Airplanes and Salty Air

Snaggletooth

Snaggletooth

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Dustin
Living on the Gulf Coast I see many old trucks that are very rusty from the Salt Air from the gulf. But I never have seen any aircraft with any rust at all. So I'm guessing that Salty air does not effect planes??
 
Aluminum does not rust. It does, however, corrode, which is the equivalent. It can be a serious problem and one to be carefully looked at during a prebuy inspection of planes that live on a salt water coast.
 
Ya can't fly a corroded/rusted aircraft. Either it gets maintained properly, fixed, or becomes unairworthy and dies.
 
or (C) sits green on the ramp in Savannah for 6 months before paint and interior are completed and is then sold to an unsuspecting owner for 40.5 mil.
Rotor&Wing said:
Or (A) Flies night cargo for 20 years afterward, or (B) gets fresh paint and gets sold by Barron Thomas. :rolleyes:
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Aluminum is the most common metal in the earth's crust. It's all over the place. But it's never, ever found in nature as a metal; it's always an oxide or sulphate or something like that. That's because it reacts so easily with so many other elements, and it takes a lot of energy to convert it back to metal again. In 1900 aluminum was worth about as much as gold because it was so rare as a pure metal. The electrolytic refining process changed that in a hurry.

So aluminum is impatient to get back into the ground again. It won't "rust" and turn orange which is what iron does when it combines with oxygen, but it will combine with oxygen and sodium and sulfur and a whole lot of other stuff. You will see white or gray powder on it, or maybe just a lot of pitting. Sometimes you'll see little lines in the paint where filiform corrosion is happening beneath the finish. Looks like tiny mole burrows.

Much of the aluminum used in airplanes commonly has considerable copper in it (20xx series) and the copper accelerates the corrosion process. Copper-containing aluminum sheet will have a layer of pure aluminum on both sides (2% of total thickness) that will react instantly with oxygen and form a stable, inert layer of aluminum oxide to protect the underlying metal. Scratching this sheet is a very bad thing to do, as is sanding (GASP!) it. And copper can cause exfoliation corrosion within the structural member itself. Hard to spot.

Salty air is bad for an airplane. Salty moisture acts as an electrolyte between aluminum parts where they join, forming a sort of battery if the metals are dissimilar enough. The airplane rots away in invisible places. If salt gets into some joint between aluminum and steel, things get ugly indeed due to the strong electrolysis that starts there. Think steel bolts in aluminum airframe.

All of this is some of the reason we don't see aluminum cars too often. It's easier to control corrosion in steel.

Dan
 
Duh..... Not sure why that did not dawn on me! I know Aluminum don't rust.
Oi... guess I should have put more thought in to it before I asked. lol
 
Guy from my old airport bought a beautiful Cessna 150 from Florida. New paint, interior, looked like new. Seller had the pre-buy done for him by his reputable IA. Got it home and had to spend several K's putting new wings on it.
 
Snaggletooth said:
Living on the Gulf Coast I see many old trucks that are very rusty from the Salt Air from the gulf. But I never have seen any aircraft with any rust at all. So I'm guessing that Salty air does not effect planes??
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Perhaps that is because corrosion on aluminum doesn't stand out as much as red rust from ferrous metals or leave bright orange trails across white paint. Most definately does AL corrode and it can get quite serious.
 
Dan, that was a very good explanation of the how and why AL corrodes. And it's good that you mentioned the filigree type of corrosion.
 
Dan Thomas said:
Copper-containing aluminum sheet will have a layer of pure aluminum on both sides (2% of total thickness) that will react instantly with oxygen and form a stable, inert layer of aluminum oxide to protect the underlying metal.
Dan
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Anodizing also takes advantage of aluminum's corrosion properties as a protective coating.


Trapper John
 
Snaggletooth said:
Duh..... Not sure why that did not dawn on me! I know Aluminum don't rust.
Oi... guess I should have put more thought in to it before I asked. lol
Click to expand...
There is no such thing as a dumb question. Not asking is dumb.
 
Dean said:
Guy from my old airport bought a beautiful Cessna 150 from Florida. New paint, interior, looked like new. Seller had the pre-buy done for him by his reputable IA. Got it home and had to spend several K's putting new wings on it.
Click to expand...


Been there, too. A long time ago a student busted one of our 150s. We located a pair of "serviceable" wings out in Vancouver, went and looked them over, brought them home. Stripped the paint off. One of the guys noticed a little blister on the leading edge of the left wing and gave it a poke with his penknife, which went right through. The whole leading edge skin was rotten. Birds had found that they could get inside through the holes in the false spar ahead of the aileron, and had built nests next to the landing light where there was shelter from wind and rain but still nice and light. Bird poop and other moisture did the damage.

Mice, birds, anything that makes nests absolutely must be kept away from airplanes. Mouse pee is really corrosive. Mice will build nests just about anywhere, too; they don't seem to care if it's dark or light. One place I look regularly for nests is in the inboard ends of 172 elevators. Some of the later ones had that end closed off, but most are open. Push the rudder to one side and get your eyeballs right in there with a good light. Look for bits of straw and other junk, and see if there's corrosion happening at the steel elevator torque tube flanges where they're riveted to the elevator ribs. Two places in each elevator.

Airplanes flown frequently suffer less from pests than those that sit. So it's really cheaper to go flying than to let it sit. Only thing less expensive would be to sell the airplane.

Dan
 
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Shop had a 170 in for some control surface hail work, started poking around on the tail, ended up rebuilding the whole shebang for the same reasons you cited. All the nesting material (and a few deceased critters) were still in there.

Dan Thomas said:
Been there, too. A long time ago a student busted one of our 150s. We located a pair of "serviceable" wings out in Vancouver, went and looked them over, brought them home. Stripped the paint off. One of the guys noticed a little blister on the leading edge of the left wing and gave it a poke with his penknife, which went right through. The whole leading edge skin was rotten. Birds had found that they could get inside through the holes in the false spar ahead of the aileron, and had built nests next to the landing light where there was shelter from wind and rain but still nice and light. Bird poop and other moisture did the damage.

Mice, birds, anything that makes nests absolutely must be kept away from airplanes. Mouse pee is really corrosive. Mice will build nests just about anywhere, too; they don't seem to care if it's dark or light. One place I look regularly for nests is in the inboard ends of 172 elevators. Some of the later ones had that end closed off, but most are open. Push the rudder to one side and get your eyeballs right in there with a good light. Look for bits of straw and other junk, and see if there's corrosion happening at the steel elevator torque tube flanges where they're riveted to the elevator ribs. Two places in each elevator.

Airplanes flown frequently suffer less from pests than those that sit. So it's really cheaper to go flying than to let it sit. Only thing less expensive would be to sell the airplane.

Dan
Click to expand...
 
Trapper John said:
Anodizing also takes advantage of aluminum's corrosion properties as a protective coating.


Trapper John
Click to expand...

Is there an easy do-it-yourself way to get Al to anodize in a COLOR?
I'd like to get the plain Aluminum seatbelt clamps to turn a different color, without removing them from the nylon webing.
 
Dave Krall CFII said:
Is there an easy do-it-yourself way to get Al to anodize in a COLOR?
I'd like to get the plain Aluminum seatbelt clamps to turn a different color, without removing them from the nylon webing.
Click to expand...

They'd definitely have to be taken off the belts, since anodizing is an electrochemical process like plating.


Trapper John
 
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