On electrochemical corrosion of mechano-activated and thermally processed AlxCryNiz 2D decagonal quasicrystalline structures and crystalline approximants

Abstract

This article represents a pioneering study centered on the corrosion kinetics of untreated and thermally processed mechano-synthesized Al_xCr_yNi_z two-dimensional decagonal quasicrystalline structure and crystalline approximants. It sheds light on the distinguished corrosion behavior of untreated and heat-treated mechano-synthesized Al₇₂Cr₁₅Ni₁₃ and Al₈₆Cr₁₂Ni₂ alloys, including a wide diversity of miscellaneous intermetallic phases. A comprehensive characterization was performed to analyze crystallographic structure and thermal characteristics of Al_xCr_yNi_z powder particles. Electrochemical evaluations of the mechano-synthesized Al₇₂Cr₁₅Ni₁₃ and Al₈₆Cr₁₂Ni₂ specimens and their heat-treated counterparts were conducted under cyclic potentiodynamic polarization tests with 0.1 mol/L Na₂SO₄ (pH=2) and 3.5 % NaCl (pH=8.5) electrolytes at room temperature, respectively. Al₇₂Cr₁₅Ni₁₃ two-dimensional decagonal quasicrystalline phase was attained following 6 h mechano-synthesis and subsequent annealing treatment at 1035 °C. There is no evidence of quasicrystal formation in the Al₈₆Cr₁₂Ni₂ alloy system after 6 h mechano-synthesis and successive thermal processing at 445 and 570 °C. In this study, we conducted the first investigation into electrochemical performance of both Al₇₂Cr₁₅Ni₁₃ and Al₈₆Cr₁₂Ni₂ intermetallics. Both Al₇₂Cr₁₅Ni₁₃ and Al₈₆Cr₁₂Ni₂ alloys develop a protective passive film in 0.1 mol/L Na₂SO₄ electrolyte. It was determined that 6 h mechano-synthesized Al₇₂Cr₁₅Ni₁₃ sample, subjected to annealing at 1035 °C, stands out in the Al-Cr-Ni alloy systems for applications necessitating exceptional corrosion resistance, passivation behavior, and minimal susceptibility to pitting corrosion when compared to other tested counterparts. This alloy is characterized by a corrosion current density of 3.73 µA/cm² and a corrosion potential of -0.16 V(vs. Ag/AgCl), revealing a remarkably stable passive film up to a current density of 0.02 A/cm² and a potential of 2.41 V (vs. Ag/AgCl) within 0.1 mol/L Na₂SO₄ medium. Likewise, it exhibited a drastically diminished corrosion current density of 11.65 µA/cm² and a reduced corrosion potential of -0.27 V (vs. Ag/AgCl) within 3.5 % NaCl electrolyte, attributed to the formation of two-dimensional decagonal quasicrystalline phase and hexagonal δ-Al₃Ni₂ crystalline approximant at 1035 °C. It also encompassed a re-passivation current density and potential of 50.35 µA/cm² and -0.04 V (vs. Ag/AgCl), respectively, within the latter solution. Its corrosion mechanism may be ascribed to a two-step surface precipitation process: initially, Al dissolves into a hydroxide, succeeded by the formation and precipitation of Al oxides, such as NaAlO₂ and Al₂O₃∙xH₂O.