Blackening of Cast-Iron by Hydrothermal and Hot Alkaline Nitrate Treatments

In the current study, oxide layers were formed on gray cast-iron by hydrothermal and hot alkaline nitrate treatments. A uniform, homogeneous and protective magnetite layer was formed using the hydrothermal treatment on the cast-iron substrate, which had higher quality compared to the layer created by the hot alkaline nitrate treatment. The characteristics of the oxide layers were examined by X-ray diffraction (XRD), Field emission scanning electron microscope (FE-SEM), Oxalic acid test resistance, and Corrosion tests including: Open circuit potential (OCP), Potentiodynamic Polarization, and Electrochemical impedance spectroscopy (EIS). Investigations showed that the magnetite layer formed on the gray cast-iron substrate using the hydrothermal treatment passed the oxalic acid test and showed superior corrosion resistance. In the case of the oxide layer formed using the hot alkaline nitrate treatment, a lower corrosion resistance was observed due to the defective layer and also the simultaneous presence of hematite alongside with magnetite inside the oxide layer.