XPS investigation on the surface chemistry of corrosion products on ZnMgAl-coated steel.

In this work, a novel evaluation strategy for the X-ray photoelectron spectroscopy (XPS) chemical assessment is proposed to identify the corrosion products formed on the surface of hot-dip galvanized ZnMgAl coatings after exposure to standardized salt spray tests. The experiments demonstrate that the investigated system exhibits a problematic differential charging behavior between the different compounds, an effect which cannot be fully compensated for by the flood gun of the XPS system, making a reliable evaluation of the individual spectra impossible by using standard evaluation and fitting methods. For that reason, a new effective approach--taking the different charge shifts into account--was implemented and its reliability experimentally verified on model mixtures of assumed corrosion products with known composition. With this new approach, the chemical surface composition of an industrial sample of a corroded ZnMgAl coating was revealed and discussed in order to clearly demonstrate the potential of the proposed method for future corrosion studies.

Cryo ultra-low-angle microtomy for XPS-depth profiling of organic coatings.

In X-ray photoelectron spectroscopy (XPS) Ar(+) ion sputtering is usually used for depth profiling. However, for such samples as organic coatings, this is not feasible because of degradation. Also, measurement of a depth profile on a conventionally prepared cross-section is not possible if, for example, sample thickness is below the smallest available measurement spot size of the XPS system. In our approach we used a rotary microtome to cut samples under a shallow tilting angle of 0.5° to obtain an extended cross-section suitable for XPS investigations. We also used liquid nitrogen cooling to ensure an exposed area of higher quality: topography measurements with a novel optical 3D microscope and by atomic force microscopy revealed the linearity of the inclined sections. With our cryo ultra-low-angle microtomy (cryo-ULAM) preparation technique we were able to determine, by XPS, elemental and chemical gradients within a 25 µm thick polyester-based organic coating deposited on steel. The gradients were related to, for example, depletion of the crosslinking agent in the sub-surface region. Complementary reflection electron energy-loss spectroscopy measurements performed on the cryo-ULAM sections also support the findings obtained from the XPS depth profiles.

Nanoscale surface analysis on second generation advanced high strength steel after hot dip galvanizing.

Second generation advanced high strength steel is one promising material of choice for modern automotive structural parts because of its outstanding maximal elongation and tensile strength. Nonetheless there is still a lack of corrosion protection for this material due to the fact that cost efficient hot dip galvanizing cannot be applied. The reason for the insufficient coatability with zinc is found in the segregation of manganese to the surface during annealing and the formation of manganese oxides prior coating. This work analyses the structure and chemical composition of the surface oxides on so called nano-TWIP (twinning induced plasticity) steel on the nanoscopic scale after hot dip galvanizing in a simulator with employed analytical methods comprising scanning Auger electron spectroscopy (SAES), energy dispersive X-ray spectroscopy (EDX), and focused ion beam (FIB) for cross section preparation. By the combination of these methods, it was possible to obtain detailed chemical images serving a better understanding which processes exactly occur on the surface of this novel kind of steel and how to promote in the future for this material system galvanic protection.

X-ray photoelectron and scanning Auger electron spectroscopy study of electrodeposited ZnCr coatings on steel.

Zn-Cr alloyed coatings electrochemically deposited are of high interest for leading steel manufacturing companies because of their novel properties and high corrosion resistance compared with conventional Zn coatings on steel. For tuning and optimizing the properties of the electrodeposited Zn-Cr coatings, a broad range of the deposition conditions must be studied. For this reason, two different types of material were investigated in this study, one with a low electrolyte temperature and one with an elevated electrolyte pH, compared with the standard values. Because different corrosion performance and delamination behaviour of the layers were observed for the two types, advanced surface analysis was conducted to understand the origin of this behaviour and to discover differences in the formation of the coatings. The topmost surface, the shallow subsurface region, and the whole bulk down to the coating-steel interface surface were analysed in detail by X-ray photoelectron spectroscopy (XPS) and high-resolution scanning Auger electron spectroscopy to determine the elemental and the chemical composition. For better understanding of the resulting layer structure, multiple reference samples and materials were measured and their Auger and XPS spectra were fitted to the experimental data. The results showed that one coating type is composed of metallic Zn and Cr, with oxide residing only on the surface and interface, whereas the other type contains significant amounts of Zn and Cr oxides throughout the whole coating thickness.

Nanoscale analysis of surface oxides on ZnMgAl hot-dip-coated steel sheets.

In this work, the first few nanometres of the surface of ZnMgAl hot-dip-galvanised steel sheets were analysed by scanning Auger electron spectroscopy, angle-resolved X-ray photoelectron spectroscopy and atomic force microscopy. Although the ZnMgAl coating itself is exhibiting a complex micro-structure composed of several different phases, it is shown that the topmost surface is covered by a smooth, homogeneous oxide layer consisting of a mixture of magnesium oxide and aluminium oxide, exhibiting a higher amount of magnesium than aluminium and a total film thickness of 4.5 to 5 nm. Especially by the combined analytical approach of surface-sensitive methods, it is directly demonstrated for the first time that within surface imprints--created by industrial skin rolling of the steel sheet which ensures a smooth surface appearance as well as reduced yield-point phenomenon--the original, smooth oxide layer is partly removed and that a layer of native oxides, exactly corresponding to the chemical structure of the underlying metal phases, is formed.

Supercontinuum generation system for optical coherence tomography based on tapered photonic crystal fibre.

We report smooth and broad continuum generation using a compact femtosecond Ti:Sapphire laser as a pump source and a tapered photonic crystal fibre as a nonlinear element. Spectral output is optimized for use in optical coherence tomography, providing a maximum longitudinal resolution of 1.5 microm in free space at 809 nm centre wavelength without use of additional spectral filtering.

Transversal ultrahigh-resolution polarizationsensitive optical coherence tomography for strain mapping in materials.

Optical coherence tomography (OCT) and its extension, polarization-sensitive (PS-)OCT, are techniques for contactless and nondestructive imaging of internal structures. In this work, we apply PS-OCT for material characterization. We use a transversal scanning, ultra-high resolution (UHR-)PS-OCT setup providing cross-sectional as well as inplane information about the internal microstructure, the birefringence and the orientation of the optical axis within the material. We perform structural analysis and strain-mapping for different samples: we show the necessity of UHR imaging for a highly strained elastomer sample, and we discuss the effect of large birefringence on the PS-OCT images. Furthermore, we investigate high-aspect ratio photoresist moulds for the production of microelectromechanical parts (MEMS), demonstrating that transversal UHR-PSOCT is a promising tool for non-destructive strain-mapping.