difference between inert gas and active gas

Difference Between Inert Gas and Active Gas (Welding Uses and Types)

 In the welding field amongst different forums and groups, this question comes up for debate at times “What is the Difference between Inert and Active Shielding Gas”? Inert noble gases and active gases are both used in shielding gas mixtures for different applications. Gas metal arc welding (GMAW) can use either inert gas (MIG welding) or small portions of active gas (MAG welding) in shielding mixtures.

So what is the difference between inert gas and active gas when welding? Inert gas has no effect on the final weld and merely protects the arc from oxygen and contaminants. Active gas adds a slight electric conductivity and breaks down the weld pool surface tension allowing for a better penetrating weld. Due to their complete valence shells, inert gases have very low reactivity compared to the other gases in the periodic table.

Even though the term “MIG Welding” is used synonymously when “Wire Feed Welding” once active gas is introduced it technically becomes MAG Welding  (Metal Active Gas).

Keep on reading to go over shielding gases in more detail.

For reference the periodic table of elements contains 11 gases. Six of these are noble gases—helium, neon, argon, krypton, xenon, and radon—while five are active gases: nitrogen, hydrogen, oxygen, fluorine, and chlorine.

Shielding Gas

The terms inert gas and active gas can be slightly misleading since even the “inert” noble gases are not completely incapable of reacting with other elements under the right conditions. It is also possible for active gases to form semi-inert compounds like carbon dioxide.

Inert gases are used in various applications where reactivity is an undesirable trait. Active gases might be used for solvents, combustion, or other situations where such reactivity is desired.

In welding, inert gases like argon and helium help protect the weld from more active elements like oxygen and nitrogen that might damage the weld. On the other hand, active gas might be added in small portions to improve arc characteristics or increase penetration.

Shielded Metal Arc Welding (SMAW)

Arc welding was invented in 1885 as carbon arc welding, but this would soon be overtaken in the early 1900s with the invention of coated electrodes. The flux coating on the electrode is composed of carbonates and silicates that release a gas (CO2) that functions as a shielding gas, hence its name shielded metal arc welding (SMAW).

SMAW is colloquially referred to as “stick welding” because of the stiff electrode melted in with the base metal to fuse the joint together. SMAW is one of the simplest and cheapest forms of welding. 

SMAW is conducive to flux-cored welding, where the flux within the electrode wire provides a semi-inert atmosphere of CO2 for the weld. Unlike GMAW, SMAW is well-suited for outdoor use since there is less concern over a breeze altering the weld atmosphere.

The use of the semi-inert or active carbon dioxide gas makes these processes a form of active gas welding.

Gas Metal Arc Welding (GMAW)

In contrast to the flux used in SMAW, other welding methods like GMAW and GTAW pump the gas into the weld area. Gas metal arc welding (GMAW) uses a consumable electrode wire that requires the shielding gas to prevent chemical reactions like oxidation from occurring in welds. 

GMAW was invented in 1948 for welding non-ferrous metals like aluminum with helium or argon, but it was soon used to weld steel. Since then, GMAW has become the most common welding process for industrial applications. 

GMAW has two main subgroups, metal inert gas (MIG) welding and metal active gas (MAG) welding, and four main transfer modes: globular, short-circuit, spray, and pulse-spray (source).

A globular transfer is the least expensive and tends to use more active gases like CO2. The other variants rely more on inert gas, especially spray and pulse-spray. Because of its dependence on gas flow, GMAW is largely conducted indoors.

Metal Inert Gas (MIG) Welding

Metal inert gas (MIG) welding typically uses inert gases like argon or helium to prevent the contamination of weld pools by gases like oxygen or nitrogen. Once active gases are added, the welding method is technically no longer MIG welding.

Metal Active Gas (MAG) Welding

Metal active gas (MAG) welding is a form of GMAW that uses some level of non-inert or semi-inert gas. Semi-inert or active gases can be mixed with inert shielding gases in very small percentages to lower price or increase penetration without sacrificing weld quality on the right material (source).

Tungsten Inert Gas (TIG) Welding

Another form of inert gas welding is tungsten inert gas (TIG) welding, which is almost exclusively intended for use with inert gas. TIG welding is also referred to as GTAW (gas tungsten arc welding) since it also requires the use of an electric arc like GMAW and SMAW.

TIG welding uses a tungsten electrode, which will quickly be consumed by more active gases like oxygen or CO2. For this reason, inert gases are almost always used with the exception of hydrogen for nickel or austenitic stainless steel.

