Temperature-dependent passivation and pitting behavior of Fe-based bulk metallic glass and its HVOF-sprayed amorphous alloy coating in chloride solution

Fe-based amorphous alloys and their coatings offer exceptional corrosion resistance due to their disordered atomic structures, yet their temperature-dependent passivation mechanisms remain insufficiently understood. In this work, the corrosion behavior of a Fe₅₇Cr₁₅Mo₈P₁₀C₇B₃ bulk metallic glass (BMG) and its high-velocity oxygen fuel (HVOF)-sprayed amorphous alloy coating (AAC) was systematically investigated in 3.5 wt% NaCl solution over 30–70 °C, with 316 L stainless steel (316 L SS) serving as a benchmark. Comprehensive electrochemical analyses, including potentiodynamic and potentiostatic polarization, EIS, Mott–Schottky testing, and XPS depth profiling, were employed to reveal the temperature-dependent evolution of passive films and localized corrosion mechanisms. The BMG exhibited the most stable passive behavior, characterized by nearly an order of magnitude reduction in corrosion current density (~ 1.5 × 10⁻⁷ A.cm⁻²) compared to 316 L SS (~ 1.1 × 10⁻⁶ A.cm⁻²) and minimal variation with temperature, while the AAC demonstrated strong self-healing capability despite microstructural defects. In contrast, 316 L SS underwent severe pitting at approximately 65 °C, whereas both the BMG and AAC exhibited a remarkable shift in critical pitting temperature (CPT) to above 90 °C, demonstrating a significantly enhanced resistance to thermally induced localized corrosion. XPS and semiconductor analyses revealed that the BMG developed a dense Cr₂O₃/Fe₃O₄-rich passive film with low carrier density, whereas the AAC formed a hydroxide-dominated film that became increasingly defective at high temperatures. This study provides the first direct comparative insight into the thermally induced passivation degradation and pit stability of Fe-based BMGs and their coatings, establishing a diffusion-controlled model framework for evaluating pit stability. The findings highlight the crucial role of amorphous structure and coating integrity in achieving superior corrosion resistance in thermally aggressive chloride environments.