Galvanic corrosion between Aluminium and carbon fiber

[drodrig]

Hi there,

I've been reading a bit in the forum and internet about the galvanic corrosion between aluminium.

We have a carbon fiber reinforced epoxy boxes with a load and they are hung in one end. We glue an aluminium insert. The gap is 150 microns around and the glue 3M DP 410.

In theory, there is no direct contact between the carbon fiber and the aluminium, since we have the glue everywhere around the aluminium.

We can't use glass fiber in between.

I am not familiarized with the treatments but there is a company here close by which can do a chromate (Alodine) the aluminium parts.

Do you think this is safe enough to avoid the galvanic corrosion?

thanks
regards

 

[EdStainless]

Maybe.
This why modern airliners use Ti for the floor structure, so that they can use CF body and floor panels.
If this is glued to the side of a vertical surface, and if enough dirt and dust build up to bridge the glue joint, and if it gets damp, then the Al will be going away quickly.
I would look at coating the sides of the Al piece either with the glue or with a good coating (epoxy or urethane).

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P.E. Metallurgy, consulting work welcomed

 

[TugboatEng]

You describe the component as an insert. Can it be knurled for grip and then hard anodized? I have seen hard anodized components hold up well in highly galvanic connections (aluminum plate bolted to a brass fire monitor).

 

[EdStainless]

TBE, I problem is galvanic potential, Al to brass in seawater is about 0.6V difference, but Al to graphite is more like 1.2V.
It is very hard to suppress corrosion with that much driving voltage.

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P.E. Metallurgy, consulting work welcomed

 

[drodrig]

Hi,
Here some pictures (grey part: aluminium insert - brown part: carbon fiber box:

Knurling would be possible but the walls are very thin.

So anodizing would be better than chromating?

 

[Compositepro]

Galvanic corrosion between carbon fiber and aluminum is real but the concern is often overblown. Carbon and aluminum make a very good battery. But there is a tremendous difference between water and salt water. If there is not frequent exposure to salt water there is often no problem. The actual exposed area of carbon fiber is usually small due to being coated with epoxy. Carbon fiber is not a great conductor nor is fresh water. Corrosion will be concentrated near the point of contact between the two materials and one of the corrosion products are hydroxyl ions, which will chemically corrode aluminum. This tends to break the electrical contact between the aluminum and carbon fiber. Thick anodizing is non-conductive and all aluminum surfaces have some oxide layer.

 

[EdStainless]

But when you think about exposure think of all possibilities.
On aircraft interiors the drivers are cleaning agents and spilled soft drinks (cola is one heck of an electrolyte).
The worst non-seawater incidents that I have seen involved dust and dew. So think ahead.
If the face end of the box has been machined you may want to think about a thin plastic spacer there to prevent butting the Al directly against the raw CF edge with only glue as a separation.

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P.E. Metallurgy, consulting work welcomed

 

[3DDave]

Isn't there also a potential problem from the difference in CTE between the aluminum and the tube? I can picture a failure of the bond starting in a corner and that could expose bare fiber as it delaminates.

 

[EdStainless]

The CTE of the CF depends on fiber orientation, fiber loading, and resin used. They may not be that different, or they could be very different.
Good point 3DD, if the Al expands more it could split the end open, it is expands less then the bond line is gone.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, consulting work welcomed

 

[TugboatEng]

There are epoxies with rubber microspheres that might reduce any issues from differing CTE. I believe 3M DP805 is an example.