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Why can oxygen in the air damage a molten weld?

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Welder in a blue uniform and protective helmet using shielded metal arc welding to join metal pieces, with bright sparks and smoke visible at the welding point. — welding Shielded metal arc welding.

Oxygen in the air is highly detrimental to a molten weld because most metals rapidly oxidize when heated to a molten state. This reaction causes several issues that compromise the integrity of the joint:1

Inhibited Bonding: The formation of an oxide layer on the surface of the metal can prevent proper coalescence and bonding.1
Brittleness: Molten-metal droplets coated with oxide can become entrapped within the weld, which makes the resulting joint brittle.1
Compositional Changes: Valuable alloying elements added to the metal for specific properties may react so quickly upon exposure to air that the final deposited metal loses its intended composition.1

To mitigate these risks, welding processes often employ fluxes to form a protective blanket over the molten material or use inert atmospheres, such as argon, to displace the air and shield the metal from contact with oxygen.1