Role of late transition metals on pitting resistance of Zr-Ti-(Cu, Ni, Co)-Al bulk metallic glasses in 0.6 M NaCl aqueous solution

Four quanternary Zr-based bulk metallic glasses (BMGs) were selected, including the Zr46Ti2Cu45Al7, Zr61Ti2Cu25Al12, Zr55Ti4Ni22Al19 and Zr55Ti2Co28Al15, due to their robust glass-forming ability and containing a single species of late transition metal (LTM) in compositions. Their pitting resistances in 0.6 M NaCl aqueous solution were investigated to examine the role of LTM elements in the alloys, with electrochemical measurements, surface morphology observation and x-ray photoelectron spectrometry analysis. It is shown that in comparision with two Cu-bearing BMGs, Zr55Ti4Ni22Al19 and Zr55Ti2Co28Al15 BMGs exhibited significantly superior resistance to pitting. Zr61Ti2Cu25Al12, Zr55Ti4Ni22Al19 and Zr55Ti2Co28Al15 BMGs manifested distinct passivation behaviour, because of the formation of surface passive film mainly comprising of ZrO2, TiO2 and Al2O3. However, no significant differences in the electrochemical resistive properties and thicknesses of passive films were found between Zr61Ti2Cu25Al12 and Zr55Ti4Ni22Al19 BMGs. Nevertheless, at the passive film/metal interface, copper enrichment took place in Zr61Ti2Cu25Al12, whereas the nickel was slightly deficient at the interface in Zr55Ti4Ni22Al19. During pitting propagation, selective dissolution of the zirconium, titanium and aluminum over the copper took place in Zr61Ti2Cu25Al12, but it was not the case in Zr55Ti4Ni22Al19. For the two Cu-bearing BMGs, reduction of passive base metal elements in composition resulted in local selective dissolution, even absence of the passivation. (C) 2017 Published by Elsevier Ltd on behalf of The editorial office of Journal of Materials Science & Technology.