Effect of alternating oxygen concentration on corrosion behaviors of FeNiCrAl medium entropy alloys in liquid lead-bismuth eutectic at 550 °C

The Fe 40 Ni 30 Cr 20 Al 10 , Fe 45 Ni 30 Cr 15 Al 10 , Fe 50 Ni 20 Cr 20 Al 10 , and Fe 50 Ni 25 Cr 15 Al 10 medium entropy alloys (MEAs) were designed, and their corrosion behaviors were studied under alternating exposure to oxygen-saturated and low-oxygen (1.93 × 10 −7 wt%) liquid lead-bismuth eutectic (LBE) at 550 °C for 500 h. After exposure to oxygen-saturated LBE for 500 h, a nanoscale (~47 nm) oxide film formed on the surface of all four MEAs. This film consisted of an Fe-, Cr-, and Al-rich outer oxide layer and a continuous Al 2 O 3 inner layer, which effectively protected the alloys from dissolution corrosion during subsequent exposure to low-oxygen LBE. In contrast, direct exposure to low-oxygen LBE for 500 h resulted in severe dissolution corrosion in all MEAs, characterized by LBE penetration into the matrix. The Fe 45 Ni 30 Cr 15 Al 10 MEA, with its higher Ni and lower Cr content, exhibited the most pronounced corrosion damage. However, when these corroded alloys were subsequently exposed to oxygen-saturated LBE for another 500 h, a protective oxide film rapidly formed at the corrosion pits, significantly inhibiting further dissolution. The corrosion mechanisms of FeNiCrAl MEAs in liquid LBE with varying oxygen concentrations, as well as the effects of alternating oxygen concentration, were discussed in detail. • The thin oxide film formed on four FeNiCrAl medium-entropy alloys under oxygen-saturated conditions. • Dissolution corrosion occurred in four FeNiCrAl medium-entropy alloys at low oxygen concentrations. • The outer Fe-rich oxide layer is reduced and dissolved at low oxygen concentrations. • Pre-exposure to oxygen-saturated lead–bismuth eutectic improves the corrosion resistance of FeNiCrAl medium-entropy alloys.