Flux in Arc Welding: What It Is, Types, and How to Choose

Your first flux core weld looks strange: a hollow wire, a harsh arc sound, and a bead wrapped in a glassy crust you have to chip off before laying the next pass. No gas bottle, no shielding blanket, yet the molten metal comes out bright instead of oxidized brown. The reason is a powder mixture packed inside the wire, and the same basic chemistry lives in every flux-coated stick electrode and every granular SAW flux, doing four jobs at once: making shielding gas, forming a slag lid, cleaning the weld pool, and feeding it alloying elements.

What Flux in Arc Welding Is (and Why Arc Welding Needs a Shield)

It’s a blustery morning on a structural steel jobsite. Gusts of wind are whipping across an open frame, making it nearly impossible to keep any kind of loose cover over the joint. The foreman glances over to the new welder, pulls a spool of hollow wire off of the rack, and hands it over instead of a box of stick electrodes. The reasoning is simple: with this wire, they won’t need an external shielding gas bottle to get a sound weld in these conditions. The secret behind that ability is flux.

At its core, according to the CWB Group, flux is a mixture of minerals, chemicals, and alloying materials that primarily protects the molten weld metal from contamination by oxygen, nitrogen, and other atmospheric contaminants. However, its job goes far beyond just blocking the air. A properly formulated flux carries out four core functions all at the same time: shielding the weld pool from the atmosphere, deoxidation and cleaning by reacting with unwanted oxides, slag formation and bead control to shape and insulate the weld, and alloying and mechanical property control by adding elements directly into the weld metal.

With all of these duties in mind, flux is only utilized in exactly three arc welding processes: Shielded Metal Arc Welding (SMAW), which uses a flux-coated electrode, Flux Cored Arc Welding (FCAW), which uses a flux-filled tubular wire, and Submerged Arc Welding (SAW), which uses a separate, granular flux. For readers looking to get a broader overview of how these fit amongst others, our guide on the most common welding processes is a great place to start. That being said, for the purposes of this article, the focus will remain strictly on the flux itself.

Why Molten Steel Needs Protection

To understand why flux is so vital, it helps to look at the physics of the arc. When steel reaches welding temperatures, it becomes incredibly reactive and readily absorbs gases from the surrounding air. If left unprotected, nitrogen will be picked up into the molten pool, leading to brittleness and porosity. At the same time, oxygen will combine with elements in the steel to form oxides. These oxide inclusions act as crack initiation sites and drastically degrade the mechanical properties of the finished weld. Simply put, without some form of shielding, an exposed, molten weld pool has no chance of remaining sound.

There are three ways to combat this contamination. The first is external gas shielding, seen in processes such as Gas Metal Arc Welding (GMAW) and Gas Tungsten Arc Welding (GTAW), where an inert or semi-inert gas is supplied from a cylinder to blanket the arc. The second is flux-generated gas plus slag, the method used by self-shielded Flux Cored Arc Welding. Lastly, there is flux-generated gas plus external gas, known as gas-shielded Flux Cored Arc Welding, or FCAW-G. These hybrid setups pair the benefits of a flux core with the added protection of an external shielding gas, a topic that is covered more in depth by our dual-shield FCAW explainer.

This is also where the two classes of FCAW become important. FCAW-S (Self-Shielded) relies entirely on the flux inside of the wire to generate its shielding gas and slag, making it ideal for outdoor, windy jobs such as the one on our jobsite. FCAW-G (Gas-Shielded) utilizes both the flux and an externally supplied gas for protection. It’s worth noting that these classes often call for different polarity conventions, something that will always need to be verified against the manufacturer’s data sheet for the wire being used. In fact, FCAW as a whole was developed in the 1950s as a way to raise productivity over SMAW whilst keeping much of the portability that flux shielding provides.

A Word on Fume Exposure

While flux is integral to creating a sound weld, its chemistry also has major implications for the welder’s air. The National Institute for Occupational Safety and Health (NIOSH) states that an exposure limit for total welding emissions cannot be established because the composition of welding fumes and gases varies for different welding processes, base metals, filler metals, fluxes, and coatings. The components can also interact with one another, creating a vastly different fume profile from one job to the next. For this reason, ventilation can never be an afterthought when working with flux-based processes.

All in all, flux isn’t just a “cover” for the weld. It’s the chemistry engine behind SMAW, FCAW, and SAW. By generating its own protective atmosphere, cleaning the pool, forming a slag lid, and feeding in alloying elements, it allows sound, strong welds to be made in places where external gas shielding simply wouldn’t be practical. These are the foundational roles that every flux family, from coated electrodes all the way down to granular SAW flux, is built upon.

The Chemistry Inside Welding Flux: What Is Actually in the Powder

Welding flux looks like simple powder, but it is a precisely balanced blend of minerals, oxides, halides, and alloys. Every ingredient has a job, and once it is combined with the right binder or manufacturing process, it becomes the chemistry engine that makes the arc behave and the weld metal come out right. Break down what goes into the powder and how it is formed, and it becomes much easier to see why one consumable runs like silk while another builds a weld tough enough for critical service.

