Understanding the nature of passivation film formed during corrosion of Fe39Mn20Co20Cr15Si5Al1 high entropy alloy in 3.5 wt% NaCl solution

In metallic materials, the nature of passivation film plays an important role in governing corrosion behavior. The present work focuses on a detailed analysis of the passivation film characteristics of a transformation induced plasticity Fe39Mn20Co20Cr15Si5Al1 (at%) high entropy alloy (HEA) in 3.5 wt% NaCl solution at room temperature. The characteristics of the passivation film were assessed using a potentiodynamic polarization test, electrochemical impedance spectroscopy (EIS), and X-ray photoelectron spectroscopy (XPS). The microstructural analysis of the alloy achieved using electron backscattered diffraction revealed that it consisted of two different phases: γ phase (face-centred cubic crystal) and ε (hexagonal closed pack), with the area fraction of ε phase being 96.5%. The electrochemical experiments showed that the HEA maintained excellent uniform corrosion resistance but low pitting corrosion resistance. The scanning electron microscopy revealed deep pits covered with perforated layers, and the average pit size and pit depth was determined to be 202.8 µm and 168 µm, respectively. The EIS revealed a bilayer type of passivation film formations at different voltages. For the passivation film grown at −0.1 V, the upper layer film thickness was 2.08 nm and the inner layer film thickness was 3.14 nm. For the passivation film grown at −0.05 V, the upper and inner film thicknesses were 1.78 nm and 3.10 nm. The passivation film resistance was found to be one order of magnitude higher for the passivation films grown at −0.1 V and −0.05 V than that for the naturally grown passivation film. The XPS of the sample that corroded during the potentiodynamic polarization test revealed that the passive layer contained Fe, Mn, Co, Cr, Si, and O elements as metal oxides and hydroxides with Mn and Cr based chemical species present as dominant species. Their oxidation states were estimated to be present as Fe2+, Fe3+, Mn3+, Co2+, Cr3+, Si4+, O2-, and OH1- along with their corresponding metallic states. According to the proposed corrosion mechanism, a bilayer type of passivation film forms on the alloy. The film’s upper layer is rich in Mn and Cr and the inner layer is devoid of Mn and Cr. The detection of two particles in the same pit with two different chemical compositions seems to corroborate the formation of a bilayer passivation film and its chemical compositions.