Why do metals need to be cleaned before soldering ? Is flux always necessary?

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Would like to understand the reasons from a science point of view

In: Chemistry

Metals usually get an oxide layer that acts as preventive layer to avoid further corrosion. The oxide layer is supposed to be removed to expose pure metal beneath for welding (soldering?)

Flux isn’t always necessary, as some conditions don’t require anything to protect metal. Some examples would include welding in space, since there’s no oxygen to protect the metal from, or welding gold as gold does not oxidize. It might still be a good idea to clean the surfaces before welding, however, since oils and dust can still cause issues.