Galvanic reactions 1

[rpmag]

WRT galvanic reaction between steel and aluminium alloy, is the rate of corrosion of the anode (alloy sheet) more affected by the mass of the cathode or the area of surface contact with the cathode Steel spaceframe chassis?

 

[swall]

Surface area rather than mass. But, what kind of fasteners are used to join the sheet aluminum to the steel? And remember, if you don't have continuous exposure to an electrolyte, the potential for galvanic corrosion is minimal.

 

[rpmag]

I am using stainless steel rivets. Both the stainless and mild steel are more nobel that the aluminium which would then become the anode. The sheets would also be bonded, thus the primary contact point would be the rivets.
However the adhesive may also contribute to the reaction. It seems that a filled polyurethane contains a significant amount of carbon black. I have been informed by Henkel that this will contribute to the reaction if used. Mind you epoxy will not contribute.
Your thoughts?

 

[swall]

With ss rivets and the aluminum skin, you have a large anode area and a small cathode area. This is the "least worse" situation. The adhesive won't aggravate the situation, since I assume it will be mainy on the inside of the aluminum sheet, which would be the dry side. What environment will this vehicle be exposed to and how long do you want it to last? If you really want to be safe, you could put a sealant on the rivets or in the rivet hole before you set them, which would help isolate the rivets from the aluminum.

 

[rpmag]

I would like the panels (Ali skinned honeycomb panels) to last as long as possible, i.e. years! The panels are unlikely to have great exposure as it will have a body over the top, but naturally wheel wells etc will have exposure. I had thought of using epoxy on the rivet prior to setting, but I feel it will probably mostly squeeze out.

 

[pauljohn]

In the yachting industry where stainless fittings and aluminum spars are a constant scource of headaches we find that 'monel' rivets last longer, although the shear strength is lower.

 

[rpmag]

I have that information too from Carroll Smith's books, but they are not as easy to find locally and have a much lower performance. I shall look into it further.

 

[trisha26]

if you are talking about fasteners, then you really wont have a problem as there is always a corrosion protection coating on the fastener, besides steel and aluminium on the galvanic series are not too far apart and you also need a large delta for this to take effect, along with area and the right environment.