Electrochemical investigations on the corrosion behaviour of duplex cast stainless steels in flue-gas desulphurisation plants

This paper deals with results from corrosion examination on duplex cast sta inless steels from original components in the flue-gas desulphurization (FG D) of the VEAG-power stations Boxberg III and Jaenschwalde under practice a nd laboratory conditions. In practice the mentioned components have a bette r corrosion resistance in the FGD medium of the Boxberg power station. The same results were obtained by laboratory examinations, getting a shift in c ritical potentials of a few hundred millivolts. The tests in a by-pass of t he FGD confirm these results in tendency. However, partially significant di fferences to laboratory examinations were established on specimens with pre -damaged original surfaces. This can be traced back to the more severe test conditions in the FGD, like solids and flow. The repassivation behaviour o f the materials under investigation was examined by means of electrochemica l noise analysis. The values of the free corrosion potentials in the Jaensc hwalde power station were higher than potentials, necessary for a safe repa ssivation of occurring pits. This cause the failures on the components. Rem edy steps are discussed in this paper in detail. The free corrosion potenti al in the Boxberg power station is about 200 mV below the repassivation pot ential. Because of that, short-time fluctuations of system parameters leadi ng to enhanced corrosion risk can be tolerated.