Effect of Annealing Temperature on Microstructure and Corrosion Resistance of HVOF-Sprayed Fe-Based Amorphous–Nanocrystalline Coatings

In this work, a Fe-based amorphous coating (Fe44Cr18Mo7Ni4Cu2B20Si3C2) was fabricated via high-velocity oxygen fuel (HVOF) spraying, and the effect of annealing temperature on its microstructure, phase composition, and corrosion resistance in 3.5 wt.% NaCl solution was investigated. The as-sprayed (AS) coating exhibits an amorphous content of 85.71% and a porosity of 1.37%. Annealing at 540 °C (H540) reduces porosity to 0.98% without significant crystallization, whereas annealing at 640 °C (H640) and 740 °C (H740) triggers extensive crystallization (amorphous content drops to 24.18% and 19.20%), and porosity increases to 1.82% and 2.17%. Electrochemical tests show that corrosion resistance deteriorates progressively with increasing temperature. icorr increases from 3.56 μA/cm2 (AS) to 50.0 μA/cm2 (H740), while Rp decreases from 8472 to 669 Ω·cm2. EIS reveals that the AS coating is dominated by the inner barrier layer (Rb >> Rt), whereas annealing causes a drastic collapse of Rb (from 8.374 × 104 to 5.011 Ω·cm2). This degradation is attributed to crystallization-induced grain boundaries and reduced effective Cr content, which impair passive film integrity and accelerate corrosion.