Surface of alloyed steels is often composed of stable oxides formed during the recrystallization annealing performed just before hot-dip galvanizing. This annealing is carried out at about 800°C under reducing atmosphere (N₂ - 5 vol.% H₂) containing low water partial pressure (dew point around -40°C). During this treatment native iron oxides are reduced but, less-noble alloying elements in steel such as Si, Mn, Al, … segregate to the surface and form oxide particles or films. These oxides affect steel reactivity during immersion in alloyed zinc bath.

To precisely simulate all phenomena occurring during these former process steps, it is crucial to reproduce representative conditions of industrial lines. Thus, a high temperature wettability device (HTWD) was designed to study these gas–metal interactions and liquid metal wettability phenomena. This device is directly connected to our X-rays photoelectron spectroscopy (XPS) equipment. From now on, the identification of elements present on the top surface and their bonding states is possible before and/or after contact with the liquid phase, thus avoiding contamination by the ambient atmosphere. This setup is particularly useful to better understand and improve the coatability of advanced high-strength steels (AHSS) [1].

Within this scope, selective oxidation of binary FeAl alloys (with 1.5 and 8 wt.% of Al) was studied under annealing atmosphere dew points varying from -60°C to +10°C. Surface chemistry was analysed in-situ and ex-situ by X-rays Photoelectron Spectroscopy, as well as ex-situ by FEG-Auger microscope and Secondary Ion Mass Spectrometry. Thin top surface oxide films and cross-sections extracted with a Focused Ion Beam Microscope were characterized in a Transmission Electron Microscope.

The chemistry of oxides observed on the FeAl alloys top surfaces is well predicted by thermodynamic model, where at lower dew points the alloy’s surfaces were covered by Al₂O₃ layer and at higher ones the surface oxide was FeAl₂O₄. As supposed with the increase of the atmosphere dew point, internal oxidation is favoured by demonstrating two morphologies: alumina nodules and a dense alumina layer at the oxidation front. In parallel with the internal oxidation, metallic iron particles appear on the surface of the samples annealed at higher dew points. These particles can cover a large part of the steel surface. The evolution of both internal and external oxidations coupled to the surface wettability is discussed, as well as the mechanisms of metallic particles formation.

References

[1] M-L. Giorgi, J. Diawara, S. Chen, A. Koltsov, J-M. Mataigne, Influence of Annealing Treatments on Wetting of Steels by Zinc Alloys, J. Mater. Sci., 2012, 47 (24), 8483.