The Core Ingredients and Their Jobs

No two flux formulations are identical, but most share a common family of constituents:

• Silica (SiO₂) and Alumina (Al₂O₃): Common in acid-based fluxes, these oxides are key slag formers. They create a fluid, glassy slag that covers and insulates the weld bead.
• Titania (TiO₂): The staple of rutile formulations, titania stabilizes the arc, improves bead wetting, and produces a smooth, easily removable slag. That is a big part of why rutile wires feel so welder-friendly.
• Lime/Calcium Oxide (CaO), Magnesia (MgO), and Iron Oxide (FeO): These adjust slag viscosity and basicity. In basic fluxes, lime and magnesia are crucial to driving impurities out of the weld metal.
• Calcium Carbonate (CaCO₃): Heated, it decomposes and releases carbon dioxide. That reaction is one source of the shielding gas cloud around the arc.
• Fluorides (such as Calcium Fluoride, CaF₂): Most prominent in basic fluxes, they lower slag melting temperatures, keep slag fluid at lower heat inputs, and play a large role in deoxidizing the weld pool.
• Sodium and Potassium Compounds: Powerful arc stabilizers found in flux coatings and in some flux cores. They help strike and hold a smooth, consistent arc.
• Deoxidizers (Silicon and Manganese) and Alloying Additions: Flux does more than shield the puddle. These additions scavenge oxygen, tie up unwanted elements, and can be tailored for specific strength or toughness requirements.

The Three Jobs of Flux Chemistry

Whatever the recipe, flux chemistry works toward three core metallurgical jobs:

1. Forming the shielding gas: As the flux reaches arc temperatures, compounds such as calcium carbonate decompose and release gas. That shroud displaces oxygen and nitrogen from the atmosphere, protecting the molten weld pool.
2. Forming the slag: The oxide and fluoride components melt into a molten blanket. This lid insulates the cooling bead, slows the cooling rate, helps control bead shape, and traps impurities before they solidify into the brittle crust you chip off.
3. Metallurgically conditioning the weld metal: The most overlooked job. Deoxidizers, fluorides, and alloying elements interact directly with the pool, cleaning out oxides, controlling the level of non-metallic inclusions, and adding alloy content. That directly impacts the strength and toughness of the finished weld.

Basicity: The Key to Toughness

One of the most useful ways to classify flux is by basicity index. As noted by CWB Group, acid fluxes fall below 0.9, neutral fluxes sit between 0.9 and 1.2, and basic fluxes run above 1.2.

The logic behind the scale is straightforward: as basicity rises, the oxygen content of the weld metal falls, and with it the inclusion level. Fewer inclusions mean greater impact toughness. That is why basic fluxes are relied on for critical, low-temperature service, while acid and rutile fluxes prioritize arc smoothness and positional ease over that level of metallurgical refinement.

How Flux is Manufactured For Each Process

The chemistry may be similar, but the way flux reaches the arc changes completely depending on the process it was built for.

Shielded Metal Arc Welding (SMAW): Extruded Coatings

For stick electrodes, the dry mix of minerals, fluorides, deoxidizers, and alloys is combined with a sodium and potassium silicate binder into a thick paste. That coating is extruded onto a bare steel core wire and baked. This single coating must do everything: supply the shielding gas, form the slag, and carry the alloying additions into the weld. Within SMAW, CWB identifies three broad coating families: cellulosic (such as E4211), rutile (such as E4913), and basic (such as E4918). Of the basic types, CWB notes that the EXX18, EXX28, and EXX16 classifications are the ones classified as low hydrogen.

Flux Cored Arc Welding (FCAW): Roll Formed Cores

For flux cored wire, flux takes a tubular form. A thin metal strip is rolled into a U shape, the blended flux compound is loaded into the channel, and a second set of rolls closes it into a tight tube. According to CWB, there are three basic manufacturing types for the cross-section of flux cored wire: butt, folded, and overlap. The seam style directly affects wire consistency, moisture resistance, and feedability through the liner. These formulations show up across many positional consumables, including those featured in an all-position welding rod family.

Submerged Arc Welding (SAW): Fused Granular Flux

SAW uses no coating or core at all. Instead, a separate granular flux is fed from a hopper. As stated by CWB, SAW flux is produced by melting proportioned materials such as silica sand, metal oxides, and halide salts at roughly 1,500 to 1,700 degrees Celsius. The molten mix is then chilled, ground, and screened to a specific particle size, and the arc runs fully submerged beneath the blanket. Unused flux can be recovered, sieved, and reused, a trait unique to this process. CWB also notes that basic SAW fluxes are typically built from calcium, magnesium, sodium, potassium, and manganese oxides, plus calcium carbonate and calcium fluoride, while silica and alumina form the bulk of acid-based SAW fluxes.

Every grain, oxide, and fluoride in a flux is there for a reason. The binder or the manufacturing method is simply the vessel that delivers it. Once you understand those building blocks, predicting why one flux favors a silky, out-of-position arc and another is built to meet the strictest toughness requirements becomes far easier.

The Main Flux Families and How Each One Behaves

Ask any fabricator to name their favorite electrode and you will get an opinion, not a spec. The stick that lights instantly and lays a fat, flat bead but fills the booth with smoke. The wire that runs quiet and flat until you tip it overhead. The basic wire the inspector insisted on, with its arching, spitting arc and a slag that runs ahead of the puddle. Those personalities come from the flux, and only four families cover nearly everything you will ever buy: cellulosic, rutile (titania), basic (lime), and metal-cored or powder types.

Cellulosic: the hot, forgiving, smoky one

Cellulosic electrodes and wires use an organic binder, usually a cellulose compound, that holds the coating together and drives nearly everything about how they weld.

The arc strikes almost effortlessly and runs hot and fast, which gives cellulosics a strong, penetrating arc profile. Beads come out smooth and slightly convex with good wetting into the joint. They are genuinely all-position capable, which is why E6010 and E6011 have survived for decades in vertical and overhead structural work, shipbuilding, and pipe fitting.

The catch is the fume. Cellulosic consumables produce more fume and more spatter than the other families, and the organic binder is a moisture source, meaning higher diffusible hydrogen in the deposit. That is why many codes restrict them for critical, fatigue-loaded, or pressure-retaining service. A good tool in the right hands, not the automatic choice.

