carbon fiber with aluminium, titanium and copper

[aerospace2013]

thread327-339948

Hello

I'm referring to the thread above.

it was mentioned that carbon fibers create an electric potential with aluminum. How can we prevent this? can we spray the aluminum with epoxy before embedding the Al inside the carbon?

Also, does the same happen with titanium and copper or not

I appreciate your help guys

Mr Blakmax could you please help.

 

[RPstress]

You mention embedding the aluminum inside carbon. Is this a layer of metal in a composite laminate? A washer or some other sort of component?

Aluminum has fairly negative electrode potential (of common metals only zinc and magnesium are worse). You need to electrically insulate the aluminum from galvanically dissimilar materials (especially carbon; only platinum and gold are more positive or noble). The aluminum must not touch the carbon and must not be joined to it by a conductor (e.g., moisture, and the moisture that infuses into a thermoset polymer matrix can be enough to give problems, especially if the resin has microcracks or crazing).

Aluminum should be being protected from the environment anyway with anodize, prime and paint, but this is not enough if carbon is about. A better material than just epoxy is needed to protect aluminum from carbon. As was noted in that thread a layer of thin glass fabric with primer on the aluminum seems usually to last ok even though the glass is fairly porous. The additional distance the glass adds may be a significant factor (if you could arrange (and guarantee) a really thick gel coat roughly twice the thickness of the glass layer it might work about as well).

Titanium and good stainless steel have better adhering oxide layers than aluminium plus their constituent elements are less negative than aluminium, and generally titanium and good stainless don't need additional protection (over and above their protective oxide) and can be used bare. Some of the martensitic and precipitation hardening stainlesses are not so good; for instance, it may be worth protecting 440A, B or C or 17-4PH, although in practice they'll usually be ok. Guaranteed good stainlesses are A-286, as used for many aerospace fasteners, and austenitic (300-series).

Copper (and brass and bronze) is not so galvanically negative as aluminium but is more so than titanium, and copper has no protective oxide layer and it should be reasonable well insulated from any carbon. A brass bush can be inserted into a hole in a thick carbon laminate with a coating of primer and a sealant.

Whoever is responsible for design rules concerning avoidance of galvanic corrosion should have set down pretty clear ones for what needs protecting from what and how, especially carbon fiber.

 

[aerospace2013]

Thank you very much Mr RPstress for your reply

What I'm trying to do is to place aluminium , titanium and copper small wires ( with diameter around 0.25mm) between the carbon fiber tows and then pour the epoxy on them to form a laminate for specific purposes.

Does your explanation still apply here?

so you are suggesting not to use the aluminium, but copper and titanium will be OK?

Appreciate your reply

 

[RPstress]

With wires between the tows some contact between the carbon and the wire would seem very likely. I would expect the aluminium to disappear after a few months (maybe years) service in an environment with ordinary moisture, at least close to where it was touching the carbon. If you could arrange to keep it very dry it would last longer (just keeping it indoors would help a lot).

I think the titanium should be ok. The copper...maybe! The copper mesh (usually a bronze alloy) used for lightning strike resistance on carbon aerofoils seems to last ok for many years with regular soakings. The copper's always separated from the carbon by a layer of epoxy about 0.005" thick, but it may get squidged down to zero in some places (the epoxy forms a thin layer on top of the copper, too).

A test might help. If you could set up some tap water or seawater with a lot of carbon and the relevant wires in firm contact with the carbon in the water, and maybe elevate the temperature a bit you might get more of a feel for how long it might last (you'll need a control with no carbon).

How much do you need there to be no corrosion activity? With just wires I assume there's no safety issues, so presumably it's just money or some sort of experiment success/failure at stake. I don't know how much the conductivity of the wires may be affected; I would assume that as long as there's a continuous uncorroded length they'll work passably well (if that's what you want them to do).

