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Types of electrodes and their importance in welding quality

Understanding how different types of electrodes affect weld quality and durability.

Published: October 26, 2024

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Introduction

Welding is a fundamental process in the fabrication and construction of metallic structures, and one of the most important aspects to guarantee the quality of the welded joints is the choice of the right electrode to ensure an efficient and stable fusion, affecting the strength, durability and visual quality of the welds made. There are several types of electrodes, each designed for specific applications and particular welding conditions. They vary in composition, coating, base metal (substrate), required welding positions, and behavior depending on their purpose.

In this article, an exposition of the different types of electrodes for electric arc welding, their characteristics, and how they influence the quality of the weld is made.

What is a welding electrode?

It is one of the elements used to establish the electric arc necessary to melt and bond metals. Its main function is to conduct the electric current to the welding point, creating an electric arc with the necessary heat to melt both the base metal and the filler material (in case the electrode is consumable). The electrode can be coated, flux cored (FCAW) and bare rod.

Classification of electrodes according to their purpose

They can be classified into consumable and non-consumable:

Consumable electrodes: During the welding process, these electrodes melt, forming part of the filler material. Among them are coated electrodes, which are widely used in manual SMAW (Shielded Metal Arc Wheeling) welding processes. These electrodes have a coating that helps stabilize the arc, protect the molten metal from contamination, and add alloying elements that improve the mechanical properties of the weld.
Other examples of consumable electrodes are those used in MIG/MAG (Metal Inert Gas / Metal Active Gas) welding, where a continuous bare wire feeds the arc and becomes the filler material, and flux-cored cored electrodes, also known as flux-cored wire or FCAW (Flux-Cored Arc Welding), is a type of consumable electrode used in electric arc welding that consists of a hollow wire containing a core composed of materials that, when melted, generate gases and slag that protect the weld puddle from the air.
Non-consumable electrodes: In this case, the electrode is not consumed during welding and only serves as a conductor of electricity to generate the electric arc that melts the base material and the uncoated rod that is applied manually by the welder. A typical example is the tungsten electrode used in the TIG (GTAW) process. This type of electrode allows precise control of the arc and is used in applications where high quality and joint accuracy are required, such as in the welding of materials like aluminum and stainless steel.
Carbon electrodes: They are used in cutting and drilling processes and to remove material from metal parts. These electrodes are useful in maintenance and repair work, especially in operations where fast material removal is required. Industries that frequently use these electrodes include metal structure manufacturing and heavy equipment maintenance, where an effective solution for metal removal is needed.

Classification according to coating

The coating of the electrodes is decisive in the quality of the weld since it influences the arc stability, the protection of the weld pool, and the mechanical characteristics of the final weld. There are several types of coatings, and each has a specific purpose:

Cellulosic coating: Containing cellulose and other organic materials, this type of coating provides a penetrating arc and is ideal for welding in all positions, especially in vertical down welding. It is common in pipe and structural welding where mobility is crucial.
Ruthyl coating: These coated electrodes contain mainly titanium dioxide (rutile), are easy to handle, and provide a good weld bead appearance. They are suitable for welding in horizontal and vertical upward positions, and are often used on light structures.
Basic coating: Contains calcium carbonate and calcium fluoride. This type of electrode, also known as low hydrogen, is essential in critical structural welding applications because it minimizes the possibility of cracks and defects. It is especially useful in welding high-strength steels.
Acid coating: Formed by iron oxides and other acidic substances. It provides a stable and smooth arc, with good bead finish and is used in jobs where good bead appearance and cleanability are required, but low penetration.
Oxidizing coating: Composed of iron oxides that promote oxidation during the welding process. Provides good control of the weld puddle and is used on specific jobs where precise control of metal oxidation is required.

