Influence of grain size on the corrosion resistance of lean duplex DX2202

摘要

Lean Duplex Stainless Steels (DSS) may be delivered in various forms, including hot rolled (HR) and cold rolled (CR) flat products. Disparities in corrosion resistance between these production stages have been observed, and microstructure parameters are suspected to explain it. This work aims at evaluating the influence of microstructure, especially grain size and grain boundary length, on the anodic dissolution rates in acidic media and on pitting resistance of Lean DSS DX2202, taking into account its biphased nature. HR and CR specimens of the same heat were studied. All of them were annealed and pickled. Additionally, different grain-coarsening heat treatments were carried out to obtain a larger spectrum of grain size. Electrochemical etching and optical microscopy, together with the creation of an image processing program, allowed evaluating grain size in each phase separately on the plane of the plate surface. Anodic dissolution rates were obtained from potentiodynamic tests in pH 1.0, 260 g.L~(-1) NaCl, and pitting resistance was evaluated by a statistical method in 0.5 M NaCL A linear evolution of the critical current of dissolution of ferrite as a function of grain size has been found and the role of grain boundary length is discussed. Furthermore, the contribution of other parameters related to heat treatment, such as nitrogen enrichment in austenite measured by Electron Probe Microanaryser (EPMA) and residual stresses determined by X-ray Diffraction (XRD) are debated.