what the hell is this?

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#1

Look at the image that I have attached. This is off a part that I was making. There is an aluminium insert forming the core part of the piece. Surrounding the aluminium insert is a 3M DP490 2 part epoxy adhesive. There is additional adhesive in places to ‘fill in some holes’ as the metal insert was designed for a metal part and I couldnt be bothered to make my own and given it was €4, it also wasnt cost effective.

Around the aluminium and adhesive in places, there is a film adhesive with a glass carrier for GC reasons. Surrounding that is numerous layers of carbon. (all surfaces were cleaned with IPA throughout)

Long and short of the story is that the part was scrapped for geometrical reasons in another section so I just went at it with the grinder to see the insides! That was approximately 1 month ago. I soaked it in some water for maybe 30 mins with the intention of leaving it a while to see if there was any corrosion. Turns out, I completely forgot about it and found it on the workshop floor about half an hour ago. Now there is a white chalk like substance all aound the adhesive/ aluminium interface. Any idea what the hell this is or why the hell its there? Is this corrosion between the aluminium and adhesive or some bizarre thing like that?

#2

The 3m DP 490 should work perfect with aluminium. We bodend aluminium inserts in carbon wishbones for example. We never had that problem.
Have you used the correct mixer/pistol? may be there was too much hardener in the adhesive and it reacted with the CO2 to carbamate.

#3

i might say you had some galvanic reaction between the Al and the CF. Water IS conductive. Maybe it’s battery acid :wink: Not sure though, looks odd. Maybe a hydro loving mold? :slight_smile:

#4

The DP490 was mixed perfectly as I was using the proper mixing nozzles, etc, so I dont think its that. Also, if it is galvanic corrosion, im pretty sure it isnt involving the carbon as its the adhesive/aluminium interface, which shouldnt galvanically corrode.

weird isnt it?!?

#5

You have corrosion. When you ground the part to explore the insides, you removed the protective layer on the aluminum which electrically isolated the materials. When you put it in a bucket of water you gave corrosion the perfect environment to occur.

#6

Yeah, Rob sounds reasonable. Ive just spoken to a metallurgist too that agrees; refractory oxide layers - shows you how little I know about metals :slight_smile:
But, the thing that is crazy is the fact that it was in moisture for 30mins, then on a dry floor for 30 days and that amount of corrosion happened in that time. Apparently it normally takes a lot, lot longer!

#7

Hi all,
galvanic corrosion on alluminium/carbon interface is in form of very light white powder that accumulates in the joint area. This happens because carbon/epoxy is electrically conducting and cathodic with respect to most air frame alloys other than titanium. To avoid galvanic corrosion on the metallic side of the joint special precaution is required and ,typically, glass fiber or aramid fiber are used as insulating layers.

Ther are tables ( galvanic scales) which list the capability of metals to resist to galvanic corrosion in presence of carbon.
I have enclosed one (among many) ; it’s written in italian but translation is intuitive
Ferro = Iron
Piombo = Lead
Ottone = brass
Argento = silver
Acciaio inossidabile = inox steel
Oro = gold
The more distant from carbon is the metal, more prone to corrosion is; as you can see , alluminium is very distant.
File1 gives just some suggestion on the basis of what required in the aeronautical field ( for those interested)

Hope this can help

#8

Oh, I thought it was IN the water for 30 days!!! Still…not much else it CAN be :slight_smile:

#9

Still galvanic corrosion. Carbon dust on the aluminium can in the short wet period create some aluminium hydroxide, which is hygroscopic. The attracted moisture does the rest.

What is the normal moisture level in your area?

#10

Not too sure what the moisture level is for the area but I was pretty sure it wasnt that high…Maybe not!!