As a part of its environmental plan, EDF is fitting the 600 MW coal-fired power station located in Le Havre with a flue gas desulphurization unit.

When this FDG unit operates (namely when the power station is used at more than 40% of its capacity), it modifies temperature and composition of flue gases. Besides, both desulphurized and raw fumes are successively discharged through the same chimney. As a consequence, EDF has to protect ducts against acid corrosion (since acids may appear when the temperature falls below the dew point) with a material able to withstand a 140°C temperature with peaks of 345°C under exceptional conditions. EDF chose nickel base alloys which have an excellent track record in other countries.

For the construction, two options have been evaluated: « wallpapering » the existing ducts with 2 mm thick nickel base alloys sheets, or building new ducts from roll-clad material. The second solution has been selected because it allows prefabricated rings under controlled conditions. The new ducts have been built from the bottom up, inserting rings at the bottom of the chimney, and then raising up to allow the next rings to be installed. This means that only a single fixed welding station is necessary inside the chimney. Finally, one of the most challenging aspect of the duct building is the welding of roll-clad alloys dissimilar metal joints. This has been the aim of comparative corrosion tests performed with different weld types, weld processes and weld metals.