aluminium corrosion

[Antonio]

During a search for bonding between al and CF I found discussions on corrosion of the aluminium under use. Would anyone be able to explain why this happens? Is it got something to do with the epoxy having an effect on the aluminium oxide?

Thanks

[Antonio]

During a search for bonding between al and CF I found discussions on corrosion of the aluminium under use. Would anyone be able to explain why this happens? Is it got something to do with the epoxy having an effect on the aluminium oxide?

Thanks

[Zac]

I'm hardly a materials guy, but I'm pretty sure that since the CF and aluminum are both conductive it sets up a galvanic cell.

[Antonio]

Thanks very much Zac, i will look into that.

[Tim.Wright]

I spent precisely one afternoon researching this before getting bored, but I think Zac is right. You get a galvanic couple between the two materials. That is why you always put a ply of glass veil (which typically will have a resin content of 90%) on the surface of any interfaces to metallic parts. Also, the bolts would be wet installed to stop corrosion inside the bolt hole where there is no glass.

That is the aerospace method of dealing with the problem. For motorsport applications though I´m not sure its such a problem as the vehicle aren´t expected to last as long as a plane. Though I really have nothing to back that up.

Tim

[Antonio]

so corrosion would only occur if the cabon fibre and aluminium were in contact? Whats the electrolyte between the two?

Also I thought an epoxy bond would create an electrical insulation between the two metals? I am sorry if i am getting this all wrong.

thanks

[Moke]

There is galvanic corrosion between carbon and metals. But unless you intend to drive your car in salt water for the next 5 years I would not worry about it too much. Al honeycomb is coated to prevent this and if you are putting Al hard points and are worried wrap them in glass or have them anodised.

[Tim.Wright]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by Antonio:
so corrosion would only occur if the cabon fibre and aluminium were in contact? ...

Also I thought an epoxy bond would create an electrical insulation between the two metals? </div></BLOCKQUOTE>

Yes I also thought the same, but it is still standard practice to design against this type of corrosion in the aerospace industry. However, I don´t think its a problem for the time frames that we would be running a car in.

I don´t think you would have to worry about your car disbonding due to corrosion the night before the competition...

Especially if you only finished it 2 nights before ;-)

But adding a few plies of glass at metallic interfaces could be considered ´good design´

[murpia]

A product exists which is designed to help this issue, it's an epoxy primer which you paint onto aluminium parts before bonding them into carbon structures. It's very thin, almost watery apparently.

Sorry but I don't have an actual supplier to suggest, though, but the normal structural adhesive suppliers should know about it.

I heard about it because a company I worked with had to rebuild an old F1 car and the engine mount inserts all fell out of the tub due to the corrosion issue. They primed the new inserts with this stuff to ensure a good life for the car after that.

Regards, Ian