Electrochemical Studies On Local Corrosion Of Two - Phase Stainless Steel 2304 And 2507 Welds

Duplex stainless steels (DSS), characterized by the two-phase microstructure of ferrite (a) and austenite (y), have been widely used because of the attractive combination of mechanical strength and corrosion resistance. Generally, the performance of DSSs largely depends on the balanced microstructure, which consists of approximately equivalent amounts of ferrite and austenite without undesirable precipitates. However, welding process in practical use will result in unbalanced microstructure and then impair the mechanical properties and corrosion resistance of DSSs. On the other hand, the crystal structures of a and y in DSSs are different and there are large amount of grain boundaries and phase boundaries, which lead to various distribution of alloy elements in two phases or boundaries. After undergoing the thermal cycle of welding (300℃～1300℃), different kinds of secondary phase such as carbides, nitrides, secondary austenite, sigma (σ) and chi (χ) will precipitate, which can also impair the corrosion resistance of DSSs. Therefore, a systematic study on the microstructure evolution, which includes phase proportion, distribution of alloy elements and precipitation of secondary phases, is in need. Based on many previous works, this article focused on a series of local corrosion which result from the thermal cycle process of DSSs’heat-affected zone (HAZ). Some novel methods including critical pitting temperature (CPT) and double loop electrochemical potentiokinetic reactivation (DL-EPR) are adopted to investigate the weakest phase in the complex microstructure of welding joint. The inner relationship between corrosion behaviors, microstructure evolution and welding procedure has been clarified. All these investigations serve as the scientific evidence for the microstructure control and procedure optimization, which are significant both in scientific investigation and industrial application.1. The microstructure evolution of plasma arc welding joint and pitting corrosion resistance of DSS2304are investigated by the CPT technique. The results reveal that the weakest area of welding seam is a-ferrite in the high temperature HAZ. The insufficient diffusion in the non-equilibrium welding process leads to lower chromium and molybdenum content in ferrite phase than base metal, which degrades the corrosion resistance of HAZ essentially. The influences of heat treatment on corrosion resistance are also studied and the optimized solution-treated temperature is confirmed at1080℃. To reveal the precipitating rule of nitride in HAZ, the specimens are electrochemically etched by oxalic acid solution and the microstructure shows that metastable pitting may initiate in nitride phase.2. The CPT method is also adopted to investigate the microstructure evolution and pitting behavior of2507super DSS under multi-pass welding. The influence of welding pass on microstructure and pitting resistance was analyzed deeply. The conclusion shows that single-pass welding can lower the corrosion resistance of the welding joint significantly, while the second pass and the third pass will heal the microstructure and improve the corrosion property. Furthermore, the influence of heat input on microstructure evolution and pitting resistance of2507super DSS is systematically investigated. As the heat input improved, pitting resistance increased firstly then decreased. When the heat input is2.5kJ/mm, the specimen acquires the best corrosion resistance with a CPT of77℃. The decrease of CPT may be caused by the precipitation of sigma phase.3. The technique of DL-EPR has been optimized to reveal the quantitative relation between intergranular corrosion sensitivity and precipitates in2507super DSS under900℃with different aging time. The optimized solution of DL-EPR test is2M H2SO4+1.5M HC1in30℃with a scan rate of1.5mV/s. The value of Ra, which can evaluate the intergranular corrosion sensitivity, increased firstly then decreased when aging time was increasing and the Ra reached to maximum when the aging time is4h. When sigma phase formed and extended, the chromium depletion area became larger, which resulted in the increase of Ra. Subsequently, the diffusion of chromium and molybdenum from the base metal healed the chromium depletion area, so the value of Ra decreased.