Effect of Surface Pretreatments on the Formation of Zr-Based Conversion Layers on Zn–Al–Mg Alloy-Coated Steel

Abstract

Zr-based conversion coatings represent an environmentally conscious alternative to traditional phosphating and chromating in the automotive industry. In this study, we employ XPS and LEIS to investigate the formation of Zr-conversion layers on Zn–Mg–Al alloy after alkaline and acidic model pretreatments. On alkaline pretreated surfaces, a Zr-oxide/oxyfluoride layer and an underlying Mg–Al–fluoride layer are formed, whereas acidic pretreatment results in only an oxidic layer. The thickness of the Zr-layer depends on pretreatment pH and immersion time. Acidic treatment achieves an approximately 23 nm-thick Zr-oxide/oxyfluoride layer after 1 min, while prolonged treatment increases the thickness of the oxidic layer for strong alkaline and acidic conditions. Mild alkaline pretreatments, however, do not benefit from extended immersion. F-induced corrosion pits are observed after mild alkaline treatment. The strong alkaline pretreatment proved to be the most efficient in creating a double-layered Zr-conversion coating with increased oxidic layer thickness over time.