If I were you I'd do this:
take two flat pieces of steel of the same (or at least similar) composition and tensile strength than those of the boiler tube. The length could be, for example, 4 inches or 100 mm. The thickness should be the same than that of the tubes.
The width will be calculated as follows: [one half the outside radius of the tube x 3,14] minus 3 mm or 1/8 inches.
Now have a blacksmith (do blacksmiths still exist?) round up the pieces to make half a cylinder out of each one. The two half cylinders should fit more or less exactly on the outside surface of the tube. Make the necessary adjustments if they don't.
Now install the two halves one in front of each other, so they will embrass (or wrap) the leaking part of the tube. Weld the two half cylinders to the tube and one to each other.
Select the electrode, or MIG wire, according to the opinions of the other gentlemen who answered your question.
Before putting the boiler in operation again, don't forget to make a hydrostatic test.
Now that you have plugged the leak, you must correct the problem that caused the tube corrosion, and the problem is poor water quality. So, talk to a competent chemist, i.e., one that knows well how to treat boiler feed water and what precautions must be taken when the water is already into the boiler, and ask for his recommendations.
Giovanni S. Crisi
Sao Paulo - Brazil