This research work investigated the effects of standard solution-annealed (SA) treatment on microstructure and chemical characteristics of Inconel 718 (IN718) superalloys fabricated using tungsten inert gas welding based wire-arc additive manufacturing (WAAM) process. A commercial wrought IN718 product was also studied for comparison. It was observed that WAAM parts developed highly-textured large columnar austenite (γ) grains along the build-up direction due to the epitaxy and high thermal dissipation rate. The micro-segregation of Nb produced an inhomogeneous γ-matrix precipitating non-hardening laves phases as a result of constitutional undercooling, high energy input and slow cooling rate of WAAM process. After the solution treatment, SA sample remained to be highly-textured solid-solution of γ-matrix and formed non-hardening delta (δ) plates with undissolved residual laves phases in Nb-enriched regions. These deprived the precipitation of beneficial hardening phases of γ″ and γ′ particles in γ-matrix during SA treatment. The high-temperature oxidation study of SA-treated samples obeyed parabolic rate law which was independent of oxidation temperatures. The oxidation mechanisms in the formation of oxide scale were found to be temperature dependent. At 800 °C, only a thin and continuous layer of Cr2O3 scale was formed externally at the air-alloy interface. A multi-layer of external Cr2O3 at air-scale interface, inner Nb-rich complex Ti0.67Nb1.33O4 scale at scale-alloy interface and internal sub-scale of Al2O3 was formed at 900–1000 °C. Due to the high lattice diffusivity of Mn+2 and Ti+4 cations through Nb-rich complex layer and Cr2O3 scale, a thin layer of TiO2 and MnCr2O4 was formed at the outermost part of Cr2O3 scale at 1000 °C. The oxidation reaction process of scale growth in IN718 alloys was mainly dominated by the outward diffusion of Cr+3 cations through Cr2O3 layer at the air-scale interface as the activation energies were found to be in the range of 250–290 kJ/mol for Cr2O3 (chromia) forming Ni-based superalloys.