Peculiarities in the Structure Formation and Corrosion of Quasicrystalline Al65Co20Cu15 Alloy in Neutral and Acidic Media

Abstract

In the present study, the structure and corrosion properties of quasicrystalline conventionally solidified Al65Co20Cu15 alloy cooled at 5 К/s were investigated. Structure was characterized by metallography, X-Ray diffraction, scanning electron microscopy, and energy dispersive spectroscopy. Corrosion properties were determined by gravimetric and potentiodynamic methods at room temperature. The investigations performed confirm the peritectic formation of stable quasicrystalline decagonal D-phase that coexists with crystalline Al4(Co,Cu)3 and Al3(Cu,Co)2 phases in the structure of Al65Co20Cu15 alloy. According to energy dispersive spectroscopy, the stoichiometric composition of D-phase is Al63Co24Cu13. The susceptibility of the Al65Co20Cu15 alloy to corrosion significantly decreases with increasing pH from 1.0 (acidic media) to 7.0 (neutral medium). A corrosion rate of the Al65Co20Cu15 alloy in the aqueous acidic solutions (pH=1.0) increases in the order HNO3®HCl®H2SO4®H3PO4. The mass of the specimens decreases in the solutions of H2SO4 or H3PO4 and increases in the solutions of HNO3 or HCl which relates to different rate ratios of accumulation and dissolution of corrosion products. The Al65Co20Cu15 alloy exhibits the highest corrosion resistance in the NaCl solution (pH=7.0) in which it corrodes under electrochemical mechanism with oxygen depolarization. The better corrosion resistance in sodium chloride solution is achieved due to the formation of passive chemical compounds blocking the surface. Free corrosion potential of the Al65Co20Cu15 alloy has value –0.43 V, the electrochemical passivity region extends from –1.0 V to –0.4 V, and a corrosion current density amounts to 0.18 mА/сm2. Depending on media, two typical surface morphologies are revealed after corrosion of quasicrystalline specimens of the Al65Co20Cu15 alloy. In the H2SO4 and H3PO4 acidic solutions, clean specimens’ surface due to its homogeneous dissolution is observed except for the more defective areas, such as boundaries of crystalline Al3(Cu,Co)2 phase containing less Co, which dissolve at a higher rate. In the HNO3, HCl or NaCl solutions, a porous layer on the surface is formed which is visually revealed as surface darkening. After staying in the NaCl solution, on the surface of the Al65Co20Cu15 alloy, the pits are also found due to preferential dissolution of components where the boundaries of Al3(Cu,Co)2 phase and flaws are located.