Exfoliation corrosion on flap mount arms

[Jesse]

Has anybody else seen exfoliation corrosion on any of the Van's-supplied parts. I'm working on a -10 that has 2 flap attach arms that are experiencing substantial corrosion. They are starting to expand flake like a biscuit. It is getting to the point that they will likely need to be replaced before long. Anybody have any ideas as to the cause? The plane was finished in 2008, is painted and has about 450hrs.

[aerhed]

First guess is someone dunked them in something they shouldn't have.

[Neal@F14]

I've seen this kind of "flakey biscuit" corrosion all over the extruded spar caps of an old Ercoupe wing center section at my airport that someone is disassembling to recover the good ribs and other parts. He said it was from a long life of living near the coast and absorbing too much salty humidity from the air. I carved some pieces out of it with my pocket knife and it just crumbles into flakes and powder very easily.

[Jesse]

That's my point. I've seen it before in an old Cessna Bird Dog, but that was an old plane that lived a long life near the coast. This is an RV-10 that was built in 2008 and has always been hangared and has only been near the coast (20-30 miles) for 3 years. And, it is painted.

[rbibb]

Sometimes I think this sort of things might be because of a particular lot of alumanim where the chemistry was a bit off and somehow it passed the QC process. Since it is internal to the structure I don't buy a faulty finish process. Need a true metallurgist to chime in here.

[Walt]

From your favorite bible:

6-17. INTERGRANULAR CORROSION.
Inter-granular corrosion is an attack on the
grain boundaries of a metal. A highly magnified
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 improperly
heat-treated and then exposed to a
corrosive environment.

6-18. EXFOLIATION CORROSION.
Exfoliation corrosion is an advanced form of inter-
granular corrosion and shows itself by lifting
up the surface grains of a metal by the
force of expanding corrosion products occurring
at the grain boundaries just below the surface.
(See figure 6-10.) It is visible evidence
of inter-granular corrosion and is most often
seen on extruded sections where grain thickness
are usually less than in rolled forms.

[rvbuilder2002]

The most likely cause was moisture entrapment during the painting process.
Was an etch and alodine painting prep. process used, with a water rinse afterwards?
If so, and not enough dry time occurred before priming/painting (particularly in humid conditions), moisture can become trapped under the paint coating, unable to evaporate.

[Jesse]

[rbibb]

That is going to be a pain to replace...

[terrykohler]

The most likely cause has already been suggested - that it's the result of the prep process. Acid is typically used as the initial step to "etch" the surface of the aluminum. If it's not properly neutralized or removed, it will continue to attack the material, and as suggested, the greater damage occurs at the grain boundaries. Prep the new pieces properly, and you shouldn't see this again.
Terry, CFI
RV9A N323TP