Yes, you can weld aluminized steel, but the aluminum coating creates challenges that regular steel doesn’t. The biggest issue is porosity: the aluminum melts and mixes into the weld pool, trapping hydrogen gas as the metal solidifies. This produces weak, spongy welds full of tiny holes. With the right process, electrode choice, and preparation, though, you can get clean, strong results.
Why the Aluminum Coating Causes Problems
Aluminized steel is ordinary carbon steel dipped in an aluminum or aluminum-silicon alloy. Type 1 has a coating containing 5 to 11% silicon blended with aluminum, designed for high-heat applications like exhaust systems. Type 2 uses nearly pure aluminum (less than 0.3% silicon) and is built more for corrosion resistance in things like HVAC ducts, siding, and roofing.
When you strike an arc on either type, that thin aluminum layer melts right into the weld puddle. Aluminum has a strong affinity for hydrogen, and as the molten metal cools and solidifies, dissolved hydrogen tries to escape as gas. If the metal freezes before the gas gets out, you end up with porosity scattered through the weld bead. Research from the Armour Research Foundation found that standard cellulose-coated electrodes produced gross surface porosity on aluminized steel, and the more cellulose in the electrode coating, the worse the porosity got. The cellulose breaks down under heat and releases hydrogen, which the aluminum traps.
Which Welding Processes Work Best
Most common welding processes can join aluminized steel, but some handle the coating better than others.
MIG (GMAW) is the most popular choice, especially for exhaust work and sheet metal fabrication. You can use ER70-S6 mild steel filler wire with a 75/25 argon/CO2 shielding gas mix. For better corrosion resistance in the finished joint, a 400-series stainless steel wire works well since it holds up to heat and resists oxidation. Current settings matter: too low and large droplets transfer erratically across the arc, creating turbulence and trapping gas. Too high and the filler metal buries gas bubbles before they can escape. Finding the sweet spot where droplet transfer is smooth and stable is key to avoiding porosity.
Stick welding (SMAW) works, but only with low-hydrogen electrodes like E7016. In controlled testing, E7016 and E7016 iron powder electrodes produced completely porosity-free welds on aluminized plate, while every cellulose-coated rod (E6010, E6012, E6020, E6024, E6027) failed with visible porosity. If you’re stick welding aluminized steel, low-hydrogen rods are non-negotiable.
TIG (GTAW) gives you the most control over heat input, which helps on thinner aluminized sheet. The main risk with TIG is contamination. Any oil, moisture, or residue on the filler rod or workpiece introduces hydrogen into the weld pool. Clean filler wire and thorough surface prep reduce this significantly.
Surface Preparation Makes or Breaks the Weld
The single most effective thing you can do is remove the aluminum coating from the joint area before welding. Grinding, sanding, or wire brushing the aluminum off the surfaces that will fuse together (the faying surfaces) produces welds comparable in quality to those made on bare, uncoated steel. This eliminates the aluminum contamination problem at its source.
If you can’t remove the coating, or if you’re working on a production line where grinding every joint isn’t practical, low-hydrogen electrodes and proper shielding gas become even more critical. Either way, make sure the surfaces are free of oil, dirt, and moisture. Even fingerprints can introduce enough hydrogen to cause pinhole porosity in the weld.
Protecting the Weld Zone Afterward
Here’s the tradeoff most people don’t think about: welding burns away the aluminum coating in and around the joint. The weld bead itself and the heat-affected zone on either side lose their corrosion protection. On an exhaust system, this means that section will rust faster than the surrounding aluminized pipe. On outdoor enclosures or ductwork, it’s an entry point for moisture.
Restoring the original coating isn’t really practical outside a factory setting. The standard fix is painting the welded area with a high-temperature paint or an aluminum-rich coating, depending on the application. For exhaust components exposed to extreme heat, a high-temp header paint or ceramic coating works. For lower-temperature applications, a cold galvanizing compound or aluminum spray paint provides a reasonable barrier. The protection won’t be as durable as the original hot-dipped coating, but it’s far better than leaving bare steel exposed.
Fume Hazards to Take Seriously
Welding aluminized steel produces fumes that contain aluminum oxide particles along with whatever else is in the base steel. Aluminum welding fumes irritate the lungs and, with prolonged exposure, can contribute to respiratory problems. If the steel contains any chromium, the welding arc converts it to hexavalent chromium, which is a known carcinogen that damages the eyes, skin, nose, throat, and lungs.
Good ventilation is the first line of defense. Work outdoors or use local exhaust ventilation that pulls fumes away from your breathing zone. In enclosed spaces or during extended welding sessions, a respirator rated for metal fumes is essential. OSHA’s respiratory protection standard (29 CFR 1910.134) applies when ventilation alone can’t bring exposure down to safe levels, which in tight quarters with aluminized steel, it often can’t.
Practical Tips for Cleaner Welds
- Grind the joint area. Remove the aluminum coating at least half an inch back from each edge you’re welding. This is the single biggest improvement you can make.
- Use low-hydrogen consumables. For stick welding, E7016 or E7018 electrodes. For MIG, keep your wire dry and use quality shielding gas.
- Store electrodes properly. Low-hydrogen rods absorb moisture from the air. Keep them in a sealed container or a rod oven. Damp rods reintroduce the hydrogen problem you’re trying to avoid.
- Dial in your amperage. Run enough current to get stable metal transfer without going so hot that you trap gas bubbles under rapidly solidifying filler.
- Protect the finished weld. Apply a corrosion-resistant coating to the weld zone as soon as the joint cools. Bare steel starts oxidizing immediately.