Welding galvanized steel sheet presents unique challenges due to the presence of the zinc coating. While galvanized steel offers excellent corrosion resistance—making it a popular choice in construction, automotive, and HVAC applications—its weldability requires careful parameter selection and post-weld treatment to maintain structural integrity and long-term durability. The primary concern during welding is the vaporization of zinc at relatively low temperatures (around 907°C), which occurs well below the melting point of steel (1370–1510°C). This leads to potential issues such as zinc fumes, porosity, and reduced coating protection at the weld zone.
For gas metal arc welding (GMAW), also known as MIG welding, on galvanized sheet metal, several key parameters must be optimized. A short-circuit transfer mode is generally preferred for thin gauge sheets (typically under 3 mm), as it provides better control, reduced heat input, and minimized burn-through risk. Using a 0.8–1.0 mm diameter ER70S-6 or ER70S-3 filler wire with a shielding gas mixture of 75% argon and 25% CO₂ (C25) helps stabilize the arc and reduce spatter. Voltage settings should be kept low—typically between 16–20 volts—and wire feed speed adjusted accordingly to maintain a smooth, consistent bead.
Travel speed is critical: too slow, and the heat causes excessive zinc vaporization and coating degradation; too fast, and incomplete fusion may occur. A travel angle of 10–15 degrees in the direction of travel, with the torch held slightly behind the weld pool, helps push zinc vapor away from the arc and reduces porosity. Pre-weld preparation should include edge cleaning and, if possible, grinding the zinc coating back 10–15 mm on both sides of the joint to reduce fume generation and improve weld quality. However, this must be done carefully to avoid removing too much base metal.
Spot welding, commonly used in automotive and light fabrication, also requires adjustments when applied to galvanized steel sheet. Higher welding currents (10–20% above uncoated steel) are typically needed to overcome the resistive zinc layer and ensure proper nugget formation. Electrode force should be increased slightly, and weld time may need to be extended to allow for consistent fusion. Using domed or truncated cone electrodes helps distribute pressure and prolong electrode life, which tends to degrade faster due to zinc adhesion. Regular electrode dressing or replacement is essential to maintain weld consistency.
One of the most critical steps after welding is zinc repair painting. The heat-affected zone (HAZ) loses its protective zinc layer, leaving the bare steel vulnerable to rust. Simply applying standard paint is insufficient—specialized zinc-rich coatings are required. These fall into two main categories: inorganic zinc silicate (IZS) and organic zinc-rich coatings. IZS offers superior performance in harsh environments and can be applied as a single coat, while organic versions are easier to apply and more forgiving in field conditions. A minimum coating thickness of 60–80 microns is recommended for long-term protection. Surface preparation is crucial: the area must be cleaned, degreased, and lightly abraded to promote adhesion. For best results, use a two-coat system: a primer with 80–90% zinc dust content followed by a compatible topcoat.
Ventilation and safety cannot be overlooked. Zinc oxide fumes generated during welding can cause metal fume fever—a condition marked by flu-like symptoms. Always use local exhaust ventilation (LEV), fume extraction torches, or respiratory protection (NIOSH-approved N95 or P100 respirators) when working in confined spaces. Welders should be trained on the hazards of zinc exposure and follow OSHA or equivalent guidelines for safe practices.
For manufacturers and fabricators, selecting high-quality galvanized steel sheet with consistent coating thickness and surface finish significantly improves weldability. At Asia Metal Ltd, we supply hot-dip and electro-galvanized sheets with precise zinc layer control (from Z60 to Z275), ensuring optimal performance in welding applications. Our products are tested for adhesion, uniformity, and dimensional accuracy, reducing variability on the production floor.
Another important consideration is the use of zinc repair paint that matches the original coating’s electrochemical behavior. For example, when repairing hot-dip galvanized steel, the repair paint should have a zinc content close to 90–95% to maintain sacrificial protection. Products like zinc silicate primers or cold galvanizing compounds (e.g., zinc-rich epoxy) are widely used in the field. According to galvanization standards, proper repair techniques can restore up to 90% of the original corrosion protection.
For spot-welded assemblies, post-weld inspection should include visual checks for weld nugget size, surface cracking, and coating integrity. Non-destructive testing (NDT) methods such as ultrasonic or peel testing can help verify weld quality, especially in safety-critical components. In automotive manufacturing, where galvanized sheet is standard, robotic welding systems with adaptive feedback loops are increasingly used to maintain consistent parameters across large batches.
Finally, consider the long-term implications of repair methods. While zinc-rich paints are effective, they are not a permanent replacement for the original hot-dip coating. For outdoor or high-humidity environments, periodic inspection and re-coating may be necessary. Using a high-performance galvanized sheet from the start—such as those offered by Asia Metal Ltd—reduces the frequency and severity of post-weld repairs, lowering lifecycle costs.
In summary, successful welding of galvanized sheet metal hinges on three pillars: optimized GMAW or spot welding parameters, rigorous safety protocols, and effective zinc restoration through proper repair painting. By combining technical precision with high-quality materials, fabricators can ensure strong, corrosion-resistant joints that stand the test of time.