Studies on Intergranular Corrosion Characteristics of Cast Duplex Stainless Steel Stabilized with Niobium

To study intergranular corrosion (IGC) characteristics, two cast grades of duplex stainless steels, 5A and 5A stabilized with niobium, were produced in a vacuum induction melting furnace. The samples were tested under three different heat-treatment conditions that were selected using a property diagram generated through the TCFE8 database of the Thermo-Calc (R) software (Thermo-Calc Software, Solna, Sweden). A Feritscope FMP30 (R) (Fischer Technology, Inc., Windsor, CT) was used to measure the ferrite phase content in the samples and compared with that specified in the property diagram. Metallurgical characteristics were analyzed through optical microscopy, scanning electron microscopy, energy dispersive spectroscopy, and X-ray diffractometry. Each heat-treated sample possessed a distinct texture due to the formation of various intermetallic phases in the alloy. The corrosion behavior of the samples was studied using an oxalic acid etch test. Double-loop electrochemical potentiokinetic reactivation and sulfuric acid tests were conducted on the niobium-stabilized samples, and the results demonstrated that the samples possess superior resistance to corrosion compared to those not stabilized with niobium. The addition of niobium produced a mixture of niobium compounds that suppressed the chromium depletion and reduced the degree of sensitization. This study demonstrated the potential of using niobium as a stabilizer in the duplex stainless steel alloy to considerably reduce IGC.