MIG Welding with 100% CO2 Shielding Gas

Ever wonder why some welders prefer MIG welding with 100% CO2 shielding gas instead of using a blend of argon and CO2 that’s designed for MIG?

Used to shield the weld from contamination, CO2 is plain-old carbon dioxide. It’s the same stuff used to extinguish fires, carbonate your beer and soda, and keep them cold.

The major advantage to MIG welding with CO2 is its low operating cost. But there are added benefits, along with a few downsides.

While it’s not for everyone, in the right situation straight CO2 is an excellent choice for a MIG shielding gas.

MIG Welding with 100% CO2 Gas Cover Image

Why is MIG welding with 100% CO2 gas popular?

Lower cost

Yes, the low cost of operation is the leading reason for MIG welding with straight CO2, also called C100. But how big is the difference when compared to a 75% Argon/25% CO2 blend? Also known as C25, it’s considered the best all-around gas blend for MIG welding on mild steels.

Here’s a chart comparing two popular sizes side-by-side. Keep in mind that C25 gas is sold by the cubic foot (cf), and you’ll buy pure CO2 by the pound. Mostly liquid while under pressure, each pound yields 8.741 cf of gas.

Specs/Use Cost – C100 vs. C25 Shielding Gas

20# CO280CF C25
H x W (in.)26 x 835 x 7
Full Wgt.45 lbs.56 lbs.
Volume174.8 cf80 cf
Time@20cfh8.8 hrs.4.0 hrs.
Initial Cost$150$315
Initial $/hr$17.25$79
Refill Cost$35$75
Refill $/hr$4$18.75

The bottom line on operating cost shows that a C25 MIG mix is around 4.5 times more costly to weld with than a straight C100 gas.

Improved portability

Besides the lower cost of 100% CO2, did you notice the difference in size? Besides providing double the welding time, a full 20 pound CO2 tank is a smaller package than the 80 cf C25. 

If portability is important to you, pure CO2 has the advantage over C25.

Hobart designed a 20-ounce CO2 kit to offer the ultimate in portability to a few of its machines. While this kit is no longer available, 20-ounce containers used for paintball guns are. These provide about 40 minutes of welding time in a very portable package.

You can choose from many other sizes to fit your needs.

Read all about: Welding Cylinder Sizes and Welding Time

More refill options

Right behind cost and portability, convenience is a big plus. You can always refill pure CO2 at your local welding supplier. But you gain more options with this gas, many with convenient evening and weekend hours (I know that’s when I run out of gas). 

  • Local home brew shops
  • Sporting goods stores that sell paintball equipment (Dick’s is one)
  • Fire extinguisher service companies
  • Aquarium suppliers

Performance

CO2 gas is inert—as is Argon—but only at low temperatures. In welding’s heat, CO2 becomes reactive and has more “bite” than argon blends. This characteristic results in both benefits and negatives. Later, we’ll get into how it welds, but for now, here are the top advantages to MIG welding with 100% CO2:

  • Better joint penetration. This helps smaller machines handle thicker metal.
  • Capable of higher welding speeds (gun travel).
  • More aggressive cleaning action that cuts through mill scale and rust.

What’s needed to MIG Weld with 100% CO2?

You’ll need a tank or two of gas in your preferred size. You won’t notice any difference between using industrial grade (99.5%) or food grade (99.9%) gas, so choose based on price and convenience.

Let’s look at a few more considerations before you jump in.

MIG welder

I guess it’s really MAG (Metal Active Gas) welding, but any true MIG welder designed to handle shielding gas will work. 

Without Argon, pure CO2 is less forgiving with a smaller sweet-spot. You’ll have more success hitting the mark using a machine with fully adjustable voltage and wire feed speed (WFS) controls, instead of fixed positions.

If you’re in the market for a welder to use with CO2, also look for adjustable inductance control like on this Lincoln. Also called slope control, this helps smooth out the arc.

100% CO2 compatible regulator

Under certain conditions CO2 can frost, or even freeze, a gas regulator. High workplace temperature and humidity levels, along with high flow rates, can contribute to this condition. 

Miller MIG welding regulator

Freezing can damage some regulators designed for Argon blends. This Miller (Hobart) regulator marked “ARGON” is not recommended for use with 100% CO2. 