Rutile: the welder-friendly default

Rutile coatings are built around titanium oxide, and they are the most common family in both stick and flux cored welding. EWI reports that rutile flux wires hold a smooth arc with good operational stability over a wide range of welding current for a given wire diameter. In plain terms, the welder gets a much wider window of acceptable settings, so more people make acceptable welds on the same wire.

Rutile also handles position better than the alternatives. The same EWI guidance notes excellent out-of-position welding capability, a lower risk of incomplete fusion defects, good out-of-position bead shape, and less fume than basic flux wires. Beads are moderately convex with good profile, slag releases readily, and spatter stays manageable.

Typical uses: structural steel erection, fillet welding, general fabrication, shipbuilding, and semi-automatic FCAW on mostly flat and horizontal joints. For a new process or a mixed-skill crew, rutile is the safest default. The SMAW stick welding guide goes deeper on consumables and positions.

Basic: the metallurgy winner

Basic, or lime, fluxes are built for weld metal properties rather than operator comfort. EWI reports that basic flux wires produce welds with good low-temperature impact toughness down to extreme temperatures such as minus 100 degrees Celsius, with diffusible hydrogen in the deposit below 5 mL per 100 g. That is why basic consumables are specified on high-strength steels and anywhere hydrogen-assisted cracking is a concern.

The trade-off is real. The solidifying slag of basic flux wires provides less support for the weld pool, which makes them less operable out of position, and they are known to have a more erratic arc. Expect a spattery, arching sound, a flatter bead, and dense slag that can feel sticky to remove. Fume runs higher than rutile. Penetration is deeper: a bonus on joint prep, a curse on thin material.

Typical uses: pressure vessels, offshore and shipbuilding critical seams, pipeline root and fill passes, low-temperature service, and high-strength or HSLA steels.

Metal-cored and powder types

These wires put metal powder in the tube with very little active flux. The result is near-100 percent deposition efficiency, minimal slag, low fume, and a smooth, attractive bead, but they need external shielding gas and usually a separate gun or torch arrangement. They shine in high-deposition automated and robotic welding where slag removal would be the bottleneck. Note that AWS A5.20/A5.20M states that metal cored carbon steel electrodes are classified according to AWS A5.18/A5.18M rather than A5.20, so look for the classification on the right standard.

How gas-shielded and self-shielded FCAW split

For gas-shielded flux cored wire, CWB confirms the fluxes are made up of two main types, rutile or titania and lime or basic, with basic selected when improved weld metal properties are required. Self-shielded wires need a more complex recipe, because they must generate their own shielding gas with no external supply.

Flux family	Arc behavior	Bead & penetration	Positions	Fume & spatter	Diffusible H	Typical uses
Cellulosic	Easy strike, hot and fast	Smooth convex, deep penetration	Excellent, incl. overhead	Highest	Highest	Structural fab, shipbuilding, pipe fitting
Rutile (titania)	Smooth, stable over wide current range	Moderate convex, moderate penetration	Best out-of-position	Low to moderate	Low to moderate	General fab, fillets, semi-auto FCAW
Basic (lime)	Erratic, arching, less stable	Flat bead, deep penetration, dense slag	Poorest out of position	Higher fume, moderate spatter	Under 5 mL/100 g	Pressure vessels, offshore, pipeline, HSLA, low-temp service
Metal-cored / powder	Quiet, very stable	Smooth flat bead, little slag	Machine and robotic work	Low fume, minimal spatter	Low	Automated and robotic welding, high deposition fillets

One last caution. Bead shape, penetration, and spatter shift with position and current, so a wire that lays a forgiving flat bead can behave very differently vertical-up or overhead. Confirm against the manufacturer’s data sheet before committing a crew to a consumable.

Where Flux Lives: Electrode Coatings, Wire Cores, and Granular SAW Flux

Not all flux is delivered the same way. The physical format of the flux determines how much slag you’ll be chipping, how visible your arc is, how much fume you’ll be breathing, and even how you store your consumables. In practice, there are only three places flux calls home: the extruded coating on a stick electrode for Shielded Metal Arc Welding (SMAW), the tubular core inside of a flux cored wire for Flux Cored Arc Welding (FCAW), and the loose, granular blanket used for Submerged Arc Welding (SAW).

SMAW: The Extruded Electrode Coating

With stick welding, the flux isn’t a separate powder you add to the joint, it’s baked directly onto the electrode. In SMAW, the flux ingredients are mixed with sodium and potassium silicate to create a binder, and the mixture is added to the bare electrode by extrusion. Once this coating is dried, it becomes the workhorse for the weld.

As the electrode burns, that coating decomposes to supply both shielding gas and slag. The gas blankets the molten pool from the atmosphere, while the slag floats on top of the bead to insulate it as it cools. This all-in-one design is what allows SMAW to be so portable, however, with that convenience comes the need for frequent rod changes, more spatter in some coatings, and a decent amount of post-weld cleanup.

FCAW: The Tubular Wire Core

For semi-automatic work, the flux gets packed inside of a hollow wire. In FCAW, the flux is added to a metal strip that has been formed into a U shape by rollers, then closed by a second U-shaped roll to make the finished tubular wire. This roll-formed construction allows for a large amount of flux chemistry to travel down the gun with every inch of wire.

This design comes with a big trade-off in productivity. Flux cored wire allows for much higher wire feed speeds and deposition rates relative to stick, making it a go-to for heavy fabrication. With that said, FCAW leaves a slag cover that must be chipped or ground away between passes, and a weld finished with slag left in place will not meet structural inspection expectations.

