Investigation into the surface selective oxidation of dual-phase steels by XPS, SAM and SIMS

Surface selective oxides created during continuous annealing (MnO, SiO2, etc.) can have a deleterious effect on coating adhesion after hot-dip galvanizing. Earlier research works have made it clear that increasing the annealing atmosphere oxidizing potential can alleviate the problem by reducing external surface selective oxidation. In the present study, increasing the water vapour content of the nitrogen-hydrogen protective gas mixture was used to raise its oxidizing potential. The technique was applied to a classical dual-phase steel (0.15% C, 1.5% Mn, 0.45% Si, 0.05% Al...) that was annealed for 60 s at 800-810degreesC in protective atmospheres of nitrogen and 5% hydrogen with water vapour contents ranging from 10 to 6000 ppm. Post-annealing surfaces were characterized by x-ray photoelectron spectroscopy (XPS), secondary ion mass spectroscopy (SIMS) and scanning Auger microscopy (SAM). In situ XPS analyses were carried out right after simulation annealing in the preparation chamber connected to the spectrometer, never returning the specimens to air. This made it possible to identify in a reliable way the elements that segregated to the surface during the treatment, and to determine their corresponding oxidation states. On the other hand, the high sensitivity of SIMS was taken advantage of to assess oxide in-depth concentration profiles (SiO2, Al2O3, FeO) as a function of the annealing conditions, and SAM was used to characterize the corresponding oxide particle morphology. External selective oxidation was thus shown to decrease with increasing water vapour contents in the atmosphere (from 80 to 6000 ppm), whereas internal oxidation increases drastically to similar to4 pm below the free surface. At 10 ppm of H2O the oxygen partial pressure is very low and the external selective oxidation results in a thin, but almost complete, coverage of the steel surface. Consequently, metallic iron cannot be observed at the surface, thus hampering hot-dip galvanizability, unless the water vapour content is raised to 6000 ppm. Various surface morphologies were observed and discussed. In the authors' opinion, basic investigations of this type are an indispensable first step to improving the response of highly alloyed steels (dual-phase, TRIP) to hot-dip galvanizing. Copyright (C) 2003 John Wiley Sons, Ltd.