Microstructural and electrochemical characterization of the passive film on a 50-kg hot rolled FeCrNiCoMn high entropy alloy

FeCrNiCoMn high entropy alloy has outstanding mechanical properties at cryogenic temperature, and the related articles surge recently. However, the studies of large-scale FeCrNiCoMn alloys have not yet been reported. This work detailed the microstructure and corrosion behavior associated with the compositional segregation of a 50-kg FeCrNiCoMn in 0.5 M H2SO4 in the form of as-cast, hot-rolled, annealed, and homogenized alloys. Interdendritic segregation was observed in the central and columnar zones of as-cast FeCrNiCoMn, where Mn was the critical element trigging the corrosion. The annealing at 900 degrees C for 1.5 h effectively improved the general corrosion resistance and passivity of the partially-recrystallized FeCrNiCoMn, whereas the complete homogenizing at 1200 degrees C for 24 h did not further improve the passivity. The structure of the passive film was characterized using Auger electron spectroscopy (AES), high-resolution transmission electron (HRTEM), and scanning transmission electron microscopy (STEM). The AES results show that the passive film on the Mn-rich and Mnpoor regions had a similar composition. Like 304 L SS, the homogenized FeCrNiCoMn had the passive film enriched of Cr and with Ni enrichment beneath the surface. The passive film on homogenized FeCrNiCoMn was degrees amorphous based on HRTEM and the thickness was estimated to be 1.66 nm by STEM. Finally, the corrosion behavior of the distinct FeCrNiCoMn alloys was discussed in detail based on the effect of casting defects, composition segregation, deformation defects, grain boundaries, and Mn-related oxide/sulfide inclusions.

Automated summary

The study of FeCrNiCoMn high entropy alloy in different conditions reveals the critical compositional segregation and corresponding improvement methods for corrosion resistance.