Not every flux cored wire is created equal either. CWB distinguishes FCAW-GS wires, which require an external shielding gas to help protect the molten metal, from FCAW-SS self-shielding wires. Additionally, self-shielded FCAW is typically run on direct current electrode negative, while most gas-shielded flux core welding is run on direct current electrode positive. For those looking to get their setup dialed in, the WeldGallery guide on welding with a flux core wire welder, as well as the self-shielded FCAW article go more in-depth on these process differences. Gas-shielded flux core wires are also available in diameters ranging from roughly 0.035 inch to 7/64 inch, giving a massive range of deposition for different joint sizes.

SAW: The Separate Granular Blanket

Lastly, there’s Submerged Arc Welding, where the flux is completely separate from the electrode. In SAW, the solid wire electrode and a separate granular flux are both consumed; the arc is created under the granular flux, and unmelted flux can be recovered and reused.

A hopper feeds this dry, granular blanket over the joint, completely submerging the arc out of sight. This is why SAW is so well known for its incredibly smooth beads, minimal spatter, and extremely high deposition rates. The downside is portability. With a hopper, power supply, and wire feeder all involved, SAW is almost exclusively a shop or heavy shop-floor process.

Format Versus Function: The Key Practical Differences

While all three formats use flux for the same core jobs, they behave drastically different on the job. SMAW grants the most freedom with no gas hose, but has the highest electrode changeover. FCAW strikes a middle ground with great deposition and all-position capability for its wires, however, it requires strict slag removal between passes. SAW offers unmatched bead quality and cleanup simplicity in flat or horizontal work, but sacrifices arc visibility and mobility. Across the board, fume levels can be significant for FCAW and SMAW, so ventilation should never be overlooked.

The Aluminum Exception

One hard limit is worth knowing before reaching for a flux cored spool. According to AWS Welding Digest, flux core wire for aluminum does not exist in a practical, commercially available form, and MIG with a spool gun or TIG is the better choice for aluminum. This isn’t an equipment preference, it’s a materials limitation, with no widely adopted aluminum flux core formulation on the market at this time.

How Flux Chemistry Lands in the Weld Metal

Flux is the chemistry engine behind three arc welding processes. Stopping at “it makes gas and slag” misses the point. What matters is what ends up in the finished weld, where code checks and failure analysis happen.

Arc heat decomposes the flux, releasing a shielding gas that blankets the arc and pool. The mineral fraction melts and floats up as slag. A third path opens: some constituents dissolve straight into the molten pool, carrying deoxidizers and alloying elements into the weld metal. That path decides whether a weld passes a Charpy impact test at minus 40 degrees Celsius or fails in a brittle snap.

Deoxidation and the Cost of Left-Over Oxides

Molten steel at welding temperature dissolves oxygen, nitrogen, and hydrogen greedily. Left alone, that oxygen reacts with silicon and aluminum to form oxides that never float out, leaving residual inclusions in the weld metal like stress risers at grain boundaries. Flux constituents such as silicon and manganese act as deoxidizers, combining with oxygen so the resulting oxides migrate into the slag. Complete deoxidation gives a dense, tough deposit; incomplete deoxidation gives oxides, inclusions, cracking risk, and less impact energy.

Basicity: The Number That Predicts Weld Cleanliness

Welders call them “basic” and “acid” fluxes because the ratio of basic to acidic oxides in the powder changes what ends up in the deposit: basic fluxes lean on lime and magnesia, acid fluxes on silica and titania. The CWB states that increasing basicity lowers oxygen content, and therefore inclusion level, in the weld metal, which in turn increases toughness. More basic means cleaner metal, and cleaner metal means better notch toughness and crack resistance. That is the whole argument for basic wire, and it is metallurgical, not marketing.

Hydrogen and the Cracking Triangle

Hydrogen ruins welds quietly. It enters from moisture in the flux, moisture on the base metal, rust and oil, and sometimes the shielding gas supply. As the weld metal cools it becomes mobile, collects at grain boundaries, and combines with a hard, restrained microstructure to produce hydrogen-assisted cold cracking.

The cracking triangle is high restraint, high hydrogen, and high hardness. Remove any one and the risk drops sharply, which is why low-hydrogen basic fluxes are specified on higher-strength steels. At a base metal minimum yield of 80 ksi, hydrogen cracking becomes a serious concern, and filler metal is chosen as much for diffusible hydrogen level as for tensile strength.

According to EWI, the two reasons to start an electrode search within basic flux based wires are excellent low-temperature impact toughness and a low diffusible hydrogen level. Basic wires can deliver impact toughness down to about minus 100 degrees Celsius with diffusible hydrogen below 5 mL per 100 g of deposit.

Rutile wins on practicality. EWI explains that rutile flux wires are the better starting point for semi-automatic FCAW where welder friendliness matters and much of the welding is out-of-position: less fume, a smoother arc, a wider operating window, and a predictable bead overhead. You trade toughness and hydrogen control for a welder who can hold the arc.

The Practical Consequences You Can See on the Bench

The same wire behaves very differently in flat than overhead. A rutile wire that lays a forgiving flat bead can throw spatter, narrow the bead, and fight the welder once rotated overhead. The chemistry has not changed, gravity and position have. If a job is 70 percent out-of-position, that changes which flux family you reach for first. Basic fluxes give a fluid, easily cleaned slag but a less supportive one, making out-of-position work harder.

Why the Strongest Wire Is Not the Right Answer

A thicker electrode does not automatically produce a better weld. Over-matching filler strength to base metal creates strength mismatch problems and raises hydrogen cracking risk, since higher-strength deposits cool harder. Base metal grade and required mechanical properties drive the choice, not the tensile number on the box.

AWS A5.20/A5.20M:2025 classifies carbon steel flux cored electrodes by weld metal mechanical properties and prescribes chemical composition, mechanical property, and usability characteristic requirements. It revised the strength requirement for many classifications from 390 MPa to 400 MPa to match the 58 ksi minimum yield strength, and adds optional supplemental designators for lower temperature toughness, diffusible hydrogen limits, and shielding gas range. The numbers on the spool are a contract with the code.

