Electrodes are an essential component of arc welding. Each method utilizes its own unique type of electrode to achieve the arc necessary for fusion. Because of this, electrodes can look very different between types of arc welding. Within each electrode family are different variations specific to certain applications. For example, some electrodes are coated—each unique chemical mixture bringing different benefits to the table.
Coated electrodes are used with stick welding, or Shielded Metal Arc Welding (SMAW). The coating on welding electrodes is important for a few reasons. It provides more control over the arc, helps protect the molten metal from impurities, and allows for cleaner, stronger welds.
Knowing why welding electrodes are coated, as well as the specific benefits to choosing one type of coating over another, can help you choose the electrode that will work best for your SMAW project.
What Are Welding Electrodes?
Welding electrodes serve as conductors of the electrical arc that enables fusion. If there’s no arc, there’s no weld pool. No weld pool, no fusion. All arc welding requires some form of an electrode.
There are two main types of electrodes: consumable and non-consumable.
Used for Gas Metal Arc Welding (GMAW), specifically MIG, and SMAW (stick welding), consumable electrodes melt and become part of the weld as you use them. These electrodes provide a streamlined way to incorporate different alloys into a weld as you work. The form these electrodes take can vary across different types of arc welding.
With MIG welding, the filler metal added to the base material also serves as the electrode. For example, the filler metal wire used with MIG welding conducts the arc to heat up the weld pool as it is added to the base metal. The filler metal is the electrode. A separate gas hose attached to the torch nozzle applies the shielding gasses to create a protective barrier for the weld pool.
In Flux Core MIG welding, the filler metal wire doubles as the electrode. The hollow wire contains a material called flux. As the metal material of the wire melts into the base material, the flux inside decomposed and gives off gas to shield the weld pool. The electrode itself melts into the weld pool, shielded by the gas given off by the flux.
SMAW, also referred to as stick welding, uses filler rods coated with consumable materials as electrodes. The rod is attached to a stinger which conducts the current through the electrode. The heat from the current causes the coating to emit a gas shield as the filler rod underneath melts into the weld pool.
These electrodes do not melt or combine with the weld. You will generally encounter these types of electrodes when Gas Tungsten Arc Welding (GTAW)—also commonly referred to as TIG welding.
A piece of tungsten serves as the electrode when TIG welding. A separate hose to the nozzle supplies the shielding gas as the electrode carries the current initiating the art. These electrodes are technically “semi-consumable” as they require periodic dressing after extended use or contamination. After breaking off the contaminated or worn tip, the tungsten is sharpened to a point and can be used once more. This process is repeated until the electrode is too small and must be replaced with a new piece.
Basically, what all the information above boils down to is this:
- With MIG welding, the electrode and the filler metal are the same. Shielding gas is fed through the nozzle to protect the weld pool.
- With TIG welding, the electrode, filler metal, and shielding gas are all separate.
- With Flux Core MIG welding and SMAW welding, the electrode, filler metal, and shielding gas are all combined into one. The shielding gas component for SMAW comes from the coating on the electrode. Flux Core MIG wire is essentially an inside-out coated electrode. The flux inside the hollow filler wire, rather than an outer coating, is what produces the shielding gas.
There are a few benefits to SMAW’s utilization of an all-in-one coated electrode. Because the flux decomposes right on top of the weld pool, the process is less sensitive to wind and draft than gas shielded arc welding. This added level of predictability when shielding the weld pool decreases the risk of ending up with porous or weak welds. Flux Core MIG welding seeks to achieve a similar level control by using the same all-in-one electrode principle. On the other hand, TIG welding and, to some degree, regular MIG welding hand require a wind-free indoor type environment to prevent weld defects caused by improper gas shielding of the weld pool.
Additionally, certain materials used to coat the electrodes for SMAW can also help create a stronger, more controlled current. This results in deeper penetration and sturdier welds.
Each different coating contributes unique properties to a project, making the selection of a coated electrode an important decision.
What Does The Coating on a SMAW Electrode Do?
Light vs. Heavy
Electrode coatings serve a variety of purposes. Different coating types and materials provide different benefits and are able to fulfill specific requirements based on project details. However, there are unifying characteristics of all coatings that differ solely based on the degree to which the electrode is coated.
Light coatings and heavy—or shielded arc coatings—share many of the same properties. The main difference between the two is that the strength of the properties is greater in heavy coatings. Because if this strength, shielded arc coatings work well with heavy-duty fabrication applications.
Benefits of Electrode Coatings
One major benefit true of all coatings is that they allow for more control of the current being used to fuse the material. While bare electrodes do exist, they are less commonly used because the arc can be much more unpredictable. A heavier coating will provide more control than a light one will, but either provide better control than bare electrodes do. This extra element of control results in significantly less spatter than you would see welding with a bare electrode, allowing for the creation of smoother, neater welds. The concentrated current resulting from an electrode coating can also penetrate deeper to create stronger welds.
