Production of magnetite thin film over steel substrate using hot alkaline nitrate blackening method

Magnetite thin film obtained from the hot alkaline nitrate blackening method has been utilized for the protection of steel substrate since the 1900s. Even though this conversion coating process has been around for decades, its application is still limited to corrosion resistance in the firearms and the fasteners industry. Furthermore, there is a lack of literature that systematically analyse the effect of the parameters involved in the blackening process. For this reason, this study aims to investigate the effect of varying the blackening bath’s nitrate concentration, temperature and dwell time on the properties of the magnetite coating, especially that of wear resistance. In order to produce the magnetite coating, mild steel specimens were placed in a boiling solution made up of sodium hydroxide and sodium nitrate dissolved in water. The blackened specimens were then characterised using a battery of test; microstructure and composition analysis using a reflecting microscope and a scanning electron microscope, pin-on-disk test to analyse the wear resistance and tribological properties and corrosion resistance test using potentiodynamic polarization scanning. Consequently, it was found that the magnetite coating improves the wear resistance and corrosion properties of the steel substrate. In addition, the best magnetite coating is obtained using a blackening bath with a nitrate concentration between 25 % and 50 %, at a temperature of 140 °C and with a dwell time between 15 minutes and 30 minutes