Corrosion behavior of Fe-based amorphous/nanocrystalline composite coating: correlating the influence of porosity and amorphicity

Recently, Fe-based amorphous coating synthesized by different thermal spraying methods are being investigated as a potential candidate for long-term surface protection of various structures attributed to their outstanding wear and corrosion resistance. Defects like porosity and crystallization are inevitable in the thermal sprayed coatings, which are introduced during the synthesis process. Corrosion behavior of these coatings is adversely affected by the presence of such defects. However, identification of a microstructural feature among amorphous content and porosity to have greater influence on the corrosion resistance of the thermal sprayed Fe-based amorphous/nanocrystalline coating has remained elusive so far. Thus, to address this problem, in-situ amorphous/nanocrystalline composite coatings were synthesized via high velocity oxy-fuel (HVOF) spraying, along with two melt-spun ribbons of different amorphous content (one fully amorphous, FA-Rib and the other having similar level of amorphicity as that of the coatings, PA-Rib). This was done to eliminate the porosity aspect of such composite coatings and investigate the extent of degradation in corrosion resistance caused by amorphicity and porosity individually. Potentiodynamic polarization and electrochemical impedance spectroscopy studies revealed that the corrosion and passive current densities and polarization resistance was greatly influenced by reduced amorphicity compared to porosity. Moreover, extent of corrosion attack increased, whereas fraction of chromium oxide and Cr substituted hematite phase in the Raman spectra of post-polarized samples decreased gradually in the order of FA-Rib < PA-Rib < the coating. Besides, analysis of the passive film by Auger electron spectroscopy exhibited an increment in the thinning of passive film, and lowered Cr/Fe ratio of the passive film in the order FA-Rib → PA-Rib → the coating. These results established amorphicity as the primary factor that affects the corrosion resistance of Fe-based amorphous/nanocrystalline coating. This study will ultimately help in designing new amorphous composite coatings with improved corrosion resistance.