Corrosion behaviour of friction stir processed, metastable, dual-phase, Fe49.5Mn30Co10Cr10C0.5, multi-principal element alloy

Abstract

The corrosion behaviour of Friction Stir Processed (FSPed), dual-phase, Multi-Principal Element Alloy (MPEA), Fe49.5Mn30Co10Cr10C0.5, was investigated. The microstructure of the alloy in the annealed condition consisted of Face Cantered Cubic (FCC) and Hexagonal Closed Pack (HCP) phases, with the HCP phase present as laminates inside the equiaxed FCC grain. Fine carbide particles were found to be precipitated at the grain boundaries and also inside the grains. FSP engendered a reduction in the grain size due to dynamic recrystallization and an increase in the fraction of strain-induced HCP phase in the microstructure. The potentiodynamic polarisation and Electrochemical Impedance Spectroscopy (EIS) studies showed that the one-pass FSPed sample exhibited best corrosion resistance compared to the annealed and two-pass FSPed samples. The reason for this was ascribed to two factors: the reduced grain size and the smaller fraction of the HCP phase induced by strain in the microstructure. Mott-Schottky results indicated the presence of n- type semiconducting passive layer on the surface of annealed and FSPed samples. The one-pass sample had the lowest defect density (ND value) compared to annealed and the two-pass FSPed sample. Scanning Electron Microscopy (SEM) studies performed on the annealed, one-pass and two-pass samples, after immersion in 3.5 wt% NaCl solution, revealed the presence of large number of pits and corrosion products in the annealed sample compared to that in the FSPed samples. Results obtained from X-ray Photoelectron Spectroscopy (XPS) studies confirmed the presence of iron oxide, chromium oxide, cobalt oxide, chromium & cobalt hydroxides as the major constituents, together with a low concentration of manganese, in the passive layer formed on the FSPed samples.