Mechanistic insight into the role of amorphicity and porosity on determining the corrosion mitigation behavior of Fe-based amorphous/nanocrystalline coating

Fe-based in-situ amorphous/nanocrystalline composite coatings were prepared from an amorphous/crystalline powder (Fe63Cr9P5B16C7) using high velocity oxy-fuel thermal spraying of optimized parameters. To investigate the individual effects of amorphicity and porosity on corrosion performance, two melt spun ribbons with no porosity having different amorphous content viz. (i) fully amorphous (FA-Rib) and (ii) partially amorphous (PA-Rib) structure with similar amorphicity as that of the coatings, were synthesized. Potentiodynamic polarization and electrochemical impedance spectroscopy studies revealed a greater influence of the reduced amorphicity on the increased passive current density and decreased polarization resistance, than that of the porosity in the fully amorphous matrix. Besides, extent of pitting increased, whereas fraction of protective phases (chromium oxide and Cr substituted hematite) in the corrosion products of post-polarized samples decreased gradually in the order of FA-Rib to PA-Rib to the coating. Moreover, results of Auger electron spectroscopy revealed that depletion of Cr and thinning of passive film increased significantly with crystallization of amorphous structure, compared to additional porosity effect. These results confirmed that corrosion mitigation ability of the coating was primarily affected by reduced amorphicity. Role of both these microstructural features in governing the corrosion mechanism of the amorphous coating was elucidated. (C) 2020 Elsevier B.V. All rights reserved.