Deposition of magnetite nano-particles on gray cast iron via hot alkaline and hydrothermal conversion methods: with regarding corrosion behaviour

In this study, the formation of Fe 3 O 4 nanoparticle coatings on gray cast iron was investigated. Two blackening methods, including a hot alkaline (open system) and a hydrothermal method (closed system), were employed. The hot alkaline method faced challenges due to the presence of near-surface graphite, leading to the formation of a porous and defective coating containing significant amounts of hematite (Fe 2 O 3 ), as confirmed by thermodynamic calculations. In contrast, a dense and uniform high-quality Fe 3 O 4 coating was produced by the hydrothermal method, and the issues related to surface graphite were effectively resolved by converting it into carbon-based nanomaterials inside the autoclave. Quantitative comparison showed that the hot alkaline coated specimens had i corr = (2.11 ± 0.04) × 10 -6 A·cm -2 , bond strength = 1.34 ± 0.04 MPa, and R a ≈ 1.42 ± 0.36 μm, whereas the hydrothermally treated samples exhibited i corr = (1.54 ± 0.03) × 10 -6 A·cm -2 , bond strength = 2.56 ± 0.03 MPa, and R a ≈ 40.36 ± 1.2 nm, indicating markedly improved corrosion resistance, adhesion, and surface quality. The resulting coatings were characterized using XRD, Field Emission Scanning Electron Microscopy (FE-SEM), Atomic Force Microscopy (AFM), Pull-off adhesion test, and corrosion tests, including Open Circuit Potential (OCP), potentiodynamic polarization, and Electrochemical Impedance Spectroscopy (EIS). A schematic illustration of mechanistic characteristics of hot alkaline (open system) and hydrothermal (close system) blackening treatment. • Hydrothermal method converts surface graphite to uniform Fe 3 O 4 coating. • Produces dense, crack-free Fe 3 O 4 with high acid resistance and fine crystals. • Shows higher R ct , lower i corr , and better chloride ion resistance. • Hydrothermal coating has stronger adhesion than hot alkaline method. • Long-term durability offsets slightly higher initial process cost.