Butt Welding Mast Extrusions

[Douglas]

Ahoy All , I had a forespar aluminium mast, that developed corrosion bubbling under the paint.
This occurred about 5 1/2' up from the bottom end of the mast , seen inside the cabin, as the mast is keel stepped .

The corrosion / bubbling paint went right around the mast about 2" wide .

In the sail track I noticed a horizontal indented line , at the middle of the corrosion , but not elsewhere .

Could it be that the mast was extended by butt welding a (+ -) 5 1/2' piece onto it , and sanded smooth then painted over ?