Introduction

One of the most common questions in MIG welding is whether to use pure CO2 or an argon-based mix as your shielding gas. Both work—but they produce different results, and choosing the right one depends on what you’re welding and what quality you need.

This guide compares argon and CO2 for MIG welding, explaining the differences in performance, cost, and application so you can make the right choice for your work.

Quick Comparison

Understanding the Gases

Pure CO2 (100% Carbon Dioxide)

CO2 is a reactive gas, meaning it participates in the welding process rather than simply shielding it. When CO2 breaks down in the arc, it releases oxygen, which reacts with the weld pool.

Characteristics:

• Produces a hotter, more penetrating arc
• Creates more spatter
• Results in a slightly oxidised weld surface
• Less stable arc with more turbulence

Argon/CO2 Mixes

Argon is an inert gas—it doesn’t react with the weld. Mixing argon with a smaller percentage of CO2 gives you benefits of both: the shielding properties of argon with some of the penetration characteristics of CO2.

Common mixes:

• 75/25 (75% Argon, 25% CO2) – Good all-round mix
• 80/20 (80% Argon, 20% CO2) – Popular general-purpose mix
• 90/10 (90% Argon, 10% CO2) – Cleaner welds, less penetration
• 95/5 or 98/2 – For stainless steel applications

When to Use Pure CO2

Pure CO2 makes sense in specific situations:

Thick Mild Steel The deeper penetration of CO2 is an advantage when welding thick sections where you need the arc to dig into the material.

Structural Work Where appearance isn’t critical and you need solid penetration—structural fabrication, heavy equipment repair.

Cost-Sensitive Production CO2 costs significantly less than argon-based mixes. For high-volume production where weld appearance isn’t a priority, the savings add up.

Outdoor Welding CO2’s higher density provides slightly better shielding in windy conditions, though it’s not a complete solution for outdoor work.

When to Use Argon/CO2 Mixes

Argon-based mixes are the better choice for most MIG welding:

Quality-Critical Work Where weld appearance matters—automotive, furniture, architectural metalwork.

Thinner Materials Less heat input reduces the risk of burn-through on thinner sections.

Reduced Spatter Less clean-up time, less wasted filler wire, better overall efficiency.

Smoother Operation A more stable arc is easier to control, making the welding process more predictable and less fatiguing.

Stainless Steel Argon-heavy mixes (typically 98/2 or argon with small oxygen additions) are essential for stainless to prevent contamination and maintain corrosion resistance.

Aluminium Pure argon or argon/helium mixes are used for aluminium—CO2 cannot be used as it would contaminate the weld.

The Real Cost Comparison

CO2 is cheaper per cylinder, but that doesn’t always mean lower overall cost. Consider:

Spatter and Clean-Up More spatter means more time grinding and cleaning. If you’re paying labour, this adds up.

Consumable Waste Increased spatter means more filler wire wasted as spatter rather than deposited as weld metal.

Rework If appearance matters and CO2 welds need additional finishing, the labour cost may exceed the gas saving.

When CO2 Wins on Cost:

• High-volume, low-appearance-requirement work
• Thick structural steel
• Situations where penetration matters more than finish

When Argon Mix Wins on Cost:

• Quality-critical work where rework is expensive
• Thinner materials where burn-through is a risk
• Applications requiring minimal clean-up

Weld Quality Comparison

Penetration

CO2: Deep, aggressive penetration due to higher arc energy Argon Mix: Moderate penetration, more controlled heat input

Bead Profile

CO2: Narrower, more finger-like penetration profile Argon Mix: Broader, more rounded profile with better fusion at the weld toes

Spatter

CO2: Significant spatter, especially at higher currents Argon Mix: Substantially reduced spatter—often 50% less or more

Surface Finish

CO2: Rougher, more oxidised surface Argon Mix: Smoother, cleaner appearance with less oxidation

Mechanical Properties

Both produce sound welds when used correctly. CO2 welds may have slightly higher oxygen content but this rarely affects structural integrity in normal applications.

Choosing the Right Mix Percentage

If you’ve decided on an argon/CO2 mix, which ratio should you use?

75/25 Argon/CO2

• Good all-round mix for mild steel
• Balance of penetration and spatter reduction
• Wide availability

80/20 Argon/CO2

• Slightly cleaner than 75/25
• Popular choice for general fabrication
• Good balance for most workshops

90/10 Argon/CO2

• Lower spatter, smoother arc
• Better for thinner materials
• Slightly less penetration on thick sections

95/5 or 98/2 Argon/CO2

• Very clean welds
• Used for stainless steel (with appropriate filler)
• Minimal spatter

Common Mistakes to Avoid

Using CO2 for Thin Material The intense heat and penetration of CO2 makes burn-through more likely on thin sections. Use an argon mix instead.

Using CO2 for Stainless Steel CO2 causes carbon pickup in stainless steel welds, compromising corrosion resistance. Use high-argon mixes (98/2 or argon/oxygen mixes).

Wrong Flow Rate Whether using CO2 or argon mix, the flow rate matters. Too low = poor shielding and porosity. Too high = turbulence and wasted gas. Typical range: 15-20 L/min.

Ignoring Your Welding Machine’s Recommendations Some machines are optimised for specific gas types. Check your manual for recommended settings.

Making Your Decision

Choose Pure CO2 if:

• You’re welding thick mild steel
• Appearance isn’t critical
• You’re cost-focused on high-volume production
• Deep penetration is your priority

Choose Argon/CO2 Mix if:

• You want cleaner welds with less spatter
• You’re welding thinner materials
• Appearance matters
• You’re welding stainless steel (high-argon mix)
• You want more stable, easier-to-control welding

For most workshops, an 80/20 or 75/25 argon/CO2 mix is the practical choice. It handles the majority of mild steel work with good results, acceptable spatter levels, and reasonable cost.

Pure CO2 has its place, but it’s increasingly a specialist choice rather than a default option.

FAQ

Can I switch between CO2 and argon mix without changing settings? You’ll likely need to adjust your voltage and wire feed speed. CO2 requires slightly different parameters than argon mixes for the same thickness of material.

Which gas is better for beginners? Argon/CO2 mixes. The more stable arc is easier to control, and the reduced spatter makes the learning process less frustrating.

Can I mix my own gases? No. Gas mixing requires specialised equipment to ensure accurate, consistent ratios. Always use pre-mixed gases from a reputable supplier.

Does argon mix work for all positions? Yes. Argon/CO2 mixes work well in all positions. The more stable arc can actually make vertical and overhead welding easier than with pure CO2.

Summary

• Pure CO2 offers deep penetration at lower cost but with more spatter and a rougher finish
• Argon/CO2 mixes provide cleaner welds, less spatter, and easier operation at slightly higher cost
• For most MIG welding of mild steel, 80/20 argon/CO2 is the practical default choice
• Always match your gas to your material—don’t use CO2 on stainless or aluminium

Related Articles:

• MIG Welding Gas Settings: Complete Setup Guide
• Best Gas for Welding Stainless Steel
• Complete Guide to Industrial Gases