copper tubing problems/important read

Gentleman and ladies. Due to the thread of the engine fire at KFFC and at the request of a maintenance question from Pierre and some questions of my own....I did some research and I found an article on copper corrosion and LO and Behold... Aluminum was a factor... granted these people are not using 100LL as a medium but water. I am asking that some of our more educated engineering types please read this article and post your thoughts. The copper tubing that I thought was overflaired... and broke of in my hand on the gascolator on the rv-4 may not of been over flaired as I first thought... and I will gladly admit my mistake if I was wrong... All I know for sure is that I touched it and it came off in my hand. In a study where copper tubing is developing leaks in the Washington DC areas this study mentions aluminum as being the ingredient that promotes pinhole corrosion!!!!

Please read and post your thoughts
Thank you,
Brian Wallis
AP/IA
http://www.nace.org/mponline/2004/0405018.pdf

It doesn't seem that dissolved aluminum or chlorine were a factor in your failure. It is well known that copper work hardens very quickly with vibration and should not be used in aircraft for plumbing any part of the fuel system without a strain loop. Even then, probably not a good idea. Steel, SS or aluminum is a better choice. There was a case not too long ago where a small SS fuel line to a transducer on a turbine engine was replaced with copper??? and the line broke after less than 200 hours. I can't find the article now but think this is roughly correct.

Brian,

Galvanic corrosion can happen between aluminum and copper, but it is the aluminum that will corrode, not the copper. However, in order for them to corrode, they must be connected by an electrolyte, for example moisture.

Annealing ridged tubes after forming and service

Can I be honest, copper tube is old world technology and I would not use it, at all, if it can be avoided. I know, factory planes have used it for years but don't think its popular anymore. Today you can use small -2 flex lines or ridged stainless steel lines.

The problem with ridged tubing, copper or aluminum, is it work hardens. As the engine vibrates, the tube becomes brittle and fails. When you flare it or bend it, it also gets harder. Every year or so you are suppose to anneal copper lines in the engine compartment. Copper lines typically use compression fittings or "solder on" fittings. Flaring copper is not typical like aluminum.

To anneal copper lines you need to make it soft first with high heat. Copper becomes soft between 1,200 to 1,650 F. You can heat it to 600-800F for a few an hour or so until evenly heated. Once the part is evenly heated or heat soaked, than allow it to air cool or water quench. Copper is not critical in the cooling phase like steel. Using a torch in the shop, probably the easiest method, get it to about 1000 F, when the copper glows a dull red in a dimly lighted work area. When you water quench, you need enough water to do the whole piece at one time.

Some say 2 hours on broil in your home oven will do it. I don't know about that. If it's big enough for the part and gets to +600F, than it will work. The idea is to heat the whole tube, not just one area, as well as cooling it all together.

BTW for you should anneal your copper spark plug washers, again torch to cherry red and quench. Spark plug washers get hard and will not seat.

Aluminum tubing is different. Some do it by using an acetylene torch, but it is easy to melt aluminum. It takes some skill. The difference between the annealing temp (650F-825F) and melting is small. One method uses a dirty acetylene flame to soot the tube up first, than a hot flame is used to burn the soot off, taking your time to remove the soot slowly and evenly. If you get too close you will melt aluminum fast. Let it air cool to 400-500F and than you can water cool. Tempilstik is a name brand of a product you paint on a surface to monitor temp, like when heating with a torch. Aluminum will not show heat (at all) like copper.

It cost more but flex lines are better. I avoid hard metal lines on things that shake.

To be fair of aluminum or copper line's, done with care work fine. I have a O-360 Lyc and I'm not going with primers. I want to avoid a bunch of 1/8" copper lines. If I end up adding them on later, I'll look for small stainless steel lines first. If I use copper, I'll solder the fittings on. The primer supply line from the gascolator and to the primer lines on the engine will be a flexible line.

metals

Talking to a teacher friend of mine... he says "when two metals are touching... like the water connection to the house, there is an insulator placed between the different metals to prevent contact and thus corrosion. There is a battery like effect on the unlike metals when joined.... and one will lose metal eventually... so please beware."
Thanks
Brian

brianwallis said:

There is a battery like effect on the unlike metals when joined

Click to expand...

Very appropriate - that's exactly what's happening. Anytime two dissimilar metals are in contact with each other AND an electrolyte is present (such as water, even from humidity in the air), the more active of the two metals will corrode. This is known as GALVANIC corrosion, and is indeed the basis for the common cells we call "batteries", at least the non-rechargeable (known as primary) kind. The only way to stop galvanic corrosion is to:

A) Separate the metals with some type of insulator, or
B) Separate the metals from the electrolyte - usually by coating the metal with something like a spray to keep moisture out.

Sidenote here - the Alclad skins used in most aluminum kits are made of an aluminum alloy, with a very thin pure aluminum cladding on the surface. Those essentially constitute two dissimilar metals, but no corrosion can take place between them because no electrolyte is present. If you scratch through the very thin (and very soft) pure aluminum cladding to expose the alloy underneath, you have now exposed both metals to moisture from the atmosphere and corrosion will take place there (slowly) unless you prime/coat that area somehow to keep moisture out.

airguy said:

Very appropriate - that's exactly what's happening. Anytime two dissimilar metals are in contact with each other AND an electrolyte is present (such as water, even from humidity in the air), the more active of the two metals will corrode. This is known as GALVANIC corrosion, and is indeed the basis for the common cells we call "batteries", at least the non-rechargeable (known as primary) kind. The only way to stop galvanic corrosion is to:

A) Separate the metals with some type of insulator, or
B) Separate the metals from the electrolyte - usually by coating the metal with something like a spray to keep moisture out.

