Relationship between Phase Occurrence, Chemical Composition, and Corrosion Behavior of as-Solidified Al-Pd-Co Alloys

The microstructure, phase constitution, and corrosion performance of as-solidified Al₇₀Pd₂₅Co₅ and Al₇₄Pd₁₂Co₁₄ alloys (element concentrations in at.%) have been investigated in the present work. The alloys were prepared by arc-melting of Al, Pd, and Co lumps in argon. The Al₇₄Pd₁₂Co₁₄ alloy was composed of structurally complex ε_n phase, while the Al₇₀Pd₂₅Co₅ alloy was composed of ε_n and δ phases. The corrosion performance was studied by open circuit potential measurements and potentiodynamic polarization in aqueous NaCl solution (3.5 wt.%). Marked open circuit potential oscillations of the Al₇₀Pd₂₅Co₅ alloy have been observed, indicating individual breakdown and re-passivation events on the sample surface. A preferential corrosion attack of ε_n was found, while the binary δ phase (Al₃Pd₂) remained free of corrosion. A de-alloying of Al from ε_n and formation of intermittent interpenetrating channel networks occurred in both alloys. The corrosion behavior of ε_n is discussed in terms of its chemical composition and crystal structure. The corrosion activity of ε_n could be further exploited in preparation of porous Pd-Co networks with possible catalytic activity.