Formation of a trivalent chromium conversion coating on AZ91D magnesium alloy

Trivalent chromium conversion (TCC) coating formation on AZ91D magnesium alloy has been investigated using scanning electron microscopy, energy X-ray dispersive spectroscopy (EDS), photoelectron spectroscoy (XPS). The was formed in a commercial SurTec 650 bath. XPS showed that coatings contained mainly Cr, Zr, O, S and F species, which were assigned to hydrated Cr(III) (Cr(OH)3/CrOOH), ZrO2, Cr2(SO4)3, CrF3, lower amounts of Mg2+ Al3+ species oxide, hydroxide fluoride. Cr/Zr atomic ratio around 0.5. above both α matrix β (Mg17Al12) grain boundary regions, it significantly thicker the cathodic phase. Raman evidenced presence Cr(VI) due oxidation by H2O2, particularly sites. Electrochemical impedance revealed corrosion protection provided barrier layer at base coating, resulted reduction about factor 2 anodic current density potentiodynamic polarization curves.