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.