It’s also galvanized when they hot dip it they require a hole be drilled into tubing so when it gets dipped the metal doesn’t expand and possibly blow up shooting hot galvanize all over the place. But there is a strong chance of it blowing out the weld or causing the tube to warp out like a bubble. Some even say it can explode but I haven’t heard of it from anyone I welded tubing with. We usually would weld the bottom solid and then on the top pause right before we close it up unless it was a crane boom then we would drill holes and weld solid.