The impact-corrosion behavior of HVAF-sprayed monolayer and hierarchical Fe-based amorphous coatings

Abstract

The impact-corrosion behavior of the monolayer and hierarchical Fe-based amorphous coatings (AMCs) fabricated by high velocity air fuel (HVAF) in 3.5 wt% NaCl solution were investigated in detail. The polarization results indicate with the impact energy increases, the monolayer coating exhibits a corrosion failure as evident by a distinct increment in passive current density, which is caused by the penetrated radial cracks. The radial cracks provide corrosion channels between the solution and the substrate. Differently, the hierarchical coating shows a gradual improvement in passive current density and metastable pitting frequency with the increase of impact energy. This is related to the IR drop, and the larger potential difference between inside and outside the crack results in the passive film being more susceptible to breakdown. Long-term electrochemical impedance spectroscopy results demonstrate that the hierarchical coating possess excellent resistance to impact-corrosion. Nevertheless, when the impact energy exceeds 21.6 J, a corrosion degradation occurs after 720 h. This failure is because that the cone cracks are extended with the energy increasing, causing pH drop and Cl− accumulation and oxygen deficiency inside the cracks. This work reveals the mechanism of impact-corrosion of Fe-based AMCs, and provides guidance for their engineering application.