| Supercritical thermal power units have become the main power units of China and the capacity is the largest in the world. But with the improvement of thermal power unit parameters, boiler four tubes (water wall tubes, reheater tubes, superheater tubes, economizer tubes) are more sensitive to ionic impurities in the water, and trace impurity ions in the water may cause corrosion of boiler tubes, if the feed water impurity ion concentration exceeds the allowable value, the corrosion and fouling of boiler tubes will be accelerated, and even lead to burst pipes. Therefore, to research water impurity ions on the corrosion of furnace tubes has an important significance on the safe and economic operation of the unit.In this paper, the conventional materials of supercritical boiler water wall tubes (15CrMoG,12CrlMoVG, T12 and T23) were used for the study, and the influence of Cl- and SO42-ions on the pitting corrosion properties of the four kinds materials in the simulated All-Volatile Treatment (AVT(O)) and Oxygenated Treatment (OT) solutions was studied by potentiodynamic polarization and AC impedance technique. The pitting corrosion properties of T23 material in the simulated All-Volatile Treatment (AVT(O)) were also studied by electrochemical noise and coupon experiments for assistant researches, and analysed by reflection microscope, X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS) for the chemical constituents of the oxide film and pitting corrosion mechanism. The main conclusions were as following:(1) At AVT(O) condition, Cl- and SO42- could induce the pitting corrosion of 12CrlMoVG, 15CrMoG and T23. and the pitting susceptibility of T12 itself is strong, so it is easy to form pitting corrosion in the absence of Cl- and SO42". The descending order of corrosion susceptibility on Cl- of four kinds of materials was T23> T12> 12CrlMoVG> 15CrMoG, and the descending order of corrosion susceptibility on SO42-was T23> T12> 15CrMoG> 12CrlMoVG.(2) At OT condition, the presence of Cl- and SO42- could strongly induce the pitting corrosion of T12 and T23, and weakly of 12CrlMoVG and 15CrMoG. The order of the corrosion resistance of four kinds of materials was T23> 15CrMoG> T12> 12CrlMoVG, and the descending order of corrosion susceptibility on SO42 was 15CrMoG> T23> T12> 12Cr1MoVG.(3) Cl- and SO42- absorbed on metal surface competing with OH- and hindering the inactivation of OH- acting on metal.(4) After the electrode was immersed in the solution, the pitting corrosion of T23 was passing into pitting induction period and growth period between 0 and 1500 seconds. After 1500seconds, the pitting was passing into a stable period.(5) Under high temperature, Fe3O4 oxide film was formed on the surface of T23 specimen. Cl-and SO42- could promote the dissolution of T23, and accelerate the corrosion of T23, but could not cause a change in the component of surface oxide film. |