Classification of non-consumable electrodes (Tungsten)

They can be classified into different types according to the following compositions:

2% thorium tungsten (WT20): Used for DC TIG welding of stainless steel, nickel alloys, titanium and copper. Thorium is a material that can generate dust harmful to health during electrode sharpening if proper precautions are not taken.
2% Cerium tungsten (WC20): They are characterized by efficient arc starting at low amperages and are suitable for both alternating current (AC) and direct current (DC), making them versatile for welding various metals, such as stainless steel and non-ferrous alloys.
2% lanthanum tungsten (WL20): Used in automated welding processes due to its higher durability and lower wear compared to other types of tungsten. It works well with alternating current (AC) and direct current (DC).
Zirconium tungsten (WZ8): Ideal for alternating current (AC) TIG welding, especially when welding metals such as aluminum and magnesium. It offers excellent resistance to contamination and provides a stable arc in AC welding.
Pure tungsten (W): Most commonly used for AC welding, especially for welding aluminum, magnesium, nickel, and their alloys. Although it is not as durable and efficient as electrodes with additives, it provides a stable arc and good melting capacity in AC welding.

Electrodes and their application according to the base metal

The choice of electrode also depends on the base metal to be welded. For example, for welding carbon steel, electrodes containing elements such as manganese and silicon, which help to deoxidize the metal and improve the mechanical properties of the joint, are often used. In the case of stainless steels, electrodes with adequate chromium and nickel content are necessary to maintain corrosion resistance.

In aluminum alloys, non-consumable tungsten electrodes are used in the TIG process because they allow precise control and prevent contamination of the base material. In addition, in structural welding of large constructions, such as bridges or buildings, electrode selection is critical to ensure that the joints have the necessary strength and ductility to withstand the structural loads and stresses.

Factors influencing weld quality

It is influenced by several factors related to the welding electrodes. These include:

Electrode composition: The proportion of chemical elements in the electrode, such as carbon, chromium, or nickel, influences the strength and ductility of the weld. It is crucial to select an electrode compatible with the properties of the base metal to avoid defects such as cracking or lack of fusion.
Coating and welding position: The electrode coating affects arc stability and molten metal fluidity, which are critical to ensure a flawless weld.
Welding position: The position in which the weld is made (horizontal, vertical, overhead) affects electrode selection, as some coatings are specifically designed to stabilize the arc in complex positions.
Current and polarity: The type of current used (alternating current (AC) or direct current (DC)) and the polarity influence how the electrode behaves during welding.
- CC with negative electrode (CCEN) provides shallower penetration and is used on thinner materials.
- DC with positive electrode (CCEP) offers deeper penetration and is more suitable for thicker materials.
- AC for electrodes specifically designed to work with alternating current, allowing for more flexible operation in workshop environments where this type of current is available.
Electrode storage conditions: Electrodes should be stored properly to avoid moisture absorption. Hygroscopic-coated electrodes, such as low hydrogen electrodes, can absorb water from the environment, which can lead to porosity and other defects in the weld. It is important to consider the following recommendations:
- Low hydrogen electrodes: They should be stored in special furnaces or in airtight containers to keep them free of moisture.
- Wet electrodes: If an electrode has absorbed moisture, it should be dried before use to avoid pore formation in the weld bead.
Environmental conditions: Humidity, wind, temperature, and contamination of the working environment affect the quality of the weld.
Base material thickness and electrode diameter: The electrode diameter should be selected according to the thickness of the base material to ensure adequate penetration, avoid defects, and maintain process stability. The relationship is the greater the thickness, the larger the electrode diameter, taking into consideration the following points:
- For thicknesses less than 3 mm: Electrode Ø 1.6 to 2.5 mm.
- For thicknesses between 3 and 6 mm: Electrode Ø 2.5 to 3.2 mm.
- For thicknesses greater than 6 mm: Electrode Ø 3.2 to 5 mm or more.
- In joints with limited access or in root welds (first pass), it may be necessary to adjust the electrode diameter to ensure good penetration without burning the edges of the base material.

Conclusion

Electrode types and their correct selection play a key role in weld quality. Knowing the properties and applications of each type of electrode, as well as the characteristics of their coatings, is essential to optimize the welding process and ensure safe and durable joints, especially in critical structural applications. Electrodes must be selected based on the base metal, environmental conditions and welding position, which together determine the efficiency and quality of the welding process.