Selective dissolution and corrosion fatigue behaviors of 2205 duplex stainless steel in H2SO4/HCl solution

• Main Authors: I-Hsuang Lo, 羅亦旋
• Other Authors: Wen-Ta Tsai
• Format: Others
• Language: zh-TW
• Published: 2007
• Online Access: http://ndltd.ncl.edu.tw/handle/02356044728598457640

博士 === 國立成功大學 === 材料科學及工程學系碩博士班 === 95 === Abstract The purpose of this study was to investigate the selective dissolution and corrosion fatigue behaviors of 2205 duplex stainless steel (DSS) in H2SO4/HCl mixed acid solution. Due to the differences in chemical composition and crystalline structure between a and r phases, the corrosion resistance and mechanical properties were dissimilar. Potentiodynamic polarization measurement and surface morphology analysis show that selective dissolution of one of the constituent phases of 2205 DSS could occur at certain characteristic potentials in H2SO4/HCl solution. Furthermore, selective dissolution would affect the corrosion fatigue behavior of 2205 DSS at various loading frequencies. Potentiodynamic polarization measurement indicated that there were two distinctly separated anodic peaks appeared in the active-to-passive transition region of 2205 DSS in H2SO4/HCl solution. After potentiostatic etching and surface morphology analysis, the anodic peak appeared at lower potential was associated with the preferential dissolution of a phase while that for r phase at a higher anodic peak potential. In the solutions with the concentration ranging from 0.25 to 2 M for H2SO4 and HCl, the anodic peak potential and the corresponding current density for selective dissolution of each phase were greatly influenced by the change of electrolyte concentration. However, the evolution of separate anodic peaks became less distinguishable when the concentrations of HCl exceeded 1.2 M. At open circuit potential (OCP), galvanic corrosion resulting from the potential difference of the constituent phases caused the enhanced corrosion of ferrite at the ferrite/austenite phase boundary. Under selective dissolution and cyclic loading condition, stress concentration sites formed in the phase preferentially corroded, which induced by the intrusion/extrusion fatigue mechanism, would be eliminated by selective dissolution. Therefore, corrosion fatigue crack was prone to initiate in the phase with lower dissolution rate. At OCP and at the potential where a�� phase selectively dissolved, corrosion fatigue crack initiated in r phase. On the contrary, corrosion fatigue crack initiated in a phase at an applied potential at which r phase dissolved preferentially. The effect of loading frequency on corrosion fatigue behavior of 2205 DSS was complicated under selective dissolution condition. At higher loading frequency, selective dissolution would eliminate stress concentration sites (or fatigue crack initiation site). Therefore, corrosion fatigue crack would not initiate in the concave phase. The cracks mainly propagated transgranularly. At lower loading frequency, corrosion fatigue crack could also initiate and propagate along a/r boundary at potential where a phase selectively dissolved. At the potential where r phase selectively dissolved and at a lower loading frequency, corrosion fatigue crack initiated and propagated transgranularly in either a or r phase depending on the extent of selective dissolution of r�� phase. The intensity of stress concentration developed was the main factor affecting the corrosion fatigue crack initiated in r phase or not. The initially transgranular mode of fracture surface became faceted as the crack continued to grow. This was attributed to the selective dissolution of either a or r phase under controlled potential condition.