Corrosion Behavior of Nanocrystalline Co and Co-P Alloys in a NaOH Solution

The corrosion behaviors of nanocrystalline Co and Co-1.1 and 2.1 wt% P (7-nm to 20-nm grain size) and their annealed alloys were studied in a 0.1-M sodium hydroxide (NaOH) solution. All tested samples exhibited active-passive-transpassive behavior. While anodic polarization curves and electrochemical impedance spectroscopy (EIS) responses for poly- and nanocrystalline Co had very similar behavior, nanocrystalline Co-P alloys showed higher passivation current and reduced polarization resistance. X-ray photoelectron spectroscopy analysis revealed that the passive film formed in the primary passivation range consisted mainly of cobalt(II) hydroxide (Co[OH]₂) with a thickness of >10 nm. The passive film formed during secondary passivation consisted of complex compounds such as Co(OH)₂, cobalt(II, III) oxide (Co₃O₄), cobalt(III) oxide (Co₂O₃), and cobalt(III) hydroxide (Co[OH]₃) with a thickness of <10 nm. Annealed nanocrystalline Co-1.1 wt% P alloys (i.e., annealing at 350°C and 800°C) had lower passivation current than the as-deposited alloys.