Electrochemical AFM Study of Corrosion Behavior of a 2507 Super Duplex Stainless Steel: Influence of Precipitated Chromium Nitrides

摘要

Precipitation of different types of chromium nitrides may occur during processing of super duplex stainless steels (SDSS), affecting the properties of the material. In this study, by using specially heat treated samples, the influence of quenched-in and isothermal types of nitrides on the corrosion behavior of a 2507 super duplex stainless steel has been investigated, by a combination of microstructure characterization, electrochemical polarization at room temperature and at 90 ℃ in 1 M NaCl solution, in-situ electrochemical AFM measurements at room temperature and post-analysis of the corroded samples.The microstructure, including precipitated nitrides, has been characterized by scanning electron microscopy and magnetic force microscopy. The isothermal nitrides particles (ca. 80-230 nm in size) precipitated along the α/γ boundaries, exhibiting higher Volta potential than the ferrite and austenite, while a local Volta potential drop was observed at the α/γ boundaries. The Volta potential difference of the nano-sized quenched-in nitride particles (ca. 50-100 nm in size) finely dispersed in the ferrite could not be resolved by the Volta potential mapping due to their very small size.In-situ electrochemical AFM measurements of the samples in 1 M NaCl solution at room temperature show stable surfaces for a wide range of applied potentials. The nano-sized quenched-in or isothermal nitrides and a small amount of sigma phase in the 2507 SDSS didn’t cause passivity breakdown of the material. During transpassive dissolution at 1.2 V_(Ag/AgCl), the ferrite matrix with quenched-in nitrides dissolved preferentially, and the α/γ boundaries with isothermal nitrides and sigma phase started to corrode. The nano-sized quenched-in nitrides finely dispersed in the ferrite of the 2507 SDSS have little influence on the corrosion resistance. In the transpassive region, isothermal nitrides appear to be slightly more deleterious than quenched-in nitrides.At 90 ℃ (above the critical pitting temperature) in 1 M NaCl solution, the isothermal nitrides and a small amount of sigma phase largely reduce the corrosion resistance of the austenite phase. The passivity is lost and fast corrosion of the austenite occurred, probably due to depletion of Cr, N and Mo in the austenite phase caused by precipitation of the isothermal nitrides and sigma phase along the α/γ phase boundaries. In contrast, the finely dispersed quenched-in nitrides reduce the corrosion resistance of the material to a much lesser extent.The results suggest that the isothermal nitrides are more detrimental than the quenched-in nitrides for the corrosion resistance of the 2507 SDSS at elevated exposure temperatures. The different formation mechanisms, sizes and distributions of the quenched-in and isothermal nitrides cause different situations of alloying element depletion, and thus have different effects on the corrosion behavior of the material.