Structure And Corrosion of Quasicrystalline Cast Al–Co–Ni and Al–Fe–Ni Alloys in Aqueous NaCl Solution

Abstract

In this work the structure and corrosion behavior of quasicrystalline cast Al₆₉Co₂₁Ni₁₀ and Al₇₂Fe₁₅Ni₁₃ alloys in 5-% sodium chloride solution (рН 6.9–7.1) were investigated. The alloys were cooled at 5 К/s. The structure of the samples was studied by methods of quantitative metallography, X-ray analysis, and scanning electron microscopy. Corrosion properties were determined by potentiodynamic method. Stationary potential values were measured by means of long-term registration of (Е,τ)–curves using
ПІ–50–1 potentiostat and ПР–8 programmer with three-electrode electrolytic cell. A platinum electrode served as counter electrode and silver chloride – as reference electrode. The made investigations confirm the formation of stable quasicrystalline decagonal D-phase in the structure of Al₆₉Co₂₁Ni₁₀ and Al₇₂Fe₁₅Ni₁₃ alloys. In Al₆₉Co₂₁Ni₁₀ alloy, at room temperature D-phase coexists with crystalline Al₉(Co,Ni)₂ phase, and in Al₇₂Fe₁₅Ni₁₃ alloy – with Al₅FeNi phase. Comparison of Vickers hardness of these phases exhibits the following sequence: H(D-AlCoNi)>H(D-AlFeNi)>H(Al₅FeNi)>H(Al₉(Co,Ni)₂). In 5-% sodium chloride solution, the investigated alloys corrode under electrochemical mechanisms with oxygen depolarization. Compared with Al₇₂Fe₁₅Ni₁₃ alloy, Al₆₉Co₂₁Ni₁₀ alloy has more negative value of stationary potential (–0,40 V and –0,48 V, respectively), and its electrochemical passivity region extends due to the inhibition of anodic processes. For both alloys, transition to passive state in the saline solution is observed. A corrosion current density, calculated from (E,lgi)-curve, for Al₆₉Co₂₁Ni₁₀ alloy amounts to 0.12 mА/сm² and for Al₇₂Fe₁₅Ni₁₃ alloy – to 0.14 mА/сm². After immersion in the saline solution for 8 days, pits are revealed on the surface of the alloys in areas, mainly where the phase boundaries and flaws are located. The number and size of pits are smaller on the surface of Al₆₉Co₂₁Ni₁₀ alloy as compared with those on the surface of Al₇₂Fe₁₅Ni₁₃ alloy. The lower corrosion resistance of Al₇₂Fe₁₅Ni₁₃ alloy may be explained by the presence of iron-containing phases in its structure. Based on obtained results, the Al₆₉Co₂₁Ni₁₀ alloy has been recommended as coating material for rocket-and-space equipment working in marine climate.