aluminum is coated onto the sheet of metal. The aluminum surface oxidizes into aluminum oxide, which unlike rust (iron oxide), forms a uniform barrier and makes it highly resistant to corrosion, as long as the integrity of the aluminum cladding is maintained.

What kicks it off, is when we drill into the metal, or rivets get loose and start smoking, galvanic corrosion occurs, etc. Anything that breaches the protective coating or interacts with the metal surface in undesirable ways. For example, water gets in between a rivet and the metal, and it starts interacting with the exposed copper and aluminum, starting corrosion. Water can also get in a lap joint, and start galvanic corrosion (this is why you DO NOT USE A PRESSURE WASHER ON A PLANE!). This process takes many years though, but once it picks up speed, it grows quick.

This is why we paint and treat the aircraft metal. Things like using Alodine, or periodically treating the airframe with something like Corrosion X. Some parts are anodized to force the oxide layer to grow as additional measure. And making sure things stay lubricated. We can't get it all, and eventually the parts will fall victim to the elements and nature's will, but we can make sure they last a very long time.

:lol:While your statement is true, I have not found the reality to be true. The most corrosion free aluminum structures I come across are bare aluminum. Put paint on an aluminum boat and you now have a continuous chase of corrosion blistering up paint. That's why lots of aluminum commercial boats are left bare.
 
:lol:While your statement is true, I have not found the reality to be true. The most corrosion free aluminum structures I come across are bare aluminum. Put paint on an aluminum boat and you now have a continuous chase of corrosion blistering up paint. That's why lots of aluminum commercial boats are left bare.

Much different alloys in the marine industry, 3003 used for boats is much different than 2024-T alclad.

Ol Tom has a ATA in marine tech, and a ATA in welding tech. And has worked in both industries Dakota crete ( shipbuilders) and Hathorn marine (small Aluminum boat builders) and two refineries, and 22 years of USN as a metal smith) comparing the two alloys is like comparing chocolate, and vanilla ice cream.
 
Intergranular corrosion is internal between crystal barriers and has to do with a material defect caused by a manufacturing error. The part needs to be replaced, you can't stop it with paint.

Hey I'm feaking glad to be wrong in this case. I was wondering why intergranular and exfoliation was not more widespread, especially with how high strength and copper alloyed aluminums like 2024 are particularly vulnerable to improper treatment.

As Tom-D said, what I described was more galvanic cell corrosion, but I was under the impression this is what started the intergranular too, as it worked its way between the aluminum cladding and 2024. Actually, now that I think about it, wouldn't the properties of the pure cladding and copper alloy result in corrosion that can look very similar to intergranular as the stuff in between breaks down and the cladding lifts away?
 
Last edited:
6-17. INTERGRANULAR CORROSION.
Inter-granular corrosion is an attack on the grain boundaries of a metal. A highly magni- fied cross section of any commercial alloy shows the granular structure of the metal. It consists of quantities of individual grains, and each of these tiny grains has a clearly defined boundary which chemically differs from the metal within the grain. The grain boundary and the grain center can react with each other as anode and cathode when in con- tact with an electrolyte. (See figure 6-9.) Rapid selective corrosion of the grain bounda- ries can occur. High-strength aluminum alloys such as 2014 and 7075 are more susceptible to inter-granular corrosion if they have been im- properly heat-treated and then exposed to a corrosive environment.
In common language a infinite number of galvanic cells acting as a line of corrosion between the layers of metal.

Sounds exactly like metal corrosion to me.. Regardless of the catalyst that starts the process, metal degrades.......:rolleyes::yes:..........
 
6-17. INTERGRANULAR CORROSION.
Inter-granular corrosion is an attack on the grain boundaries of a metal. A highly magni- fied cross section of any commercial alloy shows the granular structure of the metal. It consists of quantities of individual grains, and each of these tiny grains has a clearly defined boundary which chemically differs from the metal within the grain. The grain boundary and the grain center can react with each other as anode and cathode when in contact with an electrolyte. (See figure 6-9.) Rapid selective corrosion of the grain boundaries can occur. High-strength aluminum alloys such as 2014 and 7075 are more susceptible to inter-granular corrosion if they have been im- properly heat-treated and then exposed to a corrosive environment.
In common language a infinite number of galvanic cells acting as a line of corrosion between the layers of metal.

Electrolysis.:yes:
 
SO..... if this part is toast, will the IGC be contained within this one part or will it contaminate and spread beyond its dimensions to other aluminium parts?

