Corrosion behavior of hybrid boride–aluminide layers grown on selective laser melted Inconel 718

The geometric freedom afforded by Selective Laser Melting (SLM) is increasingly driving the use of superalloy in advanced engineering applications. However, defects inherent to SLM, such as microsegregation, unmelted particles, and microporosity, weaken the alloy's passive film continuity in chloride-containing environments, limiting its corrosion resistance. In this study, the microstructural evolution and electrochemical corrosion behavior of boron-aluminide coatings grown on SLM-Inconel 718 alloy through aluminizing (SSA), boronizing (SSB), simultaneous boroaluminizing (CBA), and sequential aluminizing-boronizing (SAB) and boronizing-aluminizing (SBA) processes were comparatively investigated. The coatings were applied at 980 °C using the pack boronizing/aluminizing method, and phase formations were analyzed by XRD, while layer morphology and elemental distributions were analyzed by SEM-EDS. Multiphase aluminide structures containing NiAl, Ni₃Al, FeAl₂, and Al₂O₃ were observed in the aluminizing process; Ni₂B, CrB, and FeB boride zones were observed in the boronizing process; and boride-aluminide mixture phases grown with varying degrees of homogeneity and continuity in the CBA, SAB and SBA processes. Layer continuity was most compact in the CBA and most disordered in the SAB. OCP and Tafel tests in 3.5% NaCl solution showed that corrosion behavior is directly related to layer integrity and porosity. According to the quantitative results, the corrosion current density was as follows: SBA < CBA < SSA < SSB < as-built IN718 < SAB. The highest performance was achieved in the SBA coating, with Icorr values ​​of 1.73 × 10⁻⁶ A/cm² and Rp of 3.06 × 10⁴ Ω being achieved thanks to the synergistic effect of the mechanical stability of the boride layer and the dense and adherent Al₂O₃ barrier formed by aluminizing processes.