For most mild steel applications, CO2 will provide adequate shielding, but when you must have a flatter bead profile, less spatter or better wetting action, you may want to consider adding 75-90% argon to your CO2 shielding gas mix. Argon is essentially inert to the molten weld metal and therefore will not react with the molten weld metal. When CO2 is mixed with argon, the reactivity of the gas is reduced and the arc becomes more stable. But, argon is more expensive. In production welding, selecting the perfect shielding gas can be a science of its own. Attributes such as material thickness, welding position, electrode diameter, surface condition, welding procedures and others can affect results.