On the effect of multiphase microstructure of ZnAlMg substrate on the Ti-based activation and phosphate conversion coating distribution

• Published in: Surface & coatings technology Vol. 369; pp. 165 - 174
• Main Authors: Tanguy, Perrine, Allély, Christian, Dragoe, Diana, Sefl, Vaclav, Stoulil, Jan, Volovitch, Polina
• Format: Journal Article
• Language: English
• Published: Lausanne Elsevier B.V 15.07.2019; Elsevier BV; Elsevier
• Subjects: Activation; Aging (metallurgy); Chemical bonds; Completeness; Conversion coating; Conversion coatings; Engineering Sciences; Inhomogeneity; Microstructure; Multiphase; Multiphase metallic coating; Organic chemistry; Phases; Phosphate conversion coating; Photoelectrons; Pre-treatment; Protective coatings; Reaction kinetics; Scanning electron microscopy; Substrates; Surface activation; Titanium dioxide; X-ray photoelectron spectroscopy (XPS); ZnAlMg; ZnAlMg; Phosphate conversion coating; Pre-treatment; X-ray photoelectron spectroscopy (XPS); Multiphase metallic coating
• ISSN: 0257-8972; 1879-3347
• DOI: 10.1016/j.surfcoat.2019.04.050

The impact of multiphase microstructure of ZnAlMg coatings on the homogeneity of the surface activation and phosphate conversion coating (PCC) is considered. Three ZnAlMg coatings and three pure phases (Zn, Zn2Mg and ZnAl) relevant for these coatings were characterized after Ti-based activation and after the complete PCC procedure. Highly alloyed ZnAl and Zn2Mg were more sensitive to the ageing of the activator bath than Zn. X-ray photoelectron spectroscopy demonstrated that Ti concentration on the surface after Ti containing activation step decreased in order Zn > Zn2Mg ≫ ZnAl and that TiO group is chemically bonded to the Zn surface while for the alloyed phases mainly TiO2 was formed. Scanning electron microscopy demonstrated that for the coatings with a fine microstructure the complete covering by PCC was achieved in standard conditions despite the differences in the activation state of different phases and different kinetics of PCC formation. This was explained by the hypothesis that the variance in the reactivity of different phases forming the coating can be leveled by the phosphate crystal size, exceeding the substrate microstructure inhomogeneity. [Display omitted] •Multiphase microstructure can affect Ti-contaning activation of ZnAlMg coating.•Alloyed phases (Zn-22%Al, Zn2Mg) are more sensitive to the activator ageing.•Quantity and chemistry of deposited Ti differ on Zn, ZnAl and Zn2Mg substrates.•Phosphate conversion coating covers homogenously multiphase ZnAlMg after 12 s at 60 °C.•Inhomogenous activation of ZnAlMg can be leveled by the size of phosphate crystals.