Corrosion performance of wire arc additively manufactured NAB alloy

Nickel-aluminum bronzes (NAB) are vital alloys, known for biofouling resistance, crucial for marine and shipbuilding industries. This study examined corrosion performance of NAB samples fabricated by wire arc additive manufacturing (WAAM) in as-built and heat-treated conditions. Microstructural analysis revealed the WAAM-NAB parts primarily consisted of the alpha-phase (copper) and three types of kappa-phases: kappa II (spherical Fe3Al), kappa III (Ni-Al in lamellar shape) within the interdendritic areas, and iron-rich kappa IV particles dispersed throughout the matrix. In contrast, casting-produced NAB showed the formation of a rosette-like kappa I phase as well. Corrosion behavior comparisons between the two NAB fabrication methods were also assessed. The microstructural characterizations revealed a rise in the size of the kappa IV particles after heat-treated at 350 degrees C for 2 h (HT1). Heat treatment at 550 degrees C for 4 h (HT2) resulted in a needle-like kappa V, coarsening of kappa II, partial spheroidization of kappa III, and reduced kappa IV precipitation. When heat-treated to 675 degrees C for 6 h (HT3), kappa II and kappa V were coarsened, kappa III was completely spheroidized, and kappa IV precipitation was significantly reduced. These microstructural features in HT2 and HT3 conditions steeply decreased their corrosion resistance compared to the WAAM as-built part. The as-built WAAM sample showed superior corrosion resistance in chloride solution, attributed to fewer kappa-intermetallic phases and a finer microstructure. The kappa-phases, irrespective of morphology, act as the cathodic areas versus the alpha-dendritic matrix, fostering microgalvanic cell formation. Consequently, precipitation of all cathodic kappa-phases draws a higher galvanic current of the anodic alpha-phase, meaning a lower corrosion resistance.