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

[Display omitted] •The 3D chemistry of the converted HDG steel system is elucidated by a combined XPS-SIMS approach.•Zr-based conversion coatings on HDG steel are constituted of a mixed Zn, Al and Zr oxyhydroxide.•The HDG steel shows Al-enrichment at both the surface and the interface with the ferrous substrate.•Both pure Zn and pure Al are required for a mechanistic understanding of the Zr-based conversion of the HDG steel.•Pure aluminium is less sensitive to Zr-based conversion compared to pure zinc. 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 ZrO2 film on top of the galvanized layer mainly made of zinc (and aluminium) as previously reported in the literature.