The Bottom Line

Flux chemistry decides oxygen level, hydrogen level, inclusions, slag behaviour, bead shape, and airborne fumes. Basic buys toughness and low hydrogen but a less forgiving arc; rutile buys arc stability and weldability but less low-temperature margin. Match that trade-off to the base metal grade, service temperature, position, and the applicable code, then verify against the data sheet and the governing WPS. The current standard explains the classification; the WPS tells you how to weld it.

Reading the Codes: What AWS Classifications Tell You About Flux

When you pick up a spool of flux cored wire or a box of stick electrodes, the string of letters and numbers can look like a foreign language. These designators do tell you a great deal about the filler metal, but they are not a shortcut to a weld procedure. To read them correctly, you need to know which American Welding Society (AWS) specification applies to the flux-containing consumable in your hands.

The Main AWS Specifications to Know

With flux playing a role in so many processes, the AWS splits its filler metal specifications to cover each delivery format:

• AWS A5.20/A5.20M:2025 – Covers carbon steel flux cored electrodes for use with or without shielding gas. Approved by ANSI on July 29, 2025, this is the 7th edition revising A5.20/A5.20M:2021. According to AWS A5.20/A5.20M:2025, it prescribes classification requirements and includes optional supplemental designators for lower temperature toughness, diffusible hydrogen limits, and shielding gas range designators. Per section 1.1 of the same standard, metal cored carbon steel electrodes are classified under A5.18, not A5.20.
• AWS A5.1/A5.1M:2025 – The specification that governs carbon steel electrodes for shielded metal arc welding, more commonly known as flux-coated stick electrodes.
• AWS A5.36/A5.36M – Introduces a new open classification system for carbon and low-alloy steel flux cored and metal cored electrodes. This open system does not preclude classification of electrodes under A5.20, A5.29, A5.18, or A5.28.
• AWS A5.18/A5.18M – Covers metal cored carbon steel electrodes and rods for gas shielded arc welding.
• AWS A5.17/A5.17M:2025 – Covers carbon steel electrodes and fluxes for submerged arc welding.

It’s also worth noting that both AWS A5.20/A5.20M:2025 and its ANSI web listing state that these specifications are written with U.S. customary units and SI units that are not equivalent, and must be used independently.

Cracking the FCAW Classification

The easiest way to learn is by example. Take the very common E71T-1C classification found on many gas-shielded flux cored spools:

• E – Electrode, designating it as a filler metal.
• 71 – The minimum tensile strength, in ksi. For this class, 71,000 psi.
• T – Tubular, indicating the electrode is flux-cored.
• 1 – Denotes the intended use and position capability for this class under its governing specification.
• C – Tied to shielding gas compatibility for that classification.

These exact definitions can change between specifications, especially with the newer open system under A5.36. With that said, always confirm the breakdown against the current standard and the manufacturer’s data sheet before assuming any of the above applies to your wire.

The SMAW Equivalent

For flux-coated stick electrodes, you’ll typically see the E60xx or E70xx series. The digits after the tensile strength indicate the coating type, welding position, and current characteristics. Most importantly for flux-related weld quality, a final suffix of 8 denotes a low hydrogen electrode (such as E7018).

Where the Real Requirements Live

One of the biggest points of confusion is qualification. Under AWS D1.1 joint design provisions, FCAW is not classified with F-numbers – F-numbers belong solely to SMAW electrodes. A classification will never take the place of a qualified procedure. The real requirements always live within the WPS, the manufacturer’s data sheet, and the governing code for the job, such as AWS D1.1, AWS D1.3, ASME Section IX, or API 1104.

Choosing the Right Flux for the Job: A Practical Decision Path

Flux chemistry gets you only halfway to a purchase. The shop-floor question is never “which flux is best” in the abstract, it is “which flux survives this job.” A pipeline crew welding 3/8 inch carbon steel in January, a fab shop running stainless, and a crew patching a vessel inside a tank all use flux cored wire, and each ends up on a different spool.

The decision unwinds in a predictable order. Work through it top to bottom and most of the ambiguity disappears before you open a box.

1. Environment — indoor shop, field work, wind, confined space
2. Position — flat and horizontal versus vertical and overhead
3. Base material and thickness
4. Required mechanical properties and service temperature
5. Hydrogen limits
6. Applicable code
7. Cost and availability

Environment Comes First

Location changes the answer more than any other variable. The AWS Welding Digest puts it plainly: flux cored arc welding handles dirty surfaces better, works outdoors in windy conditions, and penetrates deeper on thicker steel than MIG. That is the argument for self-shielded wire in the field, where the shielding gas of an FCAW-G setup gets torn away and mill scale, rust, and shop grime are the normal condition of the joint.

Indoors the trade flips. Self-shielded wires tend to produce more smoke and spatter, while gas-shielded wires are easier to control and produce more aesthetically pleasing welds in a shop environment. With gas, extraction, and clean steel, FCAW-G gives a tidier bead and a less punishing shift. The default: self-shielded outdoors in wind and on dirty steel, gas-shielded indoors where consistent beads and lower fume matter.

Position Narrows the Flux Family

Ask how much of the job is out of position. Vertical and overhead welds punish a basic flux, because basic slags are heavier, less fluid, and give the pool poor support when it is not sitting on a flat plate. Rutile slags flow and release cleanly in those positions. If nearly all of your tonnage is flat and horizontal, that constraint loosens and a basic wire becomes viable almost everywhere.

