refinement, the moving weld pool continuously
sweeps selected constituents ahead of it, and the
concentration of these selected constituents in
the pool increases continuously until the welding
is stopped and the pool solidifies. The center
of the stop point is attacked rapidly in oxidizing
acids, such as nitric acid, in a form of self-accelerating
crevice corrosion. The final solidification
pool need not extend through the thickness
of the part for full perforation to occur, as
described in the following example.