Corrosion Characteristics of Newly Designed Inconel Alloy Weldments Using Varying Nb Content in Heat Treatment

In this study, Inconel 625 weldments with different Nb contents were prepared via shielded metal arc welding followed by aging heat treatment. The microstructures of the resulting weldments were observed using scanning electron microscopy/energy-dispersive X-ray spectroscopy. The fabrication process used in this study resulted in the partial formation of the gamma '' (750 degrees C), delta (850 degrees C), and M6C carbide (950 degrees C) phases in the weldments. The electron backscattering diffraction results revealed that with an increase in the Nb content, the secondary phase fraction of the weldments increased. Potentiodynamic polarization tests were carried out to evaluate the pitting corrosion resistance of the weldments. With increasing Nb content of the weldments, the fraction of the passive region increased at 750 and 850 degrees C. The corrosion and formal potentials of the weldment with 4.26 wt% Nb decreased after the heat treatment at 950 degrees C, resulting in a relatively small passive region. Double loop-potentiokinetic reactivation tests were carried out to evaluate the sensitization of the weldments to interphase corrosion. The degree of sensitization increased with increasing Nb content when the heat treatment was carried out at 750 and 850 degrees C. The delta-phase showed lesser effect on the interphase corrosion resistance of the weldments than the gamma '' phase. The specimen with 4.26 wt% Nb heat-treated at 950 degrees C showed high degree of sensitization to interphase corrosion.