Yes it is corrosion driven. Anyways the wire appears to rot. I do not own a spot welder and the local shops want as much to replace that braided cable about two inches long as a rebuilt starter costs. If it is a repair just for yourself you can try what I did.
Take a 1/2 inch piece of soft copper pipe. Hammer it flat. Drill a hole for the bolt connection on the solinoid. Carefully work the copper to shape. Clean any remaining trace of the old wire from the spade that comes from the starter back plate. There is a fair amount of meat there so I drilled a hole through the copper and spade connection. It is too difficult if not impossible to use a weld or as there is a feedthrough grommet right there.Taking the starter apart would allow it. Plus the use of heavy braid again.
I used the largest stainless steel bolt consistant with leaving good contact area and strength. Plus two washers to increase contact area on both sides of the spade. I also had cleaned up the contact areas and used a good dielectric grease. I shaped the copper to contour the shape of the spade terminal as well. That just increases the contact area. The spade is mostly flat with small wings on either side.
It also passed enough current at the connection. Perhaps even better than the original limited area braid spot weld as the new contact area is much larger. It also did not get hot while cranking. The break in the braid when burnt out might be a little hard to spot sometimes. Even off the car. You might have to be looking for it to find it.