There's not really any aluminium to choose which would be less prone to galvanic corrosion. The best marine alloy (something like 5083 or 6082) will still corrode if there's moisture and a material with a higher potential about. It might be possible to protect the wire by some sort of passive protection (zinc maybe, possibly as a coating, which will corrode preferentially) or even perhaps an active system. Boat and ship designers may know best; there are many big ships made of aluminium interfacing with steel and stainless steel and copper alloy bushes in seawater. All the passive systems work very well, but have a life due to the zinc being used up. The people dealing with underground pipelines use passive and active corrosion protection a lot.

The environment will probably make most difference. It's possible that in an ordinary indoor environment an unlimited life might result even for the aluminum.

 

[Compositepro]

If these are signal wires just use copper wires with an enamel insulation. Copper is the best conductor, and insulated wires are readily available.

 

[aerospace2013]

Thanx a lot for the detailed explanation

I would like to order my wires from

http://www.goodfellow.com/

After I look up in their website for titanium and copper wires I found what I list in the links below.
could you please suggest me the best type with best diameter from the links below. Also, if you know any company that sells similar products with cheaper prices, could you please recommend it to me? One last thing, where can I find copper wires similar to the ones used in the aircraft radome for energy dissipation and what is the copper alloy made of?

1. titanium alloy

http://www.goodfellow.com/catalogue...eQyHuOAj&ewd_urlNo=GFCat65&Catlook=Alloy Wire

2. copper alloy wires - the have two types, insulated and non-insulated.

wire- not insulated

http://www.goodfellow.com/catalogue...eQyHuOAj&ewd_urlNo=GFCat65&Catlook=Alloy Wire

wire- insulated

http://www.goodfellow.com/catalogue...eQyHuOAj&ewd_urlNo=GFCat65&Catlook=Alloy Wire

Appreciate all the help and valuable replies

 

[RPstress]

Those links don't work. Anyway, it probably depends on what you want your wires to do and what the service environment is.

 

[aerospace2013]

the products in the link are
Titanium/Aluminium/Vanadium ( Ti90/Al 6/V 4 ) Wire
diameters available 0.1mm 0.25mm and 0.5mm

Copper/Beryllium ( Cu98/Be 2 ) Wire ( not insulated)
diameters available from 0.01mm to 1.6mm

Copper/Beryllium ( Cu98/Be 2 ) Wire (insulated)
diameters available 0.025mm 0.075mm and 0.1mm with insulation thickness around 0.01 and 0.004mm

Appreciate the continues reply and help

 

[RPstress]

Ti-6-4 should be very durable if durability's what you're most interested in. It may become subject to hot salt stress corrosion cracking at elevated temperature but not at any temperature which will leave any carbon existing. It's strong and usually has passable elongation to failure (not sure how much cold work has gone into wire; more cold work higher strength lower elongation lower toughness). Modulus is low (half steel).

CuBe is actually usually a bit stronger and has about the same modulus. Probably similar toughness and also extension to failure. It should be a bit more vulnerable to galvanic corrosion but there's not a lot in it. It'll conduct heat and electricity about 10x better than Ti-6-4 (>80 vs. 7 W/(mK) thermal conductivity & 0.04 vs. 1.8 µΩm resistivity). Similar comments to Ti-6-4 re any cold work. Dense, but I doubt that's an issue with wire.

I've no data on fatigue of CuBe so although they're similar for static strength I've no idea which might be more durable if subject to repeated bending/vibration. I suspect CuBe might be better.

Ti-6-4 is useful structurally (structural duty depends on how much bending wire's subject to) and has proven use bare with carbon fibers about. Insulated CuBe will have much better thermal and electrical props and should be about as durable corrosion-wise. If there's much water present it might be worse (depends on material and fit of the insulation). Bare CuBe will be fine if environment is pretty dry.

 

[aerospace2013]

Mr RPstress.. thank you very much for the detailed explanation and patience.

Indeed you do have a very wide knowledge in this field

Thank you again