Research On Corrosion Behaviour Of Super Austenitic Stainless Steel With High Mo And N In Typically Extreme Environments

The equipment of FGD (flue gas desulphurization) and wet process phosphoric acid suffer harsh corrosive environment, and nickel-based alloys are often used at preshent. Super austenitic stainless steel with high Mo and N (SASS) has excellent corrosion resistance and superior mechanical properties, and is expected to replace nickel-based alloys used in these harsh corrosive environments. In this paper, as the main object of study, the pitting corrosion and uniform corrosion resistence of SASS was studied, in comparasion of316L stainless steel and C-276nickel-based alloy widely used in FGD and wet process phosphoric acid equipment, to evaluate its application prospect. The pitting corrosion behavior and characteristic of SASS in simulated FGD and wet process phosphoric acid environment was studied using open circuit potential monitoring, potentiodynamic polarization, potentiostatic polarization, electrochemical impedance spectroscopy and Mott-Schottky analysis. The study was aimed at the impact of temperature on corrosion behavior of SASS, and revealing the failure mechanisms. And the structure of passive film formed on SASS in simulated FGD and wet process phosphoric acid environment was studied using XPS analysis, and revealing the mechanisms of corrosion resistence. Based on the study, the conclusions could be obtained as following.The pitting corrosion and uniform corrosion resistence of SASS is significantly better than316L, and because of its high N, Mo content, SASS has even better corrosion resistence than C-276. And SASS is expected to be used to harsh environment, such as inlet of flue gas and gas-gas heater (GGH) in FGD, and enrichment process of H3PO4in wet process phosphoric acid.With the increase of temperature, the OCP value and corrosion potential of SASS in simulated FGD and wet process phosphoric acid environment increases, and corrosion current density increases because of temperature favers cathodic reaction. Simultaneously, temperature also favors the anodic reaction, and accelerates the permeation rate of aggressive ions into passive film, and leading to the break of passive film, increasing of corrosion current density and droping of pitting potential. However, the cyclic polarization results show that area of hysteresis loop diminishes with the increase of temperature, indicating that the pitting susceptibility of SASS in simulated wet process phosphoric acid environment is low. Potentiostatic polarization results of SASS in simulated FGD and wet process phosphoric acid environment show that the current density decreases with the increase of time, and becomes stabilizing (Iss) because of the equilibrium of generation and dissolving of passive film. And Iss increases with temperature, and fluctuates significantly at80℃, which relates to the formation and repassivation of stable or metastable pitting. The pitting incubation period shortens and passivation index n decreases with the increase of temperature, indicating the protection performance and growth rate of passive film decreases.EIS analysis shows that impedance of passive film depends on temperature strongly, and resistor of passive film and polarization resistance of SASS decrease with the increase of temperature, indicating the stability of passive film deteriorate. In addition, the protection of passive film on metal substrate is mainly the dense inner layer. Mott-Schottky analysis of the passive film shows an n-type semiconducting behavior below0.6VSCE, and a p-type semiconducting behavior above0.6Vsce in FGD environment. However, it shows a p-type semiconducting behavior in simulated wet process phosphoric acid environment. The donor density in passive film is all of the order1021cm-3, and increases with temperature, indicating the increase of electron density, which agrees well with the increase of current density Ip and Iss with temperature in polarization experiment.The XPS analysis of passive film formed on SASS shows formation of difficult soluble Cr2O3could restrain the dissolve of iron. By forming denser oxide, Mo prevent cations through the passive film, and reduce the corrosion rate effectively. By forming NH3, Ligand and MH4+, N could consume H+, and improve local corrosive environment. And the passive film shows a bilayer structure, which are Cr-rich outer layer and Fe-rich inner layer.