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

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.