It's about 7 inches long ,3 inches wide, 1/8 inch thick with the only evidence of IGC on one edge for about two inches. it is sandwiched between a steel plate and aluminium with two bolts . It looks easy enough to replace once I determine the alloy.

Thanks for the informed responses and feed back!
 
Last edited:
Hey I'm feaking glad to be wrong in this case. I was wondering why intergranular and exfoliation was not more widespread, especially with how high strength and copper alloyed aluminums like 2024 are particularly vulnerable to improper treatment.

As Tom-D said, what I described was more galvanic cell corrosion, but I was under the impression this is what started the intergranular too, as it worked its way between the aluminum cladding and 2024. Actually, now that I think about it, wouldn't the properties of the pure cladding and copper alloy result in corrosion that can look very similar to intergranular as the stuff in between breaks down and the cladding lifts away?

We aircraft mechanics don't see that much inter granular corrosion, This occurs in heavier sheet stock, and forgings not in light sheet stock the Cessna and pipers are made from. items like wing spars, spacing blocks in the Cessna wing spar carry thru's have shown as inter granular but that's rare.
We mostly see filiform and direct chemical attack, showing up as a blister under the paint, or water spots on polished aluminum.
 
SO..... if this part is toast, will the IGC be contained within this one part or will it contaminate and spread beyond its dimensions to other aluminium parts?

It's about 7 inches long ,3 inches wide, 1/8 inch thick with the only evidence of IGC on one edge for about two inches. it is sandwiched between a steel plate and aluminium with two bolts . It looks easy enough to replace once I determine the alloy.

Thanks for the informed responses and feed back!

Yes, the part is toast, and no the corrosion will not be spread by its self.
When you manufacturer the new part, be certain it gets protection prior to installation. primer is the most common way to protect it. (paint it and assemble wet)
 
Yes, the part is toast, and no the corrosion will not be spread by its self.
When you manufacturer the new part, be certain it gets protection prior to installation. primer is the most common way to protect it. (paint it and assemble wet)

Wait, so I am right, we can protect against the starting of intergranular by painting, alodine, etc? I was not referring to CURING it, I mean stop it from happening in the first place.
 
We aircraft mechanics don't see that much inter granular corrosion,.


The place I have had it was in the rear fuselage float stute block, on a Cessna 180.

It's about an inch thick and it just peeled apart.
 
Wait, so I am right, we can protect against the starting of intergranular by painting, alodine, etc? I was not referring to CURING it, I mean stop it from happening in the first place.

Not really, but when you replace the part, you can try. Remember this part was already messed up when it was installed. it simply requires time to develop to where it shows to the exterior.

Paint is simply a way of isolating the part away from electrolyte and chemical attack.
 
Last edited:
Careful inspection of its counterpart on the starboard side reveals no signs of any corrosion of any type. I strongly suspect both parts are cut from the same stock. I think that one side may have been scratched upon install and after 50+ years of intermittent moisture,temperature cycles the IGC process has become apparent. The part appears to have a gold anodized surface as near as I can determine. I sits beneath a fog of Corrosion X residue. It is a spacer in compression with two bolt holes ,easy enough to fabricate and replace. I steel brushed it with a Dremel tool until the powder stopped and zinc chromated it for now.
 
Careful inspection of its counterpart on the starboard side reveals no signs of any corrosion of any type. I strongly suspect both parts are cut from the same stock. I think that one side may have been scratched upon install and after 50+ years of intermittent moisture,temperature cycles the IGC process has become apparent. The part appears to have a gold anodized surface as near as I can determine. I sits beneath a fog of Corrosion X residue. It is a spacer in compression with two bolt holes ,easy enough to fabricate and replace. I steel brushed it with a Dremel tool until the powder stopped and zinc chromated it for now.

Anodizing .... look it up.
 
Gold color probably identifies the alloy as 7075.
 
We aircraft mechanics don't see that much inter granular corrosion, This occurs in heavier sheet stock, and forgings not in light sheet stock the Cessna and pipers are made from. items like wing spars, spacing blocks in the Cessna wing spar carry thru's have shown as inter granular but that's rare.
My A&P in Michigan identified what he said was IGC in my wing spar carrythrough. Caused (or triggered) by mouse pee. (Yes, the part had to be, and was, replaced.)
 
