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

Abstract

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.