The Noble and Inert Gases

The six noble gases that occur naturally are helium, neon, argon, krypton, xenon, and radon. Noble gases have full valence shells, which is what makes them so stable and gives them their inert properties.

Noble gases are monatomic with only weak interatomic or forces like van der Waals forces. Van der Waals forces are the weakest of all bonds behind covalent bonds and powerful ionic bonds. They are caused by minor polarity shifts resulting from the shifting density of electrons

Most of the noble gases, except radon, can be extracted through fractional distillation, which is also used to separate lighter-than-air natural gas and heavier petroleum products like gasoline and diesel fuel

Neon, xenon, and krypton are rare gases, and radon is radioactive. The remaining gases argon and helium are the main inert gases used in shielding gases for gas metal arc welding and gas tungsten arc welding.

Shielding gases is composed of 100 percent inert gas are best used on non-ferrous metals like copper and aluminum. For welding steels, some mixture with an active gas is preferred to either increase weld penetration or help stabilize the arc.

Argon (Ar)

Argon is one of the most abundant gases in the earth’s atmosphere, which makes it cheap compared to other noble gases like helium. The name argon is derived from the Greek word for lazy or inactive, “αργος” (source). 

Most argon is produced by the radioactive decay of potassium-40, a factor that makes it useful in potassium-argon dating.

Argon is preferred for most applications due to its cheaper price range compared to helium. Argon also produces a very stable arc with a much lower current than helium, even if it results in lower penetration of the metal.

Helium (He)

Helium was first noted in 1868 during a solar eclipse and derives its name from the Greek sun-god and Titan Helios as a result.

The element helium was first discovered on earth in uranium ore. While it is one of the most abundant elements in the universe, it is actually relatively rare on earth, making it more expensive than argon.

The United States became the world’s largest supplier of helium and even held a monopoly for a time. One of the first major uses of helium was in balloons during World War I. Along with CO2, helium was one of the first gases used in arc welding.

Noble gases all have low melting points and boiling points, but helium has the lowest melting point of all the elements. Helium is also the only element to have no viscosity, a property known as superfluidity. Helium is second only to neon in its resistance to chemical reactions.

Helium has a high thermal conductivity and burns much hotter than argon. Helium also requires a higher gas flow rate, although it does result in a less porous weld. Due to its higher price and heat generation, helium is primarily used on metals with a high thermal conductivity like copper or aluminum.

Another downside to helium is that its very low density means that it diffuses much faster than denser argon. Helium used to be used as a preservative for such important documents as the Declaration of Independence until it was discovered that it leaked out of the case very quickly. It was then replaced with argon.

Is an Inert Gas the Same as a Noble Gas?

In the past, the terms noble gas and inert gas were largely interchangeable. Now that compounds have been formed out of noble gases, the term inert gas depends more on context. However, under most normal circumstances, a noble gas will act as an inert gas. 

When noble gases form compounds, they are technically no longer inert by definition. Xenon is the most reactive of the noble gases, and the first noble gas compound was formed using xenon. Neon, on the other hand, is the least reactive element of the periodic table and has no known compounds.

Under normal circumstances, Argon will not form a bond even with fluorine, the most electronegative element. A stable compound between the two, argon fluorohydride (HArF), was not formed until 2000 (source).

What is Active Gas?

The remaining active gases include hydrogen, oxygen, nitrogen, fluorine, and chlorine. In direct contrast to the inert gases, active gases do not have a complete valence shell. This allows them to be far more reactive with other substances.

While inert gases in welding prevent unwanted chemical reactions from occurring in welds, active gases are used in small percentages to increase heat, penetration, to control arc, and lower costs.

Welding applications for stainless steel, in particular, tend to use a small percentage of an active gas to aid with weld penetration. On its own, argon will not generate enough heat to produce sufficient penetration, while helium on its own will have a less stable arc.

Oxygen (O)

Oxygen has two unpaired electrons and is one of the most reactive elements. In fact, oxygen has the second-highest electronegativity of all the elements, behind only fluorine.

Inert gases like argon and helium are frequently used to prevent the intrusion of oxygen because of its high reactivity. Oxygen draws out the electrons of other atoms through the process of oxidation, and combustion, breathing, and rust are all forms of oxidation.

Non-inert oxygen is mixed with argon in percentages up to five percent for welding stainless steel by automated welders in assembly lines. Oxygen was first used with the spray-arc transfer method in the 1960s. The low levels of oxygen cause the mix to burn hotter and increase weld penetration for thick metals while stabilizing the arc.