I steel brushed it with a Dremel tool until the powder stopped and zinc chromated it for now.

This was a no-no for us. corrosion removal abrasive medium on aluminum was strictly by AC 43.13 chapter 6 table 6-1 (at least thats where it is now).
 
I steel brushed it with a Dremel tool until the powder stopped and zinc chromated it for now.

This was a no-no for us. corrosion removal abrasive medium on aluminum was strictly by AC 43.13 chapter 6 table 6-1 (at least thats where it is now).

Yes, particles of steel in the aluminum cause galvanic corrosion too. The aluminum will keep the iron particles from rusting.
 
Yes, particles of steel in the aluminum cause galvanic corrosion too. The aluminum will keep the iron particles from rusting.

Which is the more nobel metal? Iron or Aluminum? Which becomes the anode, and which will become the cathode? Which goes away first?
 
This was a no-no for us. corrosion removal abrasive medium on aluminum was strictly by AC 43.13 chapter 6 table 6-1 (at least thats where it is now).

I used stainless steel ,new, not used on any thing else.
 
Which is the more nobel metal? Iron or Aluminum? Which becomes the anode, and which will become the cathode? Which goes away first?

Why are you asking?
Fe 2+ + 2 e- --> Fe -0.44 V
2Al --> 2Al 3+ 3 e- -1.66 V

Iron gets reduced, electron flow from iron to aluminum
 
Last edited:
I see the error of my ways ! I'll just scrap the boat!
 

Attachments

  • 097 (2).jpg
    097 (2).jpg
    129 KB · Views: 13
Maybe I don't have to scrap the boat after all!!:D

6-136. EXAMPLES OF REMOVING
CORROSION FROM ALUMINUM AND
ALUMINUM ALLOYS.
a. Positively identify the metal as aluminum.
b. Clean the area to be reworked. Strip
paint if required.
c. Determine extent of corrosion damage.
d. Remove light to moderate corrosion
with one of the following.
(1) Non-Powered Corrosion Removal.
(a) The removal of corrosion products
by hand can be accomplished by use of
aluminum grit and silicon carbide abrasive,
such as non-woven, non-metallic, abrasive mat
(Spec. MIL-A-9962), abrasive cloth, and paper.
Aluminum wool, fiber bristle brushes, and
pumice powder are also acceptable methods.
(b) Stainless steel brush (Spec.
H-B-178, type III, class 2) may be used as long
as the bristles do not exceed 0.010 inch in diameter.
After use of this brush the surface
should be polished with 60 grit aluminum oxide
abrasive paper, then with 400 grit aluminum
oxide paper. Care should be exercised in
any cleaning process to avoid breaking the
protective film.

(b) When using abrasive blasting on
aluminum alloys, do not allow the blast stream
to dwell on the same spot longer than
15 seconds. Longer dwell times will cause excessive
metal removal. Intergranular exfoliation
corrosion is not to be removed by abrasive
blasting; however, blasting may be used with
powered corrosion removal to determine
whether all exfoliation corrosion has been removed.
 
Last edited:
The "stainless steel" brush specified is not just any old steel brush. It also specifies that the entire area must be "polished" with 60 then 400 grit aluminum oxide paper. You must have a "break-free" surface for the conversion process, and the total thickness and diameter of the material removed must still not exceed specifications. That's a pretty tall order.

If you read a little further down that paragraph, you read about all the "steel" abrasives you absolutely cannot use on aluminum.
 
The "stainless steel" brush specified is not just any old steel brush. It also specifies that the entire area must be "polished" with 60 then 400 grit aluminum oxide paper. You must have a "break-free" surface for the conversion process, and the total thickness and diameter of the material removed must still not exceed specifications. That's a pretty tall order.

If you read a little further down that paragraph, you read about all the "steel" abrasives you absolutely cannot use on aluminum.
You can also etch the aluminum to remove the contaminates. then neutralize. prime and paint.
 
Because when you understand the nobility of metals you will understand the corrosion process.

https://en.wikipedia.org/wiki/Galvanic_series

Yeah, I got that. Please look at my previous post where I listed the potentials and predicted which one gets reduced using the standard electrode potential.

The series you like to use gives the same result as the standard electrode potential.
 
Yeah, I got that. Please look at my previous post where I listed the potentials and predicted which one gets reduced using the standard electrode potential.

The series you like to use gives the same result as the standard electrode potential.

Did every one who reads this understand that ?
 
Back
Top