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

摘要

This paper deals with results from corrosion examination on duplex cast stainless steels from original components in the flue-gas desulphurization (FGD) of the VEAG-power stations Boxberg III and Jaenschwalde under practice and laboratory conditions. In practice the mentioned components have a better corrosion resistance in the FGD medium of the Boxberg power station. The same results were obtained by laboratory examinations, getting a shift in critical potentials of a few hundred millivolts. The tests in a by-pass of the FGD confirm these results in tendency. However, partially significant differences to laboratory examinations were established on specimens with pre-damaged original surfaces. This. can he traced back to the more severe test conditions in the FGD, like solids and flow. The repassivation behaviour of the materials under investigation was examined by means of electrochemical noise analysis. The values of the free corrosion potentials in the Jaenschwalde power station were higher than potentials, necessary for a safe repassivation of occurring pits. This cause the failures on the components. Remedy steps are discussed in this paper in detail. The free corrosion potential in the Boxberg power station is about 200 mV below the repassivation potential. Because of that, short-time fluctuations of system parameters leading to enhanced corrosion risk can be tolerated.