Posted by Steve Simon on February 6, 2014, 5:51 pm, in reply to "Why do two metal pieces not join when touching each other?"
You can ask the same question about almost any material! If you have two pieces that want to chemically bond (any type of bonding: ionic, covalent, metallic,...), why is it that they don't join immediately when brought together?
Although in principle they should bond, there are many possible reasons it doesn't happen in practice. It is often the case that the problem has to do with the junk on the surface of the material. For example, many materials (sodium, potassium, etc..) form oxide layers the moment they are exposed to air. If this oxide layer is removed the materials can often be "cold welded". It is also a problem that the roughness of the surface makes it hard to make a good bond --- if you imagine putting together two bumpy objects they really only touch at a very few points (unless you smash them together very hard) so they don't bond particularly well.
But in fact similar phenomena can sometimes occur when you really would rather that it did not occur! For example, in machine parts, you sometimes find that parts spontaneously weld themselves together even at low temperature (making a bit of a mess). See http://en.wikipedia.org/wiki/Galling for example.
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