If an item is for sure welded air /water tight, there might be a very small amount of condensation that might cycle between vapor and condensate due to temporary heat/cool cycles. As long as the piece doesn't set outside in the weather and fill with water prior to the sealing end, there will be very little water even if the ambient air is 99% humidity, there still wont be enough water in a item to collect a visible amount of water.
On a construction project once, we built a "dummy leg" (a piece of pipe that welds onto a larger pipe to support it) and it set outside for a while and collected a lot of rainwater. When it was put on the supporting pipe, a steam line, they didn't dump out the water before welding it tight. Later that line was put into service with 500F steam. It ran for over a week before finally building up enough pressure in the dummy leg to punch the center of the steam line (the part covered by the dummy leg) out. Engineers calculated that it developed about 15,000 PSI inside the pipe in order for it to cause the steam pipe to rupture. Everyone was surprised that the steam pipe gave way and not the dummy leg which had a flat plate on the bottom. The dummy leg was welded to a 90 ell so it was pressuring against a convex part which should have been much stronger than the flat plate.
We were lucky that it didn't explode the dummy leg pipe because people were working in the close vicinity of it. After that incident, all dummy legs had to have a minimum of 1/4" vent hole in the bottom.
If I am welding on a reinforcing plate, some call them scab plates, I always weld solid to prevent any moisture from getting under them and causing corrosion. When closing the weld, I weld up to the last 1/2" or so then let the weld cool, then finish the weld. This keeps the closure from developing a pinhole in the weld from high internal pressure from expanding gases under the plate.