Initiation and growth of modified Zr-based conversion coatings on multi-metal surfaces

In this work, a new surface pre-treatment based on a modified hexafluorozirconic acid solution with addition of copper was studied as a replacement of the phosphating process. The main purpose of this study is to compare the formation mechanism and kinetics of Zr-based conversion coatings on three different base substrates: Aluminium alloy (AA) 6014, cold rolled steel and hot dip galvanized steel. For that purpose, open circuit potential (OCP) measurements during the deposition of the conversion coatings are performed in combination with different ex-situ surface characterization techniques. The elemental distribution was analyzed by means of (Field Emission) Auger Electron Spectroscopy (FE-AES) depth profiles and mappings. Furthermore, the surface chemistry of the conversion layer was evaluated by X-ray Photoelectron Spectroscopy (XPS). The results showed that while the mechanism of formation is similar for the different substrates, the rate of formation strongly depends on the substrate type. Additionally, the thickness and lateral and in-depth elemental distribution of the Zr-based conversion layers also largely depend on the underlying metal.