Oxide
An oxide is an oxygen compound. An oxide forms when oxygen reacts with another material. Most of the time, oxides are, in their physical and chemical behavior, drastically different from that of the base material. On a metal surface, oxides are the largest hindrance for wetting to take place during soldering. Solders react, as a rule, with the base material and not with the oxides. It is for this reason, that before the soldering process oxides have to be removed from the surfaces through he application of a flux.