Abstract
In this work, the effect of treatment time on the performance of Cr(III) conversion coatings (TCC) on hot dip Zn–55Al–1.6Si (Zn55Al) coated steel sheet were investigated. The surface 3D morphology and roughness of TCCs were examined by a 3D topography instrument and the structure, chemical composition, and elemental depth distribution were studied by means of scanning electron microscope (SEM), X-ray photoelectron spectroscopy (XPS), and auger electron spectroscopy (AES). The results indicated that during the formation of TCC, the macro-roughness of Zn55Al surface was reduced, but the micro-roughness increased, which are considered to be key factors in enhancing the adhesion strength of epoxy primers. The AES depth profiles showed a two-layer TCC for both dendritic and inter-dendritic regions and chemical composition analysis of XPS showed that the surface of TCC was mainly oxides, fluoride and a small number of hydroxides. Overall, Zn55Al specimen prepared in a diluted commercial Cr(III)-based solution for 180 s at 40 °C performed a better adhesion strength to epoxy primer and had the largest polarization resistance among all TCCs in this work. Additionally, longer Cr(III) passivation process (TCP) treatment time will increase the sensitivity of the TCC to micro-cracks.
Highlights
Hot dip coated steel (Zn–55Al–1.6Si, hereinafter referred to as Zn55Al) is widely used in residential or factory decoration, parts of electrical appliances or automobiles, engineering construction, and other fields
Zn55Al treated in a dilute Cr(III) bath at 40 ◦ C for 180 s has the optimum performance in terms of the polarization resistance and adhesion strength to epoxy primer
The analysis of 3D morphology and roughness shows that the macro-roughness of Zn55Al specimen surface decreases gradually with the prolongation of trivalent chromium passivation process (TCP) treatment time
Summary
Hot dip coated steel (Zn–55Al–1.6Si, hereinafter referred to as Zn55Al) is widely used in residential or factory decoration, parts of electrical appliances or automobiles, engineering construction, and other fields. The process for obtaining hot-dip coated steel is transformed to a modern technology that requires additional surface treatment; for example, short-time immersion in specially developed chemical solutions for obtaining conversion coatings (CCs). As the bottom design of the coating system, both the improvement of corrosion resistance and adhesion strength to subsequent primers are the main points that people will pay attention to in the future. When exposed to electrolytes containing oxygen, water, and ions for a long time, the penetration of these components into the primer lead to a decrease in the adhesion of the coating. One potential advantage of chemical conversion coatings is that they can improve
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