Correlation Between Microstructure and Corrosion Behavior of Near-Equilibrium Al-Co Alloys in Various Environments

Abstract

Al-rich Al-Co alloys belong to a broad family of complex metallic alloys. These alloys contain structurally complex intermetallic phases inclusive of quasicrystals. The chemical properties of these phases are relatively unknown. The corrosion resistance of non-equilibrium Al-Co alloys in aqueous NaCl solution is influenced by aluminum content of Al-Co phases present. The only exception is structurally complex Z-Al3Co phase. To elucidate this phenomenon, the Al71Co29 and Al74Co26 alloys (metal contents are given in at.%) were prepared by arc melting and annealed at 1050 °C for 330 h. After annealing, the samples were rapidly cooled to reach near-equilibrium condition. Their phase constitution and microstructure were studied using both x-ray diffraction and energy-dispersive x-ray spectrometry coupled with scanning electron microscopy, The Al71Co29 alloy was found to consist of Al5Co2 and B2 phases. The Al74Co26 alloy was composed of Al5Co2 and Z-Al3Co phases. The samples were corrosion tested in water solutions of NaCl, HCl and NaOH by potentiodynamic polarization. In NaCl and HCl, the pitting corrosion was observed. In aqueous NaOH solution, a uniform corrosion occured. In alkaline and acidic environments, the most aggressive and the weakest corrosion attacks were found, respectively. Good corrosion resistance of the Z-Al3Co phase in chlorine anions containing environments was observed. Possible explanation of such behavior has been discussed.