During the construction of the new boiler an industrial fiber optic borescope was available to look inside the old boiler as well as the new one under construction.  These photos show the corrosion that had grown on the flues of the old boiler.  It turns out that these corrosion growths occurred at the points were flues had leaked.  Thus the corrosion process, whatever its cause, weakened the wall structure of the flue and when exposed to pressure the flue developed a leak.