Microstructure and electrochemical behavior of Pd-Cu-Ni-P bulk metallic glass and its crystallized alloys

The microstructure and electrochemical behavior of the Pd _42.5Cu ₃₀Ni _7.5P ₂₀ as-quenched bulk metallic glass and its crystallized alloys after heat-treatment at 623 and 723K were investigated. The results revealed that the Pd _42.5Cu ₃₀Ni _7.5P ₂₀ bulk metallic glass and its crystallized alloys were spontaneously passivated in 0.144M NaCl and Hanks' solution with a low passive current density and a wide passive region. The transpassive potential of the fully crystallized alloy heat-treated at a higher temperature was lower than those of the as-quenched bulk metallic glass and partially crystallized alloy. The difference in the microstructure of the as-quenched bulk metallic glass, the partially crystallized alloy and the fully crystallized alloy was responsible for their different transpassive potentials. The wider passive region of the alloys in Hanks' solution is attributed to the presence of HCO ₃ -, HPO ₄ ^2-, SO ₄ ^2- and H ₂PO ₄ - ions in the solution, which act as corrosive inhibitors.