The main purpose of the flux in FCAW (flux core) or SMAW (stick) is to decompose into CO2 gas during combustion and shield the molten weld puddle from the oxygen and nitrogen contamination of the weld puddle. Run a GMAW (MIG) bead with no shielding gas and the result is a weld bead more pocked marked then the surface of the moon. Secondly the decomposed flux (slag) protects the cooling weld bead from reacting with O or N. It also helps the weld bead cool at a slower rate which prevents brittleness. Any decomposed flux (slag) in the weld bead is called "inclusions" and are considered weld defects. The primary result of O or N contamination is voids or holes in the weld bead which reduce strength. The flux is not there to clean weld surfaces as it would be in a noncolesing process such as soldering or brazing. Paint, rust and other contaminates are removed by the welding heat as the arc is 10000 degrees and the weld puddle is close to 3000 degrees. Different types of electrodes will have various materials added to the metal alloy and flux to aid in contaminate removal. Some electrodes have vary little added and depend on clean base metal. Anything added would be considered a contaminate in its self. I did a quick check at the American Welding Society (AWS) and a welding engineering text (Blodgett) and when dealing with mild steel (90% of all steels) flux(slag) inclusions are not a major corrosion concern. The voids and porosity of the slag is a concern because the resulting voids allow other corrosive contamiates in. That said in certain enviroments elements exist that will combine with oxides in the slag and promote corosion. Aggresive cleaning is always reccommended.

O and N are reactive gases which readily combine with other elements and the heat generated during the welding process aids this process. Noble gases such as argon and helium are inert and are also used as welding shield gases. CO2 is reactive, but in such a way that it aids in penetration and considered positive in many welding processes. A small amount (2%) of O is sometimes added to gas mixtures used to weld stainless steel.

The body of knowledge is huge and there are just about as many misconceptions as truths.

Welding is all about HEAT, too much and you get holes. Too little heat and you get no penetration and no strength.

Jim

Jim

Good luck with your project!