Passive Film Properties Evolution Involving Oxygen Vacancies Transportation of FeCrMoCBY Amorphous/Nanocrystalline Composite Coating with Different Solution Temperatures.

Electrochemical and passive film evolution peculiarities including compositional alternation, film thickening and diffusion process of oxygen vacancies of Fe48Cr15Mo14C15B6Y2 amorphous/nanocrystalline composite coating were investigated in borate buffer solution with different solution temperatures. A lower stability of the passive film could occur at a higher temperature as manifested by a greater corrosion current density and passive current density. The formation of metal mechanical mixtures as the corrosion products could cause inner stress and deteriorate the stability of passive film, which were also active sites easily invaded by corrosive ions. This effect would induce metastable pitting in these active sites due to the dissolution of large amounts of metal mechanical mixtures into the solution accompanied with local rupture of passive film and initiation of pitting. Reduction in impedance value at elevated temperatures was caused by the increment in Fe-containing n-type semiconducting species and depletion of Mo-containing oxides, which can suppress the growth of insoluble oxide Cr2O3. AFM topographic images indicated that the distribution of agglomerated oxides with larger size on the surface gradually dispersed in larger region with increasing temperature, suggesting that the micro/nanoscale hierarchical structures of passive films were changed. The promoted annihilation effect of oxygen vacancies at the passivated substances/electrolyte interface resulted in the acidification effect by accelerating the production of more hydrogen ions at a higher temperature. [ABSTRACT FROM AUTHOR]