Material and Thickness Set the Process

UTI describes flux cored arc welding as a good technique for materials no thinner than about 20 gauge, including carbon steel, low-alloy steels, high-nickel alloys, cast iron, and stainless steels. Below that gauge the wire is fatter than the plate and the heat has nowhere to go, so switch to MIG or TIG. Aluminum is the other hard stop: no practical, commercially available aluminum flux core wire exists, so a spool gun or TIG is the route. Our look at aluminum flux core welding shows what that dead end looks like in practice.

The Rutile Versus Basic Decision

Once you know the service temperature and any hydrogen limits, the flux family decision makes itself. The EWI guidance states the rule: begin the electrode search within rutile flux based wires when the application is semi-automatic FCAW where welder friendliness is a concern, or involves a large percentage of out-of-position welds; begin within basic flux based wires when the application requires excellent low-temperature impact toughness or a low diffusible hydrogen level.

Read that as a default, not a law. Welder friendliness means a stable arc, easy slag release, and a bead that does not punish a tired operator. Low-temperature toughness and hydrogen control are non-negotiable on pressure equipment, offshore structures, and anything under a code that caps diffusible hydrogen. When both point the same way, choose rutile. When the requirement is a hard number, choose basic.

Strength Class Has to Match the Base Metal

With the family settled, match classification strength to the plate. E70XX covers most structural carbon steel; E71XX and E80XX come in when the base metal or design load demands more. The minimum yield requirements behind those classifications live in AWS A5.20/A5.20M:2025. Pairing a stronger electrode with weaker plate is not an upgrade, it is a mismatch, and mismatched strength classifications are a code problem rather than a quality win.

Wire Diameter and Starting Amperage

Wire diameter	Typical starting amperage band	Where it usually fits
0.045 in	about 180 to 300 A	Light sheet and thin structural fillet work
0.062 in	about 250 to 400 A	General fabrication, plate up to about 3/8 in
0.075 in	about 300 to 500 A	Heavier plate, structural and pipe work
0.135 in	about 400 to 600 A	Plate 3/4 in and thicker, production and shipbuilding

Diameter drives the usable current window on the data sheet. Below the range the arc starves; above it you get stubbing and spatter. Bigger wire also means fewer passes per joint and a heavier slag to chip, so choose the smallest diameter that still carries your required amperage.

Code Before Cost

Before you compare prices, check the code. The WPS, the manufacturer’s data sheet, and the governing standard decide what is legal. AWS D1.1, D1.3, ASME Section IX, and API 1104 each carry their own expectations, and a classification on the spool never replaces a qualified procedure.

Cost and Throughput

Cost often favors FCAW more than people expect. UTI notes FCAW has the highest deposition rate of any welding method, so more filler metal goes into the joint per unit time. Add the higher wire feed speed, the larger bead per pass, and fewer slag-chipping breaks, and labor savings often outweigh consumable cost. The comparison against GMAW is closer: MIG deposits fast and needs no chipping, but the shielding gas and the machine add cost. FCAW-G wires are generally less expensive per pound than FCAW-S wires and run from about 0.035 inch up to 7/64 inch, so the indoor choice is often the cheaper consumable too. See our coverage of the advantages of stick welding against MIG, and size your power supply against the wire rather than the other way around with our welding equipment guide.

Safety: Flux Fumes, Fluorides, and Hexavalent Chromium

When working with flux, the biggest hazard isn’t slag or spatter. It’s what you’re breathing. Because flux is the fume source for three common arc welding processes, fume control has to be treated as its own, non-negotiable part of the job. From stainless steel fabrication to field repairs, the same chemistry that protects your weld can produce some of the most toxic airborne hazards in the shop.

The Order Of Fume Production

Not every arc creates the same amount of fume. According to OSHA’s fact sheet FS-3647 on controlling hazardous fumes and gases during welding, arc welding processes rank in order of decreasing fume production: Flux Core Arc Welding (FCAW), then Shielded Metal Arc Welding (SMAW), then Gas Metal Arc Welding (GMAW).

That ranking is tied directly to the flux. Decomposing a flux coating or core generates the bulk of the particulate in FCAW and SMAW, while GMAW relies mainly on external shielding gas. The process alone never sets the dose, though. Base and filler metal, rod composition, job location, work practices, air movement and your ventilation controls all move the numbers.

Hexavalent Chromium: A Hidden Threat

Chromium is one of the more concerning byproducts of welding. It shows up in stainless steel, nonferrous alloys, chromate coatings and some consumables. Arc temperatures oxidize it into its hexavalent state, Cr(VI), which becomes part of the fume and is a known human health hazard.

OSHA regulates exposure under 29 CFR 1910.1026 and 1926.1126. The permissible exposure limit for hexavalent chromium is 5 micrograms per cubic meter as an 8-hour time-weighted average, with an action level of 2.5 micrograms per cubic meter. OSHA’s compliance directive also notes that stainless steels generally contain 12 to 30 percent chromium, so any work on these materials carries elevated Cr(VI) risk.

Ventilation, Flux Chemistry, and Real World Exposure

Ventilation is the most effective control, but the data shows how hard this hazard is to tame. According to NIOSH’s engineering controls database, total welding fume concentrations ranged from 2 to 60 mg/m3 without local exhaust ventilation. With ventilation in place, they dropped to 3 to 13 mg/m3.

Those numbers remain concerning, and CPWR research presented to NIOSH supports that. 83 percent of personal breathing zone air samples exceeded the proposed hexavalent chromium REL, and 90 percent of those still exceeded it with local exhaust ventilation running. The leading cause? Hoods placed 2 to 3 feet from the weld, when several inches is far better.

This is where flux chemistry comes in. In OSHA’s final hexavalent chromium rule, experiments showed that eliminating or reducing sodium and potassium in the flux cuts hexavalent chromium production in the fume. That change is not yet commercially widespread because of possible weld quality concerns, a clear trade-off between fume toxicity and weldability.

