Aluminium Foil = Aluminium Tube?

[Iceman]

I was just looking at picture of an Alcoa plant, a large Aluminium producer, and I notice the seem to store it in huge rolls of the material.

Anyway, it got me to thinking... When we have a big roll of foil, if we left it long enough, would it continue to be a roll of a thin sheet, or would it eventually just become a solid tube?

Something like Ice seems to eventually stick to other pieces of ice to form larger blocks. What properties determine how this happens, why don't two pieces of the same element, placed on each other just kind of 'merge'?

[gorcee]

Given long enough time in contact, they probably will.

Aluminum is interesting. Aluminum is highly reactive with oxygen; this is why aluminum powder can make an explosion more powerful.

However, when aluminum is in contact with air, it undergoes a process called passivation. Essentially, this forms a thin layer on the surface metal of aluminum oxide. This is why aluminum is so weather resistant: the actual aluminum is rarely in contact with the air.

As a result, rolls of it would probably resist the "contact welding" that could happen with other materials/coatings.

edit: a quick glance at the Wiki page reminds me that aluminum oxide is a good insulator, which would prevent ionic transport between sheets.

[BeerBottle]

Yes, cold welding is when two lumps of the same metal fuse together like you described, because the two crystal lattices merge into one. Of course in everyday situations you'll have noticed that blocks of steel don't stick together when they touch, the reason is that in the everyday world the surfaces of everything is covered in layers of organic carbon, adsorbed water etc, which means the surfaces can't fuse and they can't form a good enough contact. Cleaning the surfaces won't work as at atmospheric pressure the surfaces are contaminated nanoseconds after you clean them. To get cold welding to work on contact you need clean surfaces in a vacuum. There were worries this could be a big problem with spacecraft, but apparently studies have shown even in the vacuum of space you still need to try hard to get optimal conditions for contact cold welding.

Another option is to apply high pressures to force the metals together through their surface layer of organic goo.

What happens with ice is different though - ice blocks typically stick together if the surface has slightly melted then refrozen, forming a bond between two blocks. Alternatively, water from moist air can condense and freeze on the surfaces of the blocks, eventually sticking the together.