Corrosion behavior of Zr-1Sn-0.35Fe-0.15Cr-0.10Nb-0.03Ge alloy in 400℃ super-heated steam with oxygen

Abstract:

In order to explore the corrosion behavior of Zr-1Sn-0.35Fe-0.15Cr-0.10Nb-0.03Ge (mass fraction, %) alloy in 400 ℃ super-heated steam with different oxygen contents, Zr-1Sn-0.35Fe-0.15Cr-0.10Nb-0.03Ge alloy specimens were put into static autoclave and dynamic autoclave for super-heated steam corrosion tests under deaeration, 300 ppb dissolved oxygen (DO) and 1000 ppb DO environment at 400 °C/10.3 MPa, respectively. The scanning electron microscope, transmission electron microscope, X-ray photoelectron spectroscope were used to characterize the microstructure of alloys and oxide films, as well as the valence state of the alloying element. Results show that compared with the deaeration environment, the average corrosion rate of Zr-1Sn-0.35Fe-0.15Cr-0.10Nb-0.03Ge alloy in 300 ppb DO and 1000 ppb DO environments increases by 23.5% and 29.4%, respectively, indicating that DO can accelerate the corrosion of the alloys, and the higher the DO content, the more obvious the corrosion acceleration effect. DO not only promotes the oxidation of Fe, Cr, Sn and Ge in the oxide film, but also accelerates the reaction process of Zr→ZrOx→ZrO2 in the transition layer at the O/M interface. Based on the above two aspects, a mechanism of DO accelerating the corrosion of zirconium alloys at 400 °C/10.3 MPa is proposed: The increase in DO content in the corrosive environment leads to an increase in the concentration of O2- and OH- participating in the reaction in the oxide film. On one hand, this promotes the oxidation of alloying elements, leading to the increase of defects in number and local stress in the oxide film; on the other hand, this also accelerates the evolution of Zr→ZrOx→ZrO2 at the O/M interface, resulting in higher stress in the oxide film and providing less time for stress release during oxidation. The roles of the two aspects both promote the generation of pores and cracks, and accelerate the diffusion of O2- and OH-, thus accelerate the corrosion of zirconium alloys.

Key words:zirconium alloy; corrosion; DO; microstructure