The obvious downside is that the risk of oxidation and rust becomes greater in low carbon steels as the oxygen level increases. The oxygen can even corrode the electrode if deoxidizers are not used.

Carbon Dioxide (CO2)

When oxygen is mixed with carbon to form carbon dioxide (CO2), the result is a semi-inert gas that can be combined with an inert gas for a more cost-effective shielding gas. CO2 was first used in GMAW in 1953 to help weld steel at a lower cost.

In addition to being relatively cheap, CO2 also results in deeper weld penetration. Unfortunately, carbon dioxide does cause more spatter around the weld, and it requires a higher voltage setting than argon.

There are several shielding gas mixtures that contain different levels of CO2 ranging from 100 percent to 2 percent depending on its intended use. These mixes are referred to by their CO2 percentage; for example, C100 or C2.

The most common mixture that covers the widest number of applications is a mixture of 75 percent argon to 25 percent CO2 called C25. The low cost and balance of properties make this mix a favorite of hobbyists.

There are also tri-mixes that contain some combination of helium, argon, carbon dioxide, or oxygen for use on stainless steel.

Hydrogen (H)

Hydrogen is the most abundant element in the universe and among its most reactive; one needs only consider the hydrogen bomb as an example. Hydrogen is flammable and reacts violently with oxygen, and hydrocarbons are a major component of fossil fuels.

Like oxygen, the active gas hydrogen can be mixed with argon or other gases in small percentages, under 10 percent, to weld stainless steel. Higher percentages of hydrogen up to 25 percent can be used on nonferrous metals.

Companies like Hydrostar produce hydrogen and argon blends for TIG welding nickel or austenitic stainless steel. The added hydrogen increases the heat of the weld while also allowing for higher weld speed. Hydrogen is also cheap while creating a clean weld area (source).

However, using hydrogen in any application other than austenitic stainless steel will cause the metal to be brittle. Austenitic steels are non-magnetic and contain either nickel or high levels of magnesium and nitrogen.

Nitrogen (N)

Nitrogen is another abundant element found on earth. Nitrogen is neither flammable nor

toxic, and it is found in all living organisms

Nitrogen is an interesting case since, as an element, it is not a noble gas. However, nitrogen is found in nature as a diatomic substance (N2) where the three unpaired electrons of its elemental form are paired with one another in its molecular form (source).

In its molecular form, nitrogen is semi-inert, and its abundance makes it cheaper than fully inert gases like argon.

Nitrogen functions as a semi-inert gas at low temperatures and is used in GMAW on austenitic stainless steel. However, at higher temperatures over 1832°F (1000°C), nitrogen forms nitrous oxides, which can be hazardous to the welder.

Nitrogen is used in low percentages of 5 percent and in some tri-mixes with argon and CO2. Like CO2, nitrogen aids with arc stability and weld penetration.

The Halogen Gases Chlorine (Cl) and Fluorine (F)

Halogens are active nonmetals, but they are only found as compounds in nature. Two of the halogens, chlorine, and fluorine, are gases at room temperature and are both highly reactive as well as toxic in this form.

The most reactive element of the periodic table is fluorine. If steel wool comes into contact with fluorine, it will catch fire. Both chlorine and fluorine only have one unpaired electron, and fluorine has the highest electronegativity of all the elements, followed by oxygen and chlorine (source).

Both form salts and fluoride salts are used in drinking water in very low amounts. Fluoride is actually necessary in small amounts for bone strength in mammals. This is why it’s also used in dental products (source).

Fluorine has a long history of metallurgy. Fluorine received its name from the Romans who used it as a flux to aid the flow of molten metal. The name comes from the Latin word meaning “to flow.”

As for chlorine, some fluxes used for soldering contain ammonium chloride or zinc chloride.

Experiments have been conducted using chlorine and fluorine in shielding gas at two percent or lower. (source). The compounds sulfur hexafluoride (SF6) and Dichlorodifluoromethane (CF2CI2) have both been used as shielding gases (source).

Sulfur hexafluoride can be used to shield molten metal alloys of magnesium and aluminum when casting (source).

Final Thoughts

The main distinguishing factor between inert gases and active gases is the closed valence shells of inert elements which makes them highly resistant to chemical reactions.

Active gases are more reactive, although oxygen and nitrogen can form semi-inert molecules. Nitrogen does this with other nitrogen atoms, while oxygen combined with carbon can form CO2.

Which gas mixture you use as a shielding gas will depend on your particular project and the equipment and materials you have on hand. High levels of expensive inert gas won’t be necessary for smaller hobby projects where CO2 may do just fine.

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