Design of Ni-based amorphous alloy corrosion-resistant to high temperature hydrochloric acid

A corrosion-resistant Ni-based amorphous alloy was developed by combining the Inoue's three principles and the dissolution-ionization-diffusion-deposition (DIDD) model. The calculations showed that a rapid localized increase of pH value at the metal-solution (M-S) interface is due to the dissolution of both Ta and Nb in the Ni-NbTa-Fe-P alloy system and is beneficial to the formation of two-layered structure of corrosion inhibition in the passivation film, in which the dense Ta/Nb mixed oxides formed initially in the passivation film while the extra mixed-phosphates appeared in the inner layer neighboring the substrate. The 140 d immersion experiment in the 80 degrees C/32 wt% hydrochloric acid and the electrochemical testing showed that the (Ni57.5Nb19.5P8Fe14.9)87Ta13 amorphous alloy had the lowest corrosion rate together with the best resistance to the metastable pitting. The structure of passivation film was mainly composed of dense Ta-Nb mixed oxides with an inner phosphate layer, which was consistent with the theoretical calculations. The work indicated that the DIDD model could be useful in designing Ni-based amorphous alloys for extremely corrosive service environments.