Welding Shielding Gas Selection Guide

Shielding gas does two things: it protects molten weld metal from atmospheric contamination, and it shapes how the arc behaves. The wrong gas choice causes porosity, excessive spatter, poor bead shape, or wasted money. The right choice depends on the process, base metal, and what you are optimizing for.

MIG Welding Carbon Steel

The three common options for MIG welding carbon steel are 100% CO2, 75/25 Ar/CO2, and 90/10 Ar/CO2.

100% CO2 is the cheapest option. It produces deep penetration and works well on thicker material. The downsides are more spatter, a rougher arc, and a wider, more convex bead profile. Many production shops use straight CO2 when appearance does not matter and they want maximum penetration.

75/25 Ar/CO2 is the most popular mix for general fabrication. It balances penetration with a smoother arc, less spatter, and a flatter bead profile. This is the default recommendation for most hobby and shop welding.

90/10 Ar/CO2 produces the smoothest arc and least spatter but reduces penetration. It is preferred for thin material, automotive panels, and situations where appearance matters. The cost per cubic foot is slightly higher than 75/25.

MIG Welding Stainless Steel

Standard 75/25 Ar/CO2 should not be used for stainless steel. The 25% CO2 causes excessive carbon pickup, which destroys the corrosion resistance of low-carbon grades like 308L and 316L.

Tri-mix (90He/7.5Ar/2.5CO2) is the industry standard for MIG welding stainless. The helium adds heat for better wetting, the small amount of CO2 stabilizes the arc, and the argon provides shielding.

98/2 Ar/CO2 is a simpler alternative that works well for most stainless MIG welding. Easier to source than tri-mix and acceptable for non-critical applications.

TIG Welding

TIG welding almost always uses 100% argon. This applies to carbon steel, stainless steel, aluminum, and most other metals.

The exception is when welding thick aluminum, where an argon/helium mix (75Ar/25He or 50/50) adds heat to improve penetration and travel speed.

Back purging with argon is required when TIG welding stainless pipe or tubing. Without a purge, the root side of the weld oxidizes (sugaring), which destroys corrosion resistance.

Flux-Core and Stick

Self-shielded flux-core wire (E71T-11, E71T-GS, E71T-8) and stick electrodes do not require external shielding gas. Their flux coatings or cores generate their own shielding atmosphere.

Gas-shielded flux-core wire (E71T-1, E81T1-Ni1) requires either 75/25 Ar/CO2 or 100% CO2, depending on the wire manufacturer's recommendation. Check the datasheet for the specific wire you are using.

Gas Flow Rates

For MIG welding, set flow at 25-30 CFH for most indoor work. Increase to 35-40 CFH in drafty conditions. More is not better. Excessive flow creates turbulence that actually pulls air into the shielding envelope.

For TIG welding, set flow at 15-20 CFH for most work. Use a gas lens to extend the laminar flow zone and improve coverage at lower flow rates.

For back purging stainless pipe, flow depends on pipe diameter. A general starting point is 10-15 CFH through the pipe, but you need enough to displace the air volume inside.

Classification system defined by AWS, gas manufacturer technical data.