Analysis of the microstructure and corrosion behaviour of electrodeposited Fe and Fe alloys

Corrosion is material deterioration owing to materials' interaction with their surroundings. When the surface of metals/alloys is converted to a stable form having its oxide, hydroxide or sulphide state, corrosion progression (the deterioration of the material) could be ceased. This process is called passivation which is the main point of this thesis. This thesis aimed to obtain an iron-based alloy coating that can be passive. As Zn is a passivable element, it was used with Fe for alloy coating in this research. Fe and Fe alloys (Fe-Zn, Fe-Mn, Fe-Mn-Zn) have been electrodeposited potentiostatically on AISI 4140 steel in aqueous and ionic liquid media for corrosion resistivity. Ionic liquids containing solely anions and cations (without solvent) have been employed as metals and alloys electrodeposition baths. The corrosion behavior of resulting films obtained in ionic liquid media was compared with that of the films electrodeposited from aqueous solutions. A scanning electron microscope, an energy dispersive spectrometer, X-Ray Diffraction and a Fourier Transform Infrared Spectrometer were used to characterize the surface morphologies, compositions, and structures of the synthesized coatings. The corrosion behavior of the alloy coatings was determined in NaCl solution by linear sweep voltammetry. Bulk steel material itself is oxidized at the potential window in which metals/alloys fabricated in this work oxidize in NaCl solution. Therefore, inert substrate (graphite and platinum) was coated by the metals/alloys and analyzed in terms of corrosion behavior. It was found that growth conditions have a considerable effect on the corrosion properties of coatings. Fe, Mn and Fe-Mn obtained from ionic liquid deposition bath could have pitting-like corrosion. Zn based coatings (pure Zn, Zn-Fe, Zn-Mn or Zn-Fe-Mn) electrodeposited from ionic liquid could have passivation behavior. Fe-Zn modified electrode obtained from aqueous solution has passivation. However, ternary Zn-Fe-Mn alloy electrodeposited from an aqueous solution cannot show passivation behavior.