A combined XPS/ToF-SIMS approach for the 3D compositional characterization of Zr-based conversion of galvanized steel

The present study investigates the surface and in-depth characteristics, and deposition mechanism of zirconium-based conversion coatings on hot dip galvanized (HDG) steel substrates for automotive applications, based on zinc galvanizing baths with small additions of aluminium . This contribution provides scientific proof of the chemical heterogeneity both at the surface and along the depth of the Zr-based conversion coatings on galvanized steel. To this purpose, the definition of two derivative model systems – pure zinc and pure aluminium – was indispensable due to Al-enrichment phenomena observed at the galvanized steel surface. A 3D evaluation of the sample chemistry before and after formation of the Zr-based conversion coatings was conducted by a combined approach based on X-ray photoelectron spectroscopy (XPS) and time-of-flight secondary ion mass spectrometry (ToF-SIMS) with 1-keV Cs + sputtering. The experimental results provide direct evidence of the fact that Zr-based conversion coatings on the HDG steel are constituted by a mixed Zn, Al and Zr oxyhydroxide in combination with Zr oxy-/hydroxyfluoride on top of a complex fluoride (hydr)oxide Zn/Al layer, rather than a pure ZrO 2 film on top of the galvanized layer mainly made of zinc (and aluminium) as previously reported in the literature.