For most mild steel applications, CO2 will provide ample shielding, but if you need a flatter bead profile, less spatter, or better-wet motion, you may want to add 80 to 95 percent of Argon to your CO2 shielding gas mix. Argon is inert to the molten welding metal and thus will not react to the molten welding metal. When CO2 is mixed with Argon, the gas's reactivity decreases, and the arc becomes more stable. But Argon is more expensive than that. Choosing the best shielding gas may be a science of its own in the manufacture of welding. Results can be affected by characteristics such as material thickness, welding position, electrode diameter, surface condition, welding techniques, etc.