It is my understanding that metals are a crystal lattice of ions, held together by delocalized electrons, which move freely through the lattice (and conduct electricity, heat, etc.).
If two pieces of the same metal are touched together, why don’t they bond?
It seems to me the delocalized electrons would move from one metal to the other, and extend the bond, holding the two pieces together. If the electrons don’t move freely from one piece to the other, why would this not happen when a current is applied (through the two pieces)?
Asked by jcw in physics.stackexchange
In some cases, they do bond. On Earth, in our humid, oxygen-rich atmosphere, metals are almost always coated with oxides, hydroxides, carbonates, or in the case of noble metals, adsorbed water and other gases, and possibly various organic compounds. These surface coatings are enough to prevent metals from bonding. In outer space, it is possible for clean metal surface to become bonded, and this can be a problem for spacecraft. The phenomenon is known as cold welding.