Effect of small addition of Mn on the passivation of Zn in 0.1 M NaOH solution

The passivation of pure Zn (99.995 wt%) and Zn–0.4Mn (0.4 wt% Mn) alloy in a deaerated 0.1 M NaOH solution (pH 12.9) was investigated by electrochemical measurements, X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM). The potentiodynamic polarization and electrochemical impedance measurements show that addition of 0.4 wt% Mn can decrease the passive current density of Zn in the passive region. XPS surface analysis indicates that there is approximately 1.0–2.0 at% Mn2+ being incorporated into the passive film on Zn–0.4Mn alloy with Mn content being higher in the outer layers. Mott–Schottky analysis shows that the incorporated Mn can decrease concentration of defects in the film. AFM observations disclose that Mn can decrease the grain size of the film. The mechanism by which Mn additions improve the passivity of Zn is that the incorporated Mn can inhibit ions transportation in the film and inhibit its growth. Meanwhile, Mn can also promote the nucleation of Zn oxides and decrease film porosity.