Trivalent Chromium Conversion Coating Formed on Hot Dip Zn-55Al-1.6Si Coated Steel

Trivalent Chromium Process (TCP) coating on hot dip Zn-55Al-1.6Si coated steel was investigated by XPS, SEM and EDS. The XPS revealed that aluminium oxides enriched the conversion coating surface. SEM and EDS analyses indicated that composition and microstructure of the conversion coating were not uniform on the dendrite and inter-dendrite regions within the Zn-Al alloy coating, and a small amount of spherical precipitation with the same composition as the conversion coating was detected in the zinc-rich inter-dendrite region. The optimal forming parameters of dilute passivator determined by salt spray test and orthogonal test is: 40°C, 50 vol.% and 3 min. In addition, the salt spray corrosion resistance of the optimal conversion coatings can reach 192 h, and the optimal TCP coating can reduce the cathodic corrosion current density in 0.05 M NaCl solution by an order of magnitude due to the electrochemical polarization measurements. (paper)