Practical Controls For The Welder

With FCAW producing the most fume of the common arc processes, engineering controls come first, ahead of respiratory protection. Where possible, use extraction guns designed for FCAW and GMAW. CPWR research confirms that balancing extraction airflow against shielding gas requirements is achievable for both processes, so you can capture fume without sacrificing gas coverage.

Engineering controls mean capturing fume at the source, not diluting it. General dilution ventilation can help in an open bay, but it does little in the corners and pockets where fume collects, and it does nothing for a job inside a booth or on a vessel.

With a fume arm, placement is everything. Keep the hood as close to the plume as you can, within several inches, while respecting your gun angle and travel speed. Jobs that can’t be controlled adequately require respirators, but respirators never substitute for ventilation. Shops chasing strict air quality standards can review approved extraction equipment in these airgas welding equipment guides.

Finally, watch the fluorides. Basic fluxes that use fluorides are excellent for low-hydrogen, tough welds, and they carry their own hazards. NIOSH requires that containers of fluoride-containing flux and filler metals carry ventilation warnings. Heed the labels. Most welding particles are fine enough to reach deep into the lungs, so fluoride fume can do more than irritate your throat and eyes. For a wider view of shop hazards, read up on the welder work environment hazards to complete your safety plan.

Troubleshooting: When the Flux Misbehaves

With flux doing so much work, it’s no surprise that when something is off, the weld will let you know. Most issues don’t come from the machine itself, but from moisture, technique, or mismatched parameters. The key is to catch them early before they turn into costly repairs.

Porosity and Wormholes

Porosity is one of the most common headaches with flux-based processes. According to AWS Welding Digest, porosity in FCAW is usually caused by contaminated base metal, damp wire, or an improper gun angle. The fix starts with a thorough surface cleaning, proper wire storage, and maintaining a steady 10 to 20 degree drag angle. On top of that, flux specific culprits such as a damp or contaminated spool, storage in a hot truck or shed, or flux that has absorbed moisture will introduce gas pockets that can grow into wormholes and hollow, wormy beads. These defects are a leading cause for the issues covered within our guide to all-position welding, as position only amplifies them if the wire is already compromised.

Lack of Fusion, Slag Inclusion, and Bead Shape

Not every crater is caused by gas. Sometimes, the weld simply isn’t tying in. AWS Welding Digest states that lack of fusion in FCAW is caused by travel speed that is too fast or voltage that is too low. With the natural slag production of flux core, technique is just as important. As noted by UTI, it is best to drag, or pull away from the completed weld rather than push, as pushing traps flux in the molten metal. Additionally, UTI stresses that welders must remove slag between each pass when using flux cored arc welding. Failing to chip this glassy layer is the number one route to slag inclusion and poor bead shape. For out of position work, this becomes especially tedious, so keeping consistent technique is vital, a concept that ties into the practices found within our breakdown of the advantages of stick welding.

Spatter, Burnback, and Arc Stubbing

Excessive spatter is almost always a parameter issue. AWS Welding Digest identifies it as being caused by too much voltage or incorrect stickout, and recommends keeping stickout between 3/4 inch and 1 1/4 inch to combat it. For burnback, the same source states it is caused by stickout that is too short or wire feed speed that is too slow. For arc stubbing and an erratic arc, these are heavily flux-related, specifically for self-shielded wires. Cold wire, damp wire, excessive drag angle, and low voltage are all common culprits, with an arc that is too long for the voltage exacerbating every one of them. According to CWB, self-shielding flux cored wires produce more smoke and spatter than gas-shielded wires and are more suited to portable field work, so some spatter should be expected, however excessive amounts almost always point to one of the fixes above.

Hydrogen Cracking and Fume Considerations

By far the most dangerous defect is hydrogen cracking, otherwise known as hydrogen-assisted cold cracking. This occurs when hydrogen from a moisture laden flux enters the weld pool, meets a highly restrained joint, and cools down to a hardened microstructure. This is the exact reason to avoid any form of moisture, as a single damp spool can supply enough hydrogen to put a high strength joint at risk. When selecting for low hydrogen work, keep in mind that CWB notes rutile flux wires produce less fume than basic flux wires, however basic fluxes are still the go-to when low diffusible hydrogen and low temperature toughness are non-negotiable. Be aware that this improved metallurgy does come with its own tradeoffs of an erratic arc.

Prevention is the Best Repair

Before chasing parameters, get into these habits: chip slag between passes, keep your gun consistent, verify polarity matches your wire, and check your machine’s volts and amps calibration. Couple these with disciplined storage for all flux consumables and you’ll prevent the vast majority of flux misbehaves before they ever begin. Treat your flux as a sensitive ingredient, not a shell, and it’ll reward you with a clean, sound weld every time.

Conclusion: What to Remember About Flux

If you only remember one thing about flux, make it this: it is the chemistry engine of three arc welding processes, and it does far more than cover the weld. Flux decides the gas that shields the pool, the slag that shapes and insulates the bead, the deoxidation and alloy transfer that set mechanical properties, and the fume you breathe for every arc hour. Change the powder inside the wire and you change the weld, the work habits, and the air quality in the bay.

That is why “which wire do I grab” is never really a wire question. It is a chemistry question, and the answers live in a few predictable places.

The decision path, in order

1. Environment first. Wind, rain, dust, and a job site with no gas hookup push you toward self-shielded flux core wire. A clean indoor shop with good extraction and a gas supply pushes you toward gas-shielded FCAW, where you get a brighter, cleaner bead and less fume.
2. Position next. Vertical and overhead work favors rutile and titania types. They hold a stable arc and give you the best out-of-position bead of any flux family. Basic fluxes weld beautifully flat and horizontal but need more operator skill overhead.
3. Then the mechanical property requirement. Service temperature, impact toughness, and hydrogen limits decide basic versus rutile. Low-temperature toughness and diffusible hydrogen limits point to basic. General fabrication with easy welding favors rutile.
4. Then the code. AWS D1.1, D1.3, ASME Section IX, API 1104, and the governing job specification decide what is actually permitted, qualified, and inspectable.
5. Cost and availability last. Wire price per pound means little if you spend the day chipping slag or fighting porosity.

