Shielding Gas for Stainless Steel Welding (Australia)

Stainless steel needs different shielding gas than mild steel — and the wrong choice degrades the properties you paid extra for. This guide covers what to use and why.

The Core Rule

CO2 degrades stainless steel's corrosion resistance. The chromium in stainless forms a passive oxide layer that gives it "stainless" properties. CO2 in the shielding gas reacts with chromium at weld temperatures, reducing the chromium available for corrosion protection.

Practical limit: Keep CO2 content under 5% for stainless MIG welding. Standard argon/CO2 mixes (12-18% CO2) are too aggressive for stainless.

What to Use

Pure argon (99.997%). Same as TIG on any material. No exceptions.

The industry standard in Australia: BOC Stainshield (or equivalent) — typically a tri-mix of helium, argon, and a small percentage of CO2. The helium adds thermal input for better fusion on stainless's lower thermal conductivity.

Why Stainless Is Different

Stainless steel conducts heat more slowly than mild steel (~30% of mild steel's thermal conductivity for 304). This means:

The right shielding gas helps manage these issues:

• Heat concentrates in the weld zone
• Distortion is more likely
• The heat-affected zone (HAZ) is narrower but hotter
• Carbide precipitation (sensitisation) occurs if the HZX sits in the 450-850°C range too long
• Helium additions increase heat input (useful for thicker sections)
• Low CO2 preserves corrosion resistance
• Proper flow rate prevents nitrogen pickup (which causes porosity)

Australian Suppliers

For stainless-specific gas mixes in Australia:

See our live gas price tracker for current pricing on argon (used as the base for most stainless mixes).

• BOC Stainshield — available in D, E, G sizes
• Air Liquide — stainless-specific mixes (quote-only)
• Supagas — may have stainless mixes available (quote-only)
• Coregas — check Bunnings Trade N Go for stainless mix availability

Common Mistakes

Using standard argon/CO2 mix (12-18% CO2). This is the most common error. The weld will hold, but corrosion resistance in the haz is compromised. On food-grade or marine stainless, this causes premature rusting.

Not enough gas flow. Stainless is more sensitive to nitrogen pickup than mild steel. Bump flow to 12-15 L/min (vs 10-12 for mild steel) to ensure adequate coverage.

Welding too hot. Excessive heat input causes sensitisation — chromium carbides form at grain boundaries, depleting the chromium that provides corrosion resistance. Use the minimum amperage that achieves fusion.

Not cleaning properly. Stainless needs cleaner preparation than mild steel. Brush with stainless-only brushes (never use a brush that's touched carbon steel — it embeds iron particles that rust).

For Food-Grade and Marine Applications

For 316 stainless in food processing or marine environments:

Back-purging — flowing argon on the underside of the weld — prevents sugaring (oxidation) on the root. For food-grade pipe work, this is non-negotiable.

• Use the highest-purity gas available (pure argon or Ar/2% O2)
• Minimise heat input (pulse MIG if available)
• Clean with stainless-specific pickling paste after welding
• Consider adding nitrogen to the shielding gas for the root pass (back-purging)

Frequently Asked Questions

Can I use regular argon/CO2 mix on stainless?

You can, but you shouldn't. CO2 above 5% degrades corrosion resistance. For non-critical applications, it works. For food-grade, marine, or structural stainless, use the right gas.

Is stainless MIG gas more expensive?

Tri-mixes and specialty blends cost 10-20% more than standard argon/CO2 mix. The price premium is small compared to the cost of replacing corroded stainless.

Do I need to back-purge stainless tube welds?

For food-grade and pressure applications, yes. For general fabrication, it improves weld quality but isn't always mandatory.

What's the difference between 304 and 316 stainless for welding?

316 contains molybdenum for enhanced corrosion resistance (especially against chlorides). Welding-wise, they use the same shielding gas. 316 is specified for marine and chemical environments.