Understanding Galvanized Steel and Its Welding Challenges
Galvanized steel is coated with a layer of zinc, which provides excellent corrosion resistance and extends the lifespan of the material. Typically, this zinc coating is applied after the steel fabrication and welding processes. However, there are situations where welding galvanized steel becomes necessary, and understanding how to do it properly is essential for safety and structural integrity.
Welding galvanized steel is feasible, but it differs from welding plain steel due to the zinc coating. The key is to prepare the material correctly by removing the zinc in the weld zone, which minimizes defects and health risks. This article explores the reasons behind the difficulties in welding galvanized steel, effective preparation techniques, suitable welding methods, and essential safety precautions to take during and after the process.
Reasons Why Welding Galvanized Steel Is More Difficult
Welding galvanized steel presents several unique challenges primarily because of the properties of zinc. Zinc melts and vaporizes at a temperature lower than that of steel, approximately 419°C (786°F). When welding, the vaporized zinc can become trapped in the weld pool, especially in joints like tee or lap welds where two galvanized surfaces meet closely at the root. This entrapment results in porosity—tiny gas pockets within the weld—that compromises its strength and quality.
Furthermore, the presence of zinc destabilizes the welding arc, causing excessive spatter and a rougher weld bead appearance compared to welding on uncoated steel. The arc instability and spatter can be mitigated with anti-spatter sprays, but the core issue remains the zinc vaporization. Another complication arises when using electrodes or filler metals with high silicone content (>0.85%). Silicone can facilitate zinc penetration into the base metal, increasing the risk of weld cracking, especially in thicker coatings or thicker plates.
To address these issues effectively, proper preparation by removing the zinc coating from the weld area is crucial. This process ensures better weld quality and reduces health hazards associated with zinc fumes.
Preparing Galvanized Steel for Welding
The most effective way to prepare galvanized steel for welding is to remove the zinc coating in the area of the weld. This step minimizes the risk of weld defects caused by trapped gases and prevents the vaporization of zinc during welding. The goal is to expose clean, bare steel in the weld zone and extend this removal approximately 2 to 4 inches beyond the joint on each side for safety and comprehensive removal.
The simplest and most accessible method involves mechanical grinding or sanding with an angle grinder. Always wear a respirator or mask when grinding galvanized steel to avoid inhaling zinc dust, which can cause metal fume fever—a flu-like illness resulting from inhaling zinc oxide fumes. Alternative methods include sandblasting or using an oxy-acetylene torch, but grinding remains the most cost-effective and straightforward approach for most applications.
It’s important to remove zinc from both sides of the joint and the surrounding area that will be red hot during welding. Proper removal prevents the formation of hazardous fumes and ensures the weld metal bonds directly to the bare steel, improving weld quality and durability.
Is It Possible to Weld Galvanized Steel Without Removing the Coating?
While it is technically possible to weld galvanized steel without removing the zinc coating, doing so significantly increases the likelihood of weld defects, health hazards, and compromised structural integrity. Most outdoor or field welding scenarios involve galvanized steel where post-weld galvanization isn’t feasible. In such cases, welding directly on coated steel is common, but it requires specific techniques and increased caution.
Among various welding options, shielded metal arc welding (SMAW) or stick welding tends to perform better because of its robustness and arc stability in outdoor environments. Nonetheless, removing the zinc coating remains the safest and most reliable approach for optimal weld quality and health safety.
Choosing the Right Welding Process for Galvanized Steel
Once the zinc coating has been adequately removed, you have the flexibility to use most welding methods suitable for mild steel. However, if you opt to weld galvanized steel with its coating intact, certain processes are more suited than others due to their characteristics and safety considerations.
Stick Welding (SMAW): The Preferred Method
Stick welding remains the top choice for galvanized steel, particularly in outdoor or industrial settings. Its robustness, arc stability, and flux coating help manage zinc fumes effectively. Electrodes with a cellulose coating, such as 6010 or 6011, are ideal because their high-temperature arcs vaporize zinc efficiently and create a protective shield through the flux. These electrodes also have low silicone content, reducing the risk of weld cracking.
For thin galvanized sheet metal, a 6013 electrode is often preferred. Unlike the cellulose-coated electrodes, 6013 doesn’t require a whip technique; instead, it is dragged along the joint similar to 7018, providing a cleaner and smoother weld bead.
MIG Welding (GMAW): Limitations and Best Practices
Metal inert gas (MIG) welding can be used on galvanized steel but presents more challenges. Since MIG wire lacks flux, it doesn’t inherently help eliminate zinc fumes or impurities. Without flux, you depend on the zinc vaporization and proper shielding gases to protect the weld pool.
