Like another posted. The problem is the acid core solder. It is making the surface "dirty". You need tinning flux not sweating flux too. They are a little different in how they work.
So clean the tank up. Then tin the tank and then tin the repair piece. Spread a good amount of the flux on the tank. Then lay the repair piece over it and then solder them together.
Something like this will not "sweat" like a copper fitting. One of the reasons is that steel does not conduct heat as well as copper. So it is hard to get the area the right temperature to work. So tin both pieces before you try to fuse them. Another thing. Everything needs to be absolutely clean. Any foreign matter at all and it will not work.