Why don't Metals all Stick Together?!

[duncanstives]

Ok this is driving me nuts... Metals are generally held together by metallic bonds where they all share a big cloud of electrons (I know this isn't totally accurate and of course there are exceptions but bear with me for the sake of argument)... I also know that metals (with the exception of certain amorphous solids like the SIM door tool for the iPhone 3gs) are organized in a specific pretty dense crystalline structure... My question is, since metallic bonds are pretty flexible with the whole electron cloud thing, what prevents two pieces of solid metal from becoming one at room temperature? I know surface morphology, oxidation layers and a bunch of stuff would prevent it under normal conditions... But if I were to somehow machine two pieces of steel using some process that made them truly flat (at an atomic level) and press them together in a perfect vacuum (also having ensure that my smoothed surface never was exposed to oxygen)what would keep the stupid things from randomly starting to share electrons back and forth thus effectively becoming one piece of solid material? Is it just because the faces would not be organized in the neat lattice and thus would be effectively not close enough for the metallic bond to be very strong? That still doesn't quite seem right because if I apply voltage across my two pieces of metal the electrons will have no problem travelling from one face to the other... Even a very small voltage would work... This is driving me nut... Somebody please help!

[studiot]

Several questions at once here.

 

I will try to sort them for you.

 

No metallic bonding is not simply a big communal cloud of electrons.

Yes there is what might be so described but the bonds remain directional and only certain electrons are able to join the party in the commune.

 

Because the bonds are directional the bonds direction (and their valences) have to match to allow alloys - bonding between dissimilar metals.

The bond's directions are reflected in the crystal structures of the metals.

 

Now to you example of the same metal.

If you machine chemically clean (oxide free) surfaces and press them together you can indeed 'weld' this way. Welding is another name for joing crystals.

This is an established technique of cold welding.

 

What else do you want to know?

[swansont]

Some don't do so easily because they gain an oxide layer which protects them. As studiot has noted, you can cold weld material (aka galling); you can wear the oxide layer off and expose the metal underneath. It's a problem in mechanical structures, especially in vacuum systems. You need to use dissimilar metals, or put anti-sieze lubricant on them. I learned this one the hard way.

[duncanstives]

Very interesting... Apparently your right: They do!

 

This is not intuitive and I have worked with many vacuum systems and never seen this phenomenon but of course have never tried to make it happen and any places where two parts are joined in vacuum systems would have some sort of butyl rubber seal rather than metal to.metal contact... You've also explained a phenomenon I just remembered: I have worked with a partial pressure hydrogen firing furnace that uses hydrogen forming gas and high temperatures to removed surface oxides... Often times stainless parts stick together a little after firing... I always wondered about it because the process is certainly below the melting point of the stainless... I bet they're welding together a bit.

Some don't do so easily because they gain an oxide layer which protects them. As studiot has noted, you can cold weld material (aka galling); you can wear the oxide layer off and expose the metal underneath. It's a problem in mechanical structures, especially in vacuum systems. You need to use dissimilar metals, or put anti-sieze lubricant on them. I learned this one the hard way.

Thanks for that link... Explains a lot!

[Enthalpy]

[Galling] You need to use dissimilar metals, or put anti-sieze lubricant on them. I learned this one the hard way.

 

And I learned the hard way that dissimilar metals is not the answer - despite being written and taught universally. My answer presently is that you need at least one non-galling alloy, or better both, and pairing has no effect at all (well, it's not just my answer; two teams investigated galling and both tell the same).

 

I experienced titanium (Ti-Al6V4, not little bit of Cr in it) gall horribly against hard chromium (no mentionable quantity of Ti in it). Which also disproves the hardness theories, as both parts were nicely hard. But you can run bronze against bronze, just fine.

[imatfaal]

Would agree with Enthalpy - Steel pedal shafts will lock into aluminium crank without anti-seize lubricant

[Sensei]

You've also explained a phenomenon I just remembered: I have worked with a partial pressure hydrogen firing furnace that uses hydrogen forming gas and high temperatures to removed surface oxides... Often times stainless parts stick together a little after firing... I always wondered about it because the process is certainly below the melting point of the stainless... I bet they're welding together a bit.

 

I have yet another explanation of such quasi-welding.

When you heat/cool object, its volume is changing, so do little roughness on it, and microscopic mountains on two surfaces jam together.

After cooling down it's extremely hard to disconnect them.

I am fighting with this everyday during distillation when two pieces of glass are nearly impossible to disconnect. Couple times thought glass will be lost.

[Fuzzwood]

Use teflon grease or sleeves.