Does anyone have details about the Galvanic Interaction between Aluminum and carbon fiber in a wet environment?

https://compositeenvisions.com/document/galvanic-corrosion-carbon-fiber-and-aluminum/

Depending on the degree to which carbon fibre matting is actually fibres and resin, carbon fibre parts electrically can be peculiar stuff. The individual fibre, length-ways, are quite good conductors, a few ohms per 10cm not unlike a pencil lead, but the points of contact between fibres are very hit and miss, sometimes creating pin-point hot spots or if the resin has got in the way, total isolation. It is not unknown, ahem, for a careless 'improvement' of a transmission antenna, by replacing a glass fibre part with a carbon fibre one in a high electric field, for some of the carbon fibre parts to be just fine while other seemingly identical parts   catch light with the induced currents.(oops!)
The sheet conductivity depends critically on the degree that the resin has been squeezed in, and I can believe on a 'plane, that the fibres will be woven, and the resin fraction will be controlled and pretty minimal, ,so making a more or less conductive sheet, especially at fixings where the part has been drilled or cut exposing the fibre ends..

Then the only question is where is carbon in the ionisation/cathodic series, and the answer seems to be "far way enough from aluminium to have problems" - After all the carbon / zinc cell works quite well with a carbon positive electrode. 

The fact the modern alkaline ones are inside out, so the carbon is the a cup for the rest of it,  and forms the body rather than a central rod, reduces the cell resistance no end compared to the ones of my childhood where the zinc was the body, and the current had to bunch up to reach the relatively high resistive carbon rod. They also leak less often than I remember  ;-)

Mike

This reference article is excellent.

Thank you 

Peter Brooks

Palm Bay FL

Hello Mike:

Looks like using carbon fiber is a lot more complex than I thought.

Thanks for the additional information.

Peter Brooks

Palm Bay FL

The fact the modern alkaline ones are inside out

The inside out cells do seem to have introduced a new problem though - the traditional holders have a spring for the -ve contact (which traditionally was the can one the cell) - but now meets a islanded -ve contact surrounded by a thin plastic sheath - any sudden movement (e.g. remote control being dropped onto the floor) and the string cuts through the plastic sheath and shorts the +ve can with the -ve contact ... resulting in a short circuit. I've had a couple of remote controls brought to me that were already too hot to hold...

    - Andy.

Hello Andy:

I assume this potential short problem can happen on all standard battery sizes (AAA, AA, C and D) but not on the multi-cell 9 V which is used in fire alarms.

Peter Brooks

Palm Bay 

true - that reminds me that I've seen vibration failures in a metal tube shaped holder, (imagine a metal bodied pocket torch) where after rattling about on a vehicle for a year or so, the battery shorted out having scuffed a hole in the insulation.

I now note that the oversleeving is similarly thin on the cheaper lithium cells as well.

Mike.

Hi Peter,

The article from Olympusmons is a great read and informs you how the corrosion occurs.  But what we need to understand is, what was the root cause?  Was it an internal arcing fault, due to wear with the cables in the conduit?  Or, as you are aware, toilets can be a highly corrosive environment, due to the corrosive fluids used, whether that be human or cleaning fluids.  Was there a leak into the conduit?  Hopefully, at least, the FAA has only found the issue in this one area on this aircraft type.  Do you know how many occurrences there has been?  Is there any similar issues on other Boeing ac?  Does Boeing manage a RCM log or conduct Fail Mode Effect Analysis?  

I hope you are able to find the root cause and rectify the problem.

John

Hello John:

The published FAA notice does not state if it was a mistake by Boeing during the planes assembly.

The notice just defines what periodic maintenance inspections must be done to find the problem.

 Peter Brooks

Palm Bay FL

It`s  common enough problem to warrant inclusion of glass fibre isolation plies in most AL-CFC component interfaces so considering what goes on it a lavatory during a flight its not surprising there is corrosion problems.

My old payloads lead at Boeing used to say the worst corrosion areas in the aircraft were the galleys and the toilets!.. urine and food & drink. Carbon can be very detrimental to the AL if not properly protected.