In addition to extra control, certain electrode coatings can also help reduce, and even eliminate, impurities. As a coating melts during welding, the materials can give off shielding gases that protect the weld pool from oxides, sulfur, and other potentially damaging substances. This flux can also serve to “clean” the metal of oxides and other impurities.
Slag(7) is yet another benefit of using coated electrodes. The material left behind after the weld can protect the still cooling metal and help increase weld strength and quality.
Different types of coatings provide different benefits. Knowing what each electrode coating does is essential to selecting the type that is right for your project. The specific functions of each element in an electrode coating are explored in depth later on.
What are they coated with?
A numbering system indicates the unique properties of each electrode. Spanning from 0-8, the first number in the code identifies the material coating the electrode. Coatings come in three general varieties: cellulose, mineral, or a mixture of the two. This number identifying the chemical makeup of the electrode coating also serves as an indicator for the type(s) of current each electrode is best for. The following numbers give information on other factors such as tensile strength and electrode position—meaning flat, horizontal, overhead, or all position electrode.
Types of Coatings:
While there are universal characteristics within each type of electrode coating, the unique chemical makeup of each individual coating will provide different properties. Be sure to research the best applications of each coating to make sure you choose one that suits your project well.
These coatings are comprised of around one third cellulose and two thirds other organic materials. When exposed to the weld arc, the materials decompose to form three separate gases—hydrogen, carbon monoxide, and carbon dioxide—that strengthen the arc. This added strength allows the current to more deeply penetrate the metal, resulting in stronger welds.
Cellulose coatings also emit a layer of gas to protect the weld pool from impurities. The gas layer creates a barrier between the metal and other elements, like oxygen, nitrogen, and hydrogen, that can create porosity in a weld. Porousness is poison for a weld, so using electrodes with cellulose coating can help ensure higher quality weld joints.
Cellulose coatings come in a variety of chemical mixtures, each with its own unique properties and best applications. While the cellulose component of the recipe is a general rule of thumb, the additional organic materials vary greatly.
Mineral coatings leave a layer of slag over the weld. While the slag might seem like an annoying side effect, it actually serves a very useful purpose. The slag from a mineral coated electrode cools much slower than a cellulose-coated electrode and the welded material underneath. This allows time for impurities to filter to the surface of the metal, preventing them from compromising the structure of the weld.
Electrode coatings with a combination of cellulose and minerals are a popular choice among fabricators because they provide the best of both worlds. Since these coatings can have anywhere from just a few components to upwards of 10 different ingredients, the chemical diversity of these coatings provides a range of significant benefits. Having both shielding gas and slag protection on a weld can be incredibly useful when working with particularly temperamental base metals.
Most Common Electrode Coatings:
While there are certain applications requiring specific electrode coatings and characteristics, these are five of the most common welding electrode coatings you’re likely to see.
Well suited for vertical positioning, cellulose electrodes leave behind a very thin, very easy to remove layer of slag. Cellulose coatings break down into hydrogen and carbon dioxide as they are heated. This provides an effective protective gas layer over the weld pool. However, this can also place the weld at risk for hydrogen embrittlement. In their purest form, cellulose coatings work best with DC. However, the addition of different elements to the coating may allow for use with AC as well. Cellulose electrodes give you all the ease of a rutile coating, but with deeper penetration and less problematic slag.
Nearly identical to cellulose, the main difference is that rutile has a higher percentage of titanium dioxide. This creates a gas shield of oxygen, nitrogen, carbon, and hydrogen, making rutile electrodes well-suited for welding low carbon steel. However, slag from rutile electrodes can tend to leave behind traces of titanium in the deposited metal. The addition of cellulose to rutile electrode coatings provides added protection over the weld pool. These electrodes give off lower levels of both spatter and fume emissions and are great for use in all positions.
Iron Oxide Electrodes:
Good for use with both AC and DC current, iron oxide electrodes produce slag that is very easy to remove from the weld. The chemical composition of this coating is high in oxygen and can tend to cause weld deposits that are weaker in overall strength. Risk of hydrogen embrittlement, though, is significantly lower than with cellulose electrodes. Iron oxide electrodes provide great arc control and allow for neat, precise bead placement.
Also referred to as hydrogen-controlled electrodes, these electrodes require a bit more care prior to welding. Electrodes must be stored in a dry location and baked before use. Failure to do so can create an unstable chemical composition in the coating resulting in a compromised weld structure. Basic electrodes deposit a low, controlled level of hydrogen which minimizes the risk of porosity and cracking in a weld. If properly stored and maintained, these electrodes a great option for working with steel.
Iron Powder Electrodes:
These electrodes are variations of other electrode coatings resulting from the addition of iron powder to a mix. Metal powders are becoming an increasingly popular addition to electrode coating mixtures as they can help increase efficiency and overall weld quality. Iron power electrodes are a common variation on cellulose electrodes that enable the electrode for use with AC.
When working with a type of welding that requires separate, coated electrodes, taking the time to understand the different options available can make or break a project. Remember to consider additional factors—such as position, tensile strength, and core metals—when deciding on an electrode. Researching the specific characteristics necessary for your fabrication application will help you decide which electrodes best suit your needs.