Effect of Mn and Cr on the selective oxidation, surface segregation and hot-dip Zn coatability

The effect of Mn and Cr on the surface selective oxidation, segregation and hot-dip galvanizing was studied in three Fe-Mn alloys with ∼0.2 wt% Al and two Fe − Mn alloys with ∼0.5 wt% Cr in an HDPS (Hot-dip Process Simulator) following industrial CGL (Continuous Galvanizing Line) conditions. The Mn content was varied from 1.5 to 3.6 wt% in the alloys. The alloys were subjected to the same thermal cycle following continuous annealing (CA) at 800 °C in N2 − 5%H2 gas atmosphere with −40 °C dew point and then hot-dipping in Zn - 0.20 wt% Al bath at 460 °C. The oxidation tendency of Mn, Al and Cr during annealing was calculated using FactSage software. GDOES (Glow Discharge Optical Emission Spectroscopy) measurements showed a higher extent and depth of segregation for Mn with increasing bulk concentration during the CA. It was observed that Cr segregates comparatively less near the annealed surface in the presence of more Mn. XPS (X-ray Photoelectron Spectroscopy) measurements revealed the formation of simple MnO in Fe-Mn-Al alloys and complex MnCr2O4 in Fe-Mn-Cr alloys. Contrary to the thermodynamic calculations, no Al-oxides were detected at the annealed surfaces of Fe-Mn-Al alloys during the XPS study. The hot-dip Zn coatability was determined by measuring the number density of uncoated spots using 3D CLSM (Confocal Laser Scanning Microscopy) and by visual inspection. The Fe-Mn-Cr alloys displayed a better hot-dip Zn-coating compared to Fe-Mn-Al alloys. The conclusions of the present study are also critically discussed considering the literature findings.