For galvanized steel, spray transfer mode with higher heat input is preferable, although it can cause increased spatter. Short circuit transfer is less effective because the arc temperature may not be sufficient to vaporize all zinc, leading to porosity and weld defects. Using wires like ER70S-3 or ER70S-2 (which contain less silicone) can help reduce cracking risks. Adjust shielding gases to include a higher percentage of CO2 (up to 20%) or a mix of 90% argon and 10% CO2 for better zinc removal and weld quality.
Additionally, techniques such as increasing root gap size, welding with a pulling technique, and welding at slower travel speeds can improve results when working with galvanized steel using MIG.
Flux-Cored Arc Welding (FCAW): An Effective Alternative
Flux-core wires like E71T-11 are well-suited for welding galvanized steel, especially outdoors. These wires contain flux inside the core, which helps in cleaning the weld area, reducing porosity, and managing zinc fumes. Self-shielding flux-core wires eliminate the need for external shielding gases, making them ideal for fieldwork.
Welding with flux-core wire generally produces stronger welds with fewer defects compared to bare wire MIG welding, especially on galvanized surfaces. Remember to switch the polarity to DC electrode negative (DCEP) for optimal results.
TIG Welding (GTAW): Not Recommended Without Proper Preparation
While TIG welding offers high precision and clean welds, it’s not suitable for galvanized steel unless the zinc coating has been removed. The vaporization of zinc can contaminate the tungsten electrode and degrade weld quality, leading to poor aesthetics and increased cleaning requirements.
If you must TIG weld galvanized steel, ensure the zinc coating is stripped away thoroughly. Use a steeper torch angle (around 70°), increase shielding gas flow (preferably 100% argon), and select an ER70S-2 filler rod for better results. However, in most cases, TIG is better suited for clean, bare steel or post-removal of zinc coating.
Effective Tips for Welding Galvanized Steel Successfully
- Remove zinc coating 2 to 4 inches on either side of the weld joint for best results.
- Opt for stick welding whenever feasible, especially outdoors.
- Leave a small root gap in lap and tee joints to allow gases to escape.
- Use a slightly larger root opening for butt joints to reduce trapped gases.
- Ensure craters are filled at the end of each weld to prevent cracking.
- Maintain a short arc length to minimize zinc vaporization and fumes.
- Connect the grounding clamp to clean, bare metal whenever possible.
- Increase amperage or slow down travel speed to provide deeper heat penetration.
- Use the thickest electrodes available to allow higher current settings.
Post-Welding Care: Protecting the Welded Area
Welding on galvanized steel damages the zinc coating, which diminishes its corrosion resistance. To prevent rust and deterioration, it’s vital to protect the exposed steel after welding. Apply a cold galvanizing spray or zinc-rich paint to the weld area, ensuring the surface is thoroughly cleaned beforehand.
Clean the weld zone with an angle grinder or abrasive pad to remove residual zinc, oil, or dirt. After cleaning, wipe the area with a solvent or thinner and then apply protective coatings. This process restores corrosion resistance and prolongs the lifespan of the welded structure.
The Health Risks of Welding Galvanized Steel
The primary health concern when welding galvanized steel is inhalation of zinc fumes, which can lead to metal fume fever—a temporary, flu-like illness characterized by chills, fever, nausea, headache, and extreme thirst. This condition typically develops within hours after exposure and lasts up to 48 hours. Although symptoms are usually mild, repeated exposure can be hazardous.
Recent research indicates that even exposure levels below current permissible limits may cause adverse health effects, emphasizing the importance of strict safety measures. The long-term effects of inhaling zinc dust or fumes remain poorly understood, underscoring the need for caution and proper protective equipment.
Myth Busting: Drinking Milk After Welding
Some believe that drinking milk can mitigate the effects of metal fume fever. However, this is a misconception. Milk does not neutralize or prevent zinc fumes from entering the body, as it affects different pathways. The most effective safety practice is to avoid inhaling fumes altogether.
To protect yourself, always use proper ventilation, local exhaust hoods, and wear appropriate PPE such as respirators or powered air-purifying respirators (PAPRs). Ensuring good ventilation and removing zinc coating before welding are the best strategies for safety.
Summary: Best Practices for Welding Galvanized Steel
- Thoroughly remove zinc coating from the weld area and surroundings.
- Prioritize stick welding for outdoor applications when working on galvanized steel.
- Maintain proper welding techniques, including appropriate gaps and travel speeds.
- Use suitable electrodes and filler materials with low silicone content.
- Apply protective coatings after welding to prevent corrosion.
- Always wear respiratory protection and ensure adequate ventilation during welding.
- Be aware of and mitigate health risks associated with zinc fumes.
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