Nanoscale Structure and Compositional Analysis of Surface Oxides on Hot–Dip Galvanized Press–Hardened Steel

Insights into the structure of surface oxides on Zn–coated press–hardened steel (PHS) are indispensable for further processing like welding or adhesive bonding. The main oxide on top of galvanized steel is Al2O3 from the primary galvanizing process. This oxide is strongly altered during final austenitization annealing before hot–forming resp. press–hardening, depending on the annealing time. Four different steel grades are investigated, whereupon two are standard galvanized and the other two are galvannealed. For both coatings, we can show that the main oxides after austenitization heat treatment are native ZnO and the initial Al2O3. Our results indicate that the main alloying component Mn forms (Mn,Zn)Mn2O4 spinel, which also contains traces of Fe and is embedded in the upper ZnO layer. Also noticeable was a varying, nanometer thick Cr enrichment at the initial Al2O3 layer depending on the availability of Cr in the steel alloy. Small precipitates of further alloy elements can be found at this (Cr,Al)2O3 layer. Further experiments with time–of–flight secondary ion mass spectrometry attached to a helium ion microscope allowed to reliably distinguish between ZnO and Zn(OH)2, which are both present in the oxide layers. All specimen show high local differences related to skin–passing prior to final heat treatment. This is especially the case for shorter annealed specimen, where thermodynamic equilibrium is not yet reached.