Unveiling the corrosion mechanism of nickel-aluminum bronze coatings on steel fabricated by wire-arc directed energy deposition from selective phase corrosion to uniform corrosion evolution

Abstract

In this work, the microstructural evolution and corrosion behavior of nickel aluminum bronze (NAB) deposition layers deposited at different distances from the steel substrate using wire-arc directed energy deposition (WA-DED) were investigated. Due to the undercooling of the organization, as the distance from the substrate was increased, the grain characteristics of the α-Cu matrix transitioned from equiaxed grains to columnar grains. Because of the addition of Fe and Cr elements and repeated heating, the bottom region (BNAB) was shown to be more complex, comprising an α-Cu matrix, β′-Cu, and various κ phases. As the samples were gradually moved away from the substrate, the content of intermetallic compounds was decreased, showing a more uniform microstructure. The enrichment of Fe and Cr in the precipitates of BNAB increased the potential volta difference between the phases, leading to exacerbated selective phase corrosion. The higher metal element (Al and Ni) content and dislocation densities of the top deposition layer were found to promote passivation behavior and reduce the density of point defects in the Cu₂O film of the top region (TNAB). The Al₂O₃ film was formed on the nano-sized κ_Ⅱ-phase rapidly, protecting the interdendritic region of TNAB. Therefore, the uniform distribution of Al₂O₃ film and the lower potential difference between different phases result in uniform corrosion of TNAB.