One For The Materials Guru's

[Guest]

Rite gent's, had to give a presentation yesterday on my project this year, part of it is to put steel thread insert's into aluminium cranks, nothing new, onza, & monty have done it amongst others.

What perplexed me was that I ended up being told that in a situation as described above, the aluminium would react/corrode in some way with the steel?

This is news to me and after looking today I can't really find any information on it. I'd like to go back after xmas with an answer ideally as to how to sort it out, but as far as im aware tensile cranks are aluminium and their insert's are steel, do they have to undergo any treatment before being put together?

I was really shocked when I was told this, I mean I use steel bolt's in my aluminium frame, and don't have any problems, so is there even an issue? What was the lecturer on about????

[26inch]

i know if you leave an alu seatpost in a steel frame it can corrode together, but as far as bolts and pedal inserts go the only thing i can think of is grease or threadlock is normally used which would help prevent corrosion but i'm sure there's something more tech than that

[Ashley Smith]

it is due to the properties of disimilar metals and the react, oxidise/ corrode................... not really a problem if you use grease of some sort............ nothing to worry about to be honest either...........

ash

[Shaun H]

The only thing I could think of that he might be reffering to is if the iron in the steel oxidized/rusted it would take electrons from the aluminium instead of oxidising fully itself causing the aluminium to oxidise and corrode as in sacrificial materials used to protect oil pipes and stuff (zinc usually).

But due to the low reactivity of the materials used (as they are alloys) I can't see that being a problem.

[Guest]

Excellent guys thank's a million, I see where your coming from about the alloy's low reactivity, that's due to a full outer sheel isn't it? Trying to remember GCSE chemistry 4 year's on isn't easy!

[smelly joe]

Not sure if someone has said this allready but, if the inserts were made of stainless steel then the steel would not be able to oxidize and rust therefore you would not have the problem of the 2 metals corroding together.

although ive never heard of aluminium and steel corroding together, certainly havent corroded together on my old As work. maybe it only haopppens over a very long period of time.

Edited December 21, 2006 by smelly joe

[durkie]

the phenomenon is known as galvanic coupling (or a galvanic couple). almost any dissimilar metals in contact with each other will corrode when there's a mobile electrolyte (water). indeed, this is a problem with the 2 euro coins i believe, because the outer ring and the inner center of the coin will start to corrode after being handled by so many sweaty hands, getting wet in the rain, etc.

the reason aluminums and stainless steels tend not to corrode under a lot of conditions is because they very readily oxidize and have a very adherent oxide layer, so almost any surface ever of aluminum is covered with a thin protective aluminum oxide layer, and almost any surface of stainless steel is covered with a thin protective chromium oxide layer (and then steel has iron oxide, but it doesn't protect so well). if you want to see the difference in how well the layers protect, go get a piece of rusty steel and scrub away the rust until you see bare metal. now get some anodized aluminum somethingorother and scrub away the anodizing, and it will be quite a bit more difficult.

but yeah in the end, just use use plenty of grease. that will help keep them from being in direct electrical contact with each other, but also the rate of this kind of corrosion is likely going to be very low if it's not being bombarded by sweat all day.

Edited December 21, 2006 by durkie

[future orange 660]

but thats anodizing not corrosion

anodizing hardens also. thats shy its harder to get off.

[Guest]

yeh anodizing make the surface much harder but I see your point anyway.

thanks durkie for the technical term for it, that will go down well in my next presentation I hope!I've been doing a bit more research and it's totally negligable really.

Thanks again folks, much appreciated.

[durkie]

but thats anodizing not corrosion

anodizing hardens also. thats shy its harder to get off.

anodizing hardens because it grows an aluminum oxide layer from the aluminum surface. aluminum oxide is one of the hardest materials out there (and definitely way harder than aluminum metal). it's still the same compound that forms naturally from exposing bare aluminum metal to oxygen. anodizing is just special because it's a somewhat thicker layer and it's colored.

the point was that aluminum oxide is tenacious and protective on the surface of aluminum, while rust is not.

[Jez]

Having had a pair of ali bolts cease inside a steel monty BB shaft, I can confirm that ali going into steel or vice versa is a bad idea

Oh and that was anodised ali as well btw, although some of the anodising had worn away.

Either way it cost me a BB. Steel crank bolts, not an issue, despite the cranks themselves being ali...I think there's more to it than meets the eye. More complicated than just two metals touching causing one or both to corrode.

[Shaun H]

the phenomenon is known as galvanic coupling (or a galvanic couple). almost any dissimilar metals in contact with each other will corrode when there's a mobile electrolyte (water). indeed, this is a problem with the 2 euro coins i believe, because the outer ring and the inner center of the coin will start to corrode after being handled by so many sweaty hands, getting wet in the rain, etc.

the reason aluminums and stainless steels tend not to corrode under a lot of conditions is because they very readily oxidize and have a very adherent oxide layer, so almost any surface ever of aluminum is covered with a thin protective aluminum oxide layer, and almost any surface of stainless steel is covered with a thin protective chromium oxide layer (and then steel has iron oxide, but it doesn't protect so well). if you want to see the difference in how well the layers protect, go get a piece of rusty steel and scrub away the rust until you see bare metal. now get some anodized aluminum somethingorother and scrub away the anodizing, and it will be quite a bit more difficult.

but yeah in the end, just use use plenty of grease. that will help keep them from being in direct electrical contact with each other, but also the rate of this kind of corrosion is likely going to be very low if it's not being bombarded by sweat all day.

Yay, that's what I said but with more words.

And yeah like I said, in most cases it's almost negligble, maybe your teacher thought you needed to take into account the long term life of whatever it is you said you were making.

[brainchild]

use copper grease on seatpins and this will NEVER happen.