Sidenote here - the Alclad skins used in most aluminum kits are made of an aluminum alloy, with a very thin pure aluminum cladding on the surface. Those essentially constitute two dissimilar metals, but no corrosion can take place between them because no electrolyte is present. If you scratch through the very thin (and very soft) pure aluminum cladding to expose the alloy underneath, you have now exposed both metals to moisture from the atmosphere and corrosion will take place there (slowly) unless you prime/coat that area somehow to keep moisture out.

Click to expand...

And to confuse everybody, since I don't know the exact answer of why:

Take your everyday refrigeration coils, which are aluminum fins with a copper tube tightly swedged through round holes in the aluminum. These coils are constantly subjected to humidity, yet don't easily corrode. How come?

Just wondering, if someone has info on the subject.

L.Adamson

Because they've already corroded.

Pure aluminum is very reactive - within seconds it will react with oxygen in the air to form aluminum oxide on the surface of the metal. This aluminum oxide layer is very tightly bonded to the surface and quite effectively blocks any further oxygen from reacting with the metal underneath. This aluminum oxide layer serves as a barrier, just as effectively as a primer coat. It not only blocks oxygen in the air, it also quite effectively blocks out the moisture that would serve as an electrolyte to support galvanic corrosion between the copper and aluminum.

What we commonly see everyday and think of as aluminum, is actually metallic aluminum with a very thin layer of aluminum oxide on the surface. Pure aluminum will only be exposed to air for a few minutes at most before forming the oxide film on it's surface.

Aluminum oxide is so effective at blocking out moisture, aluminum plates in acid electrolyte were used as the original diodes for chopping AC current into pulsed DC, long before the days of silicon rectifiers. The aluminum (with it's coating of oxide) was immersed in the electrolyte with the case of the electrolyte tank as the other electrode. Cathodic current would strip off the oxide layer and allow all further current to flow unimpeded, but when the sine wave of the AC current went back through zero, the anodic current quickly reformed the oxide layer and blocked the electrolyte from contacting the plate (aluminum oxide is a pretty good electrical insulator), stopping current flow until the voltage went back through zero to cathodic current again. The final result was AC voltage in, pulsed DC voltage out.

I found that intersting. Thanks.

gary bricker

What is everyone using to connect to the sniffle tube and to the fuel pump drain which are both copper.

airguy said:

Because they've already corroded.

Pure aluminum is very reactive - within seconds it will react with oxygen in the air to form aluminum oxide on the surface of the metal. This aluminum oxide layer is very tightly bonded to the surface and quite effectively blocks any further oxygen from reacting with the metal underneath. This aluminum oxide layer serves as a barrier, just as effectively as a primer coat. [snip]

Click to expand...

Greg,

I'd have to disagree with you. First of all, let me premise my response by the fact that I'm an engineer who designs heat exchangers for a living. Also, the last 2 years I've been working on a project which involves corrosion resistant aluminum materials.

Although it is true that aluminum oxide very quickly forms on the surface and that it is a protective barrier, it is not a perfect barrier. It will corrode under the right conditions. Mating copper and aluminum in an electrolyte will certainly do the trick.

To answer L.Adamson?s question about why aluminum fins don?t corrode in the presence of copper tubing in a refrigeration coil?well, they do corrode. It just takes a long time for it to happen. That?s a big part of an Engineer?s job; to design a product that will last the design life of the product. If the fins slowly corrode, the performance of the coil is gradually reduced, but still functional. If the tubing were to corrode, it would eventually leak refrigerant and the unit would not work at all. The tube does not corrode because the fins are sacrificial. This is a good thing.

The main reason that the fins take time to corrode is because there isn?t much surface area for corrosion to occur. The tube is expanded into the fins so tightly that no moisture can get in there. The only place for the galvanic corrosion to start is adjacent to the connection.

Many air conditioning coils are moving towards all aluminum brazed heat exchangers with parallel tubes (like a radiator). This is accomplished by using fins that are slightly sacrificial to the tube material. They are both aluminum, but different alloys. Aluminum radiators and charge air coolers have always been designed this way to resist salt spray corrosion.

I should also note that mating different metals together will have a different corrosion rates. For example aluminum corrodes very quickly when mated to copper but much less so with most other metals. Again, the dissimilar metals must be connected by an electrolyte.

True statement, I didn't mean to imply that the situation was long-term stable, it certainly is not. Any galvanic corrosion setup is stable (meaning the cathode is protected) only as long as the sacrificial anode is viable. Once the anode is gone, or the electrical connection between the two is broken, then all bets are off and you're back to having both metals corrode as appropriate for the atmosphere present. Sacrificial anodes are commonly designed to last for the lifetime of the product, or designed to be easily replaceable, like the zinc trim tabs on an outboard boat motor.

re copper primer lines

My O-360 has the usual 1/8" copper primer lines.
If they did rupture AND the primer was in and locked would there be any appreciable flow of fuel out of those lines anyway?

S_tones