Novel corrosion behaviours of the annealing and cryogenic thermal cycling it treated Ti-based metallic glasses

Effects of structural relaxation and cryogenic thermal cycling on corrosion resistance of the Ti50Zr20Be20Ni10 bulk metallic glass (BMG) was investigated in 3.5 wt% NaCl solution. The annealed Ti-based BMGs exhibited striking high pitting potentials with low corrosion current densities, while cryogenic thermal cycling treated BMGs performed poor corrosion resistance. More stable passive films were found to form on the surfaces of annealed BMGs through electrochemical impedance spectra analysis. Sub-sized pits that evolved along with micrometre-sized pits on corroded morphologies of the as-cast and cryogenic thermal cycling treated samples clarified the aggressive corrosion. Annealing treatment gave rise to the annihilation of excessive free volume within BMGs, while cryogenic thermal cycling induced the proliferation of free volume. Moreover, the variation of microhardness further reflected the fluctuation of free volume. The XPS analysis revealed that the superior corrosion resistance of the annealed samples could be attribute to the increment in the concentration of Ti, Zr and distinct decrement in the concentration of Be. It is concluded that excessive free volume would deteriorate stability of surface passive film.