Ain't likely to happen as you describe: As pointed out, a seal weld is a non-strength constant weld all the way around a joint between two surfaces to prevent (incidental water or liquid or whatever) xxx from leaking in or out - not a "pressure barrier" against fluid fluid driving through. So a seal weld might leak if a standing fluid held (a corrosive) were trapped inside for a long period of time... But this doesn't sounds like that case.
but you're describing rust (caused by water inside the pipe) "leaking out" (up somehow?) through the weld.
You (the fabricator) probably can't prevent all and every water particle from getting the (unpainted) inside of the tube steel. But you're case resides in figuring how so much could getin side so that it "leaks up" from inside in that short a period of time.
Rust coming through the walls of a weld in that short a time at the top of a post?
Can you post pictures? This doesn't sound right.
OK. Now, your mission is: find out what they used for the weld, what the tube steel material was, and how they did the welding.
Hint: A 0.030 "hard wire" is meaningless, because they didn't tell you what material the "hard wire" was made of, nor how hey welded what base metal in the tube steel. (At least there, it's reasonable to assume plain (cheap) carbon steel.)