Hydrothermal blackening on AISI 4140 steel with and without hydrazine hydrate

Abstract

Abstract The blackening coating is a conversion coating with the black colour of crystallised magnetite (Fe 3O4) on the surface of steel. The coating is to be soaked with oil or wax to prevent the corrosive environment. Recently, the technique to synthesise the black coating, hydrothermal method is introduced. In the synthesis process, hydrazine hydrate (N2H4·H2O) is often recommended as a catalyst but it is also reported toxicity. In this work, AISI 4140 was hydrothermally immersed in the FeSO4·7H2O added NaOH solution with and without addition of hydrazine hydrate. During the process, the temperature and pressure of the chamber were controlled at 140°C and 2 bar, respectively. To confirm the formation of Fe 3O4, the samples were characterised by SEM and XRD. These results showed that the coatings were composed of octahedron morphology in all conditions and the thicknesses of the coating with hydrazine hydrate concentration of 0.0, 0.1, and 0.5M were 7.05, 9.25, and 8.75 μm, respectively. With hydrazine hydrate, the Fe3O4 layer can be easily formed. However, the results of the oxalic acid spot test and potentiodynamic polarisation indicated the equivalence in quality of the coating, despite the absence of hydrazine hydrate.