Melatonin as an Accelerating Agent for Phosphate Chemical Conversion Coatings on Mild Steel with Enhanced Corrosion Resistance

Melatonin (MEL), as a new accelerating agent, is applied for phosphate chemical conversion coating on mild steel. Scanning electron microscopy has been applied to observe the morphology of phosphating crystals, showing that the incorporation of MEL results in more dense and uniform phosphate coatings. Electrochemical impedance spectroscopy and potentiodynamic polarization are performed to evaluate corrosion resistance of phosphate coatings. The results show that the corrosion rate of blank phosphate coating is 5.64 mpy, while the corrosion rate of phosphate coating accelerated with 1.25 g l −1 MEL is reduced to 0.04 mpy. In addition, the dry friction test shows that friction coefficient of the phosphate coatings with the introduction of MEL is decreased to 0.3 compared to 0.6 of the blank one, proving that MEL also reduces the friction resistance of phosphate coatings along with improved corrosion resistance.