This study examined the microstructure and electrochemical corrosion behaviour of high-entropy FeCoNiCrCu0.5 alloys annealed at various temperatures. The alloy microstructures were characterized and analyzed chemically by X-ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). Further, the effects of annealing temperatures of 350 degrees C, 650 degrees C, 950 degrees C, and 1250 degrees C with a holding time of 24h at each temperature on the alloy microstructure and properties were investigated. XRD spectra of the as-cast specimens and those heated to 1250 degrees C showed a face-centred cubic (FCC) solid-solution phase. All specimens contained a matrix, which included a Cu-depleted phase, a Cr-rich phase (second structure), and a Cu-rich phase (third structure). The Cr-rich phase precipitated in the matrix after annealing at 1250 degrees C. The microstructure of the Cu-rich phase showed spinodal decomposition with an increase in the annealing temperature from 350 degrees C to 1250 degrees C. Differential scanning calorimetry (DSC) analysis revealed that in the as-cast high-entropy FeCoNiCrCu0.5 alloy specimen annealed below 1300 degrees C, the Cu-rich (Cu-(Ni, Co, Cr, Fe)) phases precipitated in the matrix by spinodal decomposition. The electrochemical corrosion behaviours of the as-cast and annealed specimens were evaluated by potentiodynamic polarization performed in immersion tests. The as-cast and annealed specimens were severely corroded in 3.5% NaCl solution; the main corrosion mechanism was the precipitation of the Cu-rich phase in the matrix. The Cu-rich phase was susceptible to corrosion, and its potential differed considerably from that of the matrix. The Cl- ions preferentially attacked this susceptible area (Cu-rich phase). This preferential attack was attributed to the fact that the presence of copper in the alloy degraded the corrosion resistance, thereby leading to corrosion by pitting. (C) 2011 Elsevier B.V. All rights reserved.