Good luck in the long run! If the welde was clean and the plate job good, it wouldn't be rusting. Agree with the comment that is is likely a bottom out problem. Most probably from porous plating. Oxalic acid will help remove the corrosion but be sure to rinse very well. To slow up the process, perhaps seal with clear acrylic spray, keep heavily waxed, etc. Keep an eye on it for crack development.
Woody