The thin aluminized coating seems to vaporize near the weld area without making much smoke. It doesn't seem to affect the integrity of the weld joint, but it will definitely burn off (and not protect against corrosion) along and near the welded joints, kind of like galvanized coating. Welding near galvanized seems to make a lot more smoke and fumes (which you don't want to breathe), and leave more "dirty" looking residue on the nearby metals behind. I prefer an aluminized coating for mild steel exhaust components over galvanized coated mild steel. I think the aluminized coating lasts just as long if not longer than galvanized for an automotive exhaust application, and it sure welds a lot more nicely.
Of course stainless steel would be ideal for automotive exhaust components (offering the highest corrosion resistance, and therefore lasting the longest), but for this project I started with an aftermarket produced, aluminized coated mild steel downpipe to begin with, so I decided to add onto it with more mild steel tubing.
Edit: I weld aluminized or galvanized coated steel the same you way uncoated steel - DC electrode negative. But I can't remember if I ever tried AC on coated mild steel or not.
I have tried AC when silicon bronze braizing/welding over mild steel, and the AC definitely helped leave a cleaner appearance on the surface of the deposit than DC mode.
PS - when metal vaporizes when welding, it usually very quickly condenses and solidifies in the air into tiny particles, which can either appear in the air itself (as "smoke" or particulate matter), or on nearby surfaces appearing like a soot/"smut" powder of varying colors. I've seen Black, tan, red, brown, to whitish, etc, and it is my belief that the color depends on the particle's size. The smut usually easily blows or wipes off. When you see this kind of "smut" appearing on nearby surfaces during welding, I would recommend not breathing in the fumes.
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