The corrosion behaviour of manganese phosphate coatings applied to AISI 4140 steel subjected to different heat treatments

This paper aims to investigate the corrosion behaviour of manganese phosphate coatings on AISI 4140 after various heat treatments. Prior to coating, quenching and tempering treatments were applied to the substrates prepared for this process. The structural and mechanical properties of nonheat-treated and heat-treated substrates were determined using hardness test, optical microscope, scanning electron microscopy (SEM), and X-ray diffraction (XRD). The changes of hardness, microstructure and phase density of heat-treated substrates before the coating were also observed. Manganese phosphate coatings were applied to heat-treated substrates in a coating bath at a temperature of 90 °C, and electrochemical polarisation measurements of the phosphate-coated substrates were carried out. It was observed that the corrosion potential of manganese-phosphate-coated substrates, which followed the quenched treatments, were higher than that of the tempered substrates. The highest corrosion potential was obtained for the coatings on water-quenched (WQ) substrates. It was found that the heat treatment applied to the substrates yield an important improvement in the corrosion resistance of the coatings on substrates.