Composition and Structure of Surface Oxides on Press-Hardened Hot-Dip Galvanized Steel

Zink-coated press hardened steel (PHS) is due to its extremely high tensile strength and its excellent corrosive protection indispensable for structural elements in modern automotive applications. Different studies clarify the resulting microstructure of the Zn-coating after press hardening [1] of such steel parts. Also, the uppermost surface oxide layer, which can be influenced by steel matrix alloy elements, has a significant impact on the processing properties like coating or welding.
In this study, we investigated these surface oxide layers for alloy compositions of four different PHS systems with scanning and conventional transmission electron microscopy (TEM), Auger electron spectroscopy (AES) and helium ion microscopy (HIM). The complex and porous oxide-layers were stabilized by epoxy before the preparation of cross-sections and TEM-lamellae by focused ion beam milling.
The main oxides on top of the original thin Al2O3 layer, originating from the primary galvanizing process, could be determined with energy dispersive X-ray spectroscopy (EDX), selected area diffraction and AES as Zn-oxides and (Mn,Zn)Mn2O4 spinel (Fig. 1a). Also noticeable was a varying, nanometre thick Cr enrichment at this Al2O3 layer (Fig. 1b), which depends strongly on the steel alloy. Further experiments with secondary mass spectrometry (ToF-SIMS) attached to a HIM [2] allowed reliable distinguishing between ZnO and Zn(OH)2 (Fig. 2c), which are both present in our oxide layers.
Thus, we could show an influence of different steel alloy elements on the final oxide structure after press hardening. The formation of spinel is triggered by Mn, which is transported with the Zn above the Al2O3 layer, which itself is a trap for Cr, leading to a coverage of it with Cr-oxides.

Figure 1. a) AES Mapping. b) STEM-EDX Mapping c) ToF-SIMS Mapping

References:

[1] H. Järvinen et al, Surface and Coatings Technology 352 (2018), 378-391.
[2] N. Klingner et al, Ultramicroscopy 198 (2019), 10-17.

The financial support by the Austrian Federal Ministry for Digital and Economic Affairs and the National Foundation for Research, Technology and Development in the frame of the CDL for nanoscale phase transformations is gratefully acknowledged.