By: W.K. Chan, C.T. Kwok, K.H. Lo
Department of Electromechanical Engineering, University of Macau
Duplex stainless steels (DSSs), with a dual phase ferrite–austenite microstructure, have been widely used as the structural components for chemical industries, oil and gas production and offshore applications due to its outstanding corrosion resistance in chloride-containing environments. However, depletion of chromium and molybdenum is a well-known problem in DSSs, especially in welded assemblies, hot-worked and improperly heat-treated parts heated at temperatures ranging from 500 to 900°C. As the sigma phase forms, it depletes the neighboring regions of Cr and Mo, thereby lowering the Cr and Mo contents of the surface oxides over these regions. Then the DSSs become susceptible to corrosion attack. Laser surface melting (LSM) is a possible route to restore the corrosion resistance of DSSs by melting the surface with a laser beam followed by rapid solidification for homogenizing chemical compositions. Different laser parameters will cause different phase distributions in the DSSs, and hence various corrosion properties may be resulted. In present study, microstructure and corrosion behavior of aged DSS 7MoPlus (UNS S32950) were investigated in 3.5% NaCl solution at 25 oC before and after LSM. By LSM, the corrosion resistance of aged 7MoPlus was found to be significantly enhanced as reflected by the increase in the corrosion and pitting potentials and decrease in corrosion and passive current densities. The improvement in corrosion resistance of the aged 7MoPlus after LSM is mainly attributed to re-dissolution and removal of the s phase.