Preparation and characterization of colored Ti/Zr conversion coating on AZ91D magnesium alloy

• Published in: Surface & coatings technology 2015-08, Vol.276, p.239-247
• Main Authors: Yi, Aihua, Du, Jun, Wang, Jian, Mu, Songling, Zhang, Guoge, Li, Wenfang
• Format: Article
• Language: English
• Subjects: Coatings; Colorization; Conversion coating; Coordination compounds; Corrosion resistance; Magnesium alloys; Magnesium base alloys; Protective coatings; Tannic acid; Titanium; Titanium dioxide; Zirconium
• ISSN: 0257-8972; 1879-3347
• DOI: 10.1016/j.surfcoat.2015.06.069

A golden-yellow Ti/Zr conversion coating on AZ91D substrate was successfully prepared in the solution containing titanium and zirconium ions with the addition of tannic acid. The growth process, microstructure, composition and corrosion resistance of this conversion coating were characterized by EDS, SEM, XRD, XPS, FTIR and electrochemical workstation. The application of H2ZrF6 and tannic acid was the key to the coating color. The coating possessed a double-layer structure, in which the inner layer was made up of Mg(OH)2, MgO, TiO2, ZrO2, and MgF2, and the outer layer was metal–organic complex. The π electronic chromophore in the metal–organic complex produced the conversion coating color. The coating was uniform and compact, leading to improved corrosion resistance and significantly reduced corrosion current density (from 93.72μA·cm−2 to 1.047μA·cm−2). •A novel colored Ti/Zr conversion coating was obtained on the surface of AZ91D alloy.•The co-existence of H2TiF6 and tannic acid is the key factor to color the coating.•The coating mainly consists of coating Mg(OH)2, MgF2, MgO, TiO2, ZrO2 and metal–organic complex.•The metal–organic complex located in the superficial layer makes the coating golden-yellow.•The colored Ti/Zr coating exhibits excellent corrosion resistance.