Many experts recommend to use self-shielded Flux-core arc welding wires in DCEN (Direct current electro negative) mode. Reason behind this recommendation is physics of attraction and repulsion.
Air contains nitrogen and oxygen, both of which will react with iron, especially at high temperatures. As N2 and O2 are both “nonmetals” they tend to form negative ions.
If the melting wire is kept negative (DCEN), it repels the ionized oxygen and nitrogen from the wire, and attracts these negative ions to the weld puddle. So now *wire* is safe from being oxidized, but by this the weld puddle becomes the target of the oxygen and nitrogen. Unlike the wire, the puddle is (somewhat) protected by the layer of molten flux. The flux contains oxygen and nitrogen absorbers and it will also acts to slow the speed at which O and N can diffuse into the metal itself. Thus weld pool remains safe and sound welding is achieved.
If the wire is kept positive (DCEP), oxygen and nitrogen would gobble up the outer steel wire, before the flux inside had a chance to protect it, which will result in poor weld quality.
Keep reading, happy welding