This photo, looking down a group of flues, shows the sediment and corrosion that has collected at the bottom of the boiler.  It is this sediment and corrosion that when allowed to build up excessively acts as an insulator.  As an insulator the corrosion and sediment reduces the heat transfer from the flue sheet steel of the boiler to the water in the boiler and results in the steel overheating.  The steel of the flue sheet then expands more than the steel flue expands and leaking develops where the flue runs through the hole in the flue sheet.