Corrosion resistance and microstructural stability of austenitic Fe–Cr–Al–Ni model alloys exposed to oxygen-containing molten lead

The goal of the study was to determine the critical concentrations of Al, Cr and Ni, at which the quaternary Fe–Cr–Al–Ni model alloys, exposed to oxygen-containing molten Pb up to 600 °C, are corrosion resistant, while preserving the austenite structure of the alloy matrix.

Twelve alloys were designed to meet the above mentioned requirements; six of them showed corrosion resistance and preserved the austenite phase in the alloy bulk, during the exposure at 550 °C and 600 °C for 1000 h to molten Pb containing 10⁻⁶ wt% oxygen. Based on experimental results a general formula was substantiated as follows: Fe-(20–29)Ni-(15.2–16.5)Cr-(2.3–4.3)Al (wt.%). In case of temperatures below 550 °C, the critical Cr content was 14.4 wt%.

Two corundum-type crystalline structures were identified as the constituent phases of the passivating scales, one being Cr₂O₃ and the other Al₂O₃–Cr₂O₃ solid solution. The average amount of Cr₂O₃ in the Al₂O₃–Cr₂O₃ solid solution, found in the passivating scales of the Fe–Cr–Al–Ni model alloys, was estimated at ≈ 40 wt% at 550 °C and ≈35 wt% at 600 °C.

A transitional layer, consisting of Fe- and Ni-enriched austenitic matrix and exhibiting randomly distributed intermetallic B2-(Ni,Fe)Al, was formed below the oxide scale up to a depth of two microns.

The austenite, as matrix, and Ni₃(Al,Fe) as precipitates are the microstructural phases of the bulk alloys after exposure for 1000 h at 600 °C to oxygen-containing molten Pb.