Effect of Molybdenum on the Anodic Behavior of Amorphous Fe‐Cr‐Mo‐B Alloys in Hydrochloric Acid

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Abstract
The addition of a small amount of molybdenum to amorphous Fe‐Cr‐B alloys improves the corrosion resistance in hydrochloric acid. In particular, amorphous alloys containing 5 atomic percent or more molybdenum passivate spontaneously even in , showing no pitting by anodic polarization. However, an excess addition of molybdenum lowers the corrosion potential and raises the anodic current density in the passive region. This dual function of molybdenum was investigated with XPS and electrochemical methods. In the active region, an increase in the molybdenum content of the alloys decreases the anodic current density by the formation of a surface film containing large amounts of chromium and some molybdenum. The addition of molybdenum assists the formation of the passive hydrated chromium oxyhydroxide film. However, a passive film of molybdenum oxyhydroxide is unstable in comparison with the passive hydrated chromium oxyhydroxide film even at lower potentials in the passive region, and molybdenum suffers transpassive dissolution at higher potentials in the passive region. An excess of molybdenum results in a passive film whose content of molybdenum in an unstable valence state is high. Consequently, the addition of molybdenum is significantly effective in increasing the corrosion resistance, particularly in aggressive hydrochloric acid, but the lower the molybdenum content of the passive film the better is the protective quality of the film.

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