So, you’ll want to check your regulator or owner’s manual before running straight CO2 through it.

When shopping for a regulator, make sure it’s adjustable for flow (CFH). Many were designed for beverage dispensers and are set by pressure (PSI). Here’s an inexpensive flowmeter style that I like. It’s calibrated for both CO2 and Argon.

CO2 tank adapter

Welding regulators and cylinders connect using a CGA-580 style fitting, while CO2 uses the CGA-320 style. A CGA-320 to CGA-580 adapter allows mounting a welding regulator on a CO2 tank. A nylon washer seal is important to prevent leaks and freeze-ups.

Consumables

Your standard MIG gun consumables will work great with carbon dioxide shielding gas. 

Because it’s an oxidizing gas, pure CO2 can cut through minor surface contamination (rust and mill scale) better than C25. But oxidation can lead to weld porosity, so choose a MIG welding wire with strong deoxidizer additives like this solid ER70S-6

Running a dual-shielded process is another option, as you’ll find a few flux-cored, gas shielded wires designed to run with CO2.

What’s it like to MIG weld with 100% CO2 shielding gas?

Arc characteristics

OK, let’s get into the major downside of MIG welding with C100, and that’s poor arc quality

When added to Argon welding gas in small amounts of 5 to 25%, CO2 helps to stabilize the welding arc. But It’s less conductive of electricity than Argon, so at higher levels the arc becomes more erratic with a coarse, raspy quality, and the amount of spatter increases.

When in the sweet-spot, the arc is only slightly more erratic with a touch more spatter than C25. This zone is narrow with C100 and harder to dial in. Keeping the arc length short will help to reduce spatter. 

Carbon dioxide produces short-circuiting transfer at low current levels and globular transfer at the higher current levels. It cannot achieve spray transfer mode.

It takes some getting used to, but with patience, experience, and a fully adjustable machine you can get solid, consistent results.

Setup parameters

Most manufacturers provide suggested settings for welding with 100% CO2, but in general for a given WFS you’ll need more voltage. With a narrower sweet-spot, expect to spend more time fine-tuning your settings.

MIG welding with 100% CO2 parameters chart.

When comparing C100 to C25, you can see that Hobart recommends a little more voltage, or a little less WFS, for the Handler 140.

If your machine has adjustable inductance or slope, increasing this value helps to settle and smooth out the arc.

Weld appearance

The weld bead profile is typically wider with deeper penetration. While helpful on thicker materials, this strong penetration characteristic becomes harder to control when welding on thin metal, such as auto body panels.

You can increase gun travel speed to reduce penetration, while reducing welding time. 

As mentioned earlier, spatter can become an issue. Now, we’re not talking about a flux core-type mess, just some extra cleanup if the finished appearance is important.

100% C02 (C100) vs C25 MIG welding bead profile

Why do CO2 gas regulators frost up?

CO2 gas absorbs a lot of heat because of the pressure drop through the regulator. This can lead to regulator and flow meter frost or freeze-up at higher flow rates—usually above 25 cfh—especially in warm, humid conditions.

How low can I run a CO2 gas cylinder?

It’s recommended to swap out your working cylinder when pressure drops to 200 psi. Maintaining positive pressure prevents moisture and other contaminants from entering the cylinder.

Does MIG welding with CO2 gas add carbon to my weld?

Because CO2 is made of one carbon and two oxygen atoms, it’s possible for extra carbon to be added to the weld. This isn’t a problem for mild steels, but can reduce ductility of some low-alloy steels.

The big picture

C25 is a premium MIG welding gas that comes at a premium price. But, in most situations, it won’t perform C100 by anywhere near the cost difference. 

Yes, when welding with pure CO2 you give up overall arc smoothness and control on thin materials. But you gain penetration and speed. Again, in most situations, C100 is a great value.

For most hobby welders, what shielding gas you use is not as important as how much time you get to weld and improve your skills. When MIG welding with 100% CO2 gas you can afford to weld a lot more, while giving up very little in exchange.

Dave Jones
 

Dave began welding to repair equipment used in his small business. Now as a hobby, he enjoys researching, testing and writing on welding topics. Other interests include photography, RV'ing and just about anything to do with dogs—especially retrievers.