Keep your standards current

Two shortcuts will save you time. First, the acid, neutral, and basic families that CWB lays out using the basicity index let you reason quickly about expected toughness and slag behavior, even when a brand name means nothing to you. Second, remember that AWS A5.36/A5.36M uses an open classification system, so new flux core and metal cored products can be classified without waiting for a fresh fixed classification to appear in A5.20. The practical implication is simple: check the current supplement for any product you have not welded before instead of trusting your memory of an old catalog.

Safety is not a checkbox

Worth repeating because it is the part most often skipped: OSHA ranks arc welding types by fume production, and flux core arc welding produces the most fume, ahead of shielded metal arc and gas metal arc welding. Ventilation is not optional, and it is not a fan in the corner. Extraction positioned close to the arc, a fume arm on the gun, and a respirator when the controls are not enough are the baseline. If you are welding aluminum, remember that practical flux core wire for aluminum does not exist, so route that work through MIG or TIG instead, using a guide such as aluminum flux welding: how it works and when to use it to understand the differences.

Go deeper when the job demands it

• Process-level detail on SMAW stick welding and the shielding differences between self-shielded and gas-shielded flux core wire.
• The chemistry of granular submerged arc welding flux and how the arc works under the blanket.
• Defect diagnosis, including the porosity explainer and the wormhole defect article, both of which trace back to moisture and contamination in flux.

Flux is one of the few places in welding where a small change in chemistry produces a large, measurable change in the finished weld. Get the basics right, and the rest of the job gets easier.

Frequently Asked Questions

What is flux in arc welding?

Flux is a mixture of minerals, chemicals, and alloying materials that is placed around or inside the consumable to protect molten weld metal from oxygen, nitrogen, and other atmospheric contaminants. It is used in three arc welding processes: SMAW (coating on the electrode), FCAW (core inside the tubular wire), and SAW (granular flux over the joint). It does far more than shield, forming slag, cleaning the pool, and adding alloying elements along the way.

Is flux the same thing as shielding gas?

No, they solve the same problem in different ways. Shielding gas is an external blanket of argon or carbon dioxide blown over the weld, while flux generates its own shielding gas from inside the consumable and also leaves a slag cover behind. Gas-shielded flux cored welding, or FCAW-G, uses both at once, which is why it produces cleaner, more consistent welds than self-shielded wire.

What is the difference between rutile and basic flux?

Rutile (titania) fluxes are the welder-friendly option: wide operating window, stable arc, the best out-of-position behavior, and less fume. Basic (lime) fluxes deliver the better mechanical properties, with low-temperature impact toughness down to about minus 100 degrees Celsius and diffusible hydrogen under 5 mL per 100 g of deposit, but they run a more erratic arc and are harder to hold out of position.

Do I need flux to weld with MIG?

No. GMAW, commonly called MIG, uses a solid wire with no flux at all, and the shielding comes entirely from the external gas. Flux only enters the picture once you move to flux cored wire, flux-coated stick electrodes, or submerged arc welding. If you are setting up a new machine, choosing whether to run flux core capability is one of the decisions covered in our guide to choosing a welding machine.

Can you weld aluminum with flux core wire?

Effectively, no. There is no practical, commercially available aluminum flux core wire, so aluminum jobs need MIG with a spool gun or TIG with suitable filler. Flux core remains one of the strongest choices for carbon steel, low-alloy steel, and many stainless and nickel applications, and it handles dirty or contaminated steel that would defeat a gas-shielded process.

Why does my flux core weld have wormholes or hollow beads?

Wormholes usually mean too much shielding gas or the wrong polarity for the wire you are running. On a gas-shielded wire, check that your flow rate matches the wire and the nozzle distance, and confirm you are on the polarity the wire manufacturer specifies. If you are running self-shielded wire, it should not be connected to a gas supply at all.

Is flux core welding safe to do indoors?

It can be, but it needs ventilation. OSHA lists flux core arc welding as the arc welding process with the highest fume production, ahead of stick and MIG. Use general ventilation plus local exhaust or a fume extraction gun positioned close to the arc, and do not weld in a confined space without proper ventilation, because shielding gases displace oxygen and carbon monoxide can form.

How do I read a classification like E71T-1C?

The leading E means electrode, the digits give the minimum tensile strength of the weld metal in units of 10,000 psi, T identifies a tubular (flux cored) electrode, and the digit that follows indicates the intended use and position capability. The trailing letter relates to shielding gas compatibility, with C associated with carbon dioxide and M with argon and carbon dioxide mixtures. Always confirm the exact meaning against the current AWS A5.20 standard and the manufacturer’s data sheet before you weld to it.

Which flux prevents hydrogen cracking in thick, restrained steel?

Basic (low hydrogen) fluxes and wires, because they limit the diffusible hydrogen picked up by the weld metal. Basic wires are commonly specified on higher-strength steels where hydrogen-assisted cold cracking is a real risk. Keep the consumables dry, control heat input, and follow the preheat and joint geometry in your welding procedure specification.

Do I have to store flux core wire in a special way?

Yes, and it is one of the cheapest ways to prevent porosity. Flux absorbs moisture, and damp wire is one of the leading causes of gas problems in the weld. Keep spools sealed in plastic bags with desiccants, out of hot trucks and unheated sheds, rotate stock so older spools are used first, and clean off rust, oil, paint, and mill scale from the base metal before you start.