Step 1: Picture what the welding arc physically is.
The arc struck between the electrode and the workpiece is not empty space, it is an intensely hot column of ionised gas, or plasma, carrying the electric current across the gap between the electrode tip and the molten weld pool.
Step 2: Think about what feeds material into this plasma column.
At the extreme temperatures found in the arc, the tip of the electrode does not just melt, it actually vaporises to some extent, releasing metal vapour directly into the arc column itself, alongside whatever shielding gas or flux derived gases may also be present.
Step 3: See how this vapour protects the weld pool.
This metal vapour physically occupies the space right around the arc and the molten pool, effectively pushing the surrounding air out of the immediate welding zone. So it is this vaporisation of the electrode metal, forming part of the arc plasma, that contributes to shielding the weld pool from the atmosphere.
\[ \boxed{\text{vaporisation of electrode metal}} \]