Surface chemistry and pretreatment effects on Zn-Al-Mg coatings: insights into oxide and zirconium conversion layer formation

Abstract

Zn-Al-Mg hot-dip galvanized steel is one of the frequently used modern materials for an automotive industry. With high corrosion resistance and good mechanical properties, it serves as a base for consequent processes, such as painting or applying organic coatings. The influence of intermitting processes, such as chemical cleaning of the surface, surface activation and application of a conversion layer, on the Zn-Al-Mg coating surface chemistry plays an important role in later adhesion and susceptibility to corrosion. This thesis investigates the surface chemistry of Zn-Al-Mg coatings under varying chemical treatments, utilizing a combination of advanced surface-sensitive analytical techniques and others. A comparative analysis of Angle-resolved X-ray photoelectron spectroscopy (ARXPS), Low-energy ion scattering spectroscopy (LEIS), and Secondary ion mass spectrometry (SIMS) depth profiles is conducted to interpret compositional changes in Zn-Al-Mg coatings subjected to alkaline and acidic pretreatments. The findings reveal distinct chemical modifications depending on the treatment conditions, with alkaline treatments promoting the formation of mixed Mg/Al- and Zn-hydroxide layers, while acidic conditions result in the fast activation of Zn and dissolution of Mg-oxyhydroxides. Moreover, the results showed in practice high applicability of XPS and LEIS methods to study the surface chemistry of the industrially producedmaterial, and the ability of SIMS to complement these results on such complexmatrix material.Further, combination of analytical (ICP-OES) and surface-sensitive approachwas used to characterize the surface chemistry modifications and dissolution mechanism of Zn-Al-Mg coatings induced by industrially applicable cleaners with different pH, in mild and strong alkaline areas. While mild alkaline cleaner showed active Zn dissolution and Zn diffusion to the surface, strong alkaline cleaner advanced in Al dissolution, forming Mg/Zn-hydroxide on the surface. The results highlight the critical role of choosing the right cleaning conditions, depending on the preferrable effects, which could be essential for the next step in the coating process.As the next logical step in metallic coating processing the research examines theeffects of alkaline and acidic pretreatments on the characteristics of Zr-conversionlayers. The findings indicate that alkaline pretreatments result in double-layeredcoatings, consisting of an oxidic and fluoridic layer. Acidic conditions althoughform much thicker oxidic layer, deflect an insoluble fluoride layer formation. Therefore, strong alkaline pretreatment showed to be preferrable for building an uniform complex Zr-conversion coating.