7) FIB/SEM/EDX Analysis of Protective Coating on Steel

Investigation of a protective coating layer structure using EDX microanalysis on a FIB milled cross-section.

Introduction

The most widely used metallic coating for the corrosion protection of steel is a zinc coating. It offers a very good combination of galvanic and barrier protection. Zinc coatings for iron and steel provide excellent corrosion resistance in most atmospheres, in hard fresh waters, and in contact with many natural and synthetic substances. Zinc coatings are widely used to protect finished products ranging from structural steelwork for buildings and bridges, to nuts, bolts, strip, sheet, wire and tube.
Excellent corrosion protection and forming properties are claimed for a combination aluminium–zinc–silicon coated steel.

Instrumentation

• Focused Ion Beam (FIB), capable of making precisely localized cross–sections of coatings is a proper tool for coating examination.
• Scanning electron microscope (SEM) is then used for imaging of coating surface as well as for imaging of cross–sections.
  
• Together with Energy–Dispersive X–Ray Spectrometer (EDX) it allows precise structural and elemental analysis of coating. Moreover, high resolution non–destructive FIB imaging can be utilized for examination of fine bulk and interface details and for detection of possible defects.
  

Experimental Set-Up

Analysis was done on the LYRA I XMU — variable pressure SEM–FIB system equipped with EDX analyzer Quantax400 from Bruker company.
Cross–sections were milled into the sample surface using 30kV Gaion beam. Beam current 10nA was used for rough milling of the trench. Beam current 500pA was used for final polishing of the trench wall.

RESULTS OF MILLING

Figure1 shows cross–sections milled into Zn/Alcoated steel. Typical trench width was 30μm and depth 15μm, which is usually enough because of protective layer thickness (~10μm). Doing two perpendicular cross–sections gave us possibility to study anisotropical effects in coating structure. It is also possible to mill one big trench with two sides polished, however, in this case it is necessary to remove more material and to prevent redeposition problems.Figure2 shows high resolution FIBimage of cross–section edge. Here it is evident how FIBimaging brings into light fine surface details, due to its extreme surface sensitivity. Related high edge resolution, noticeable material contrast as well as grain contrast originated due to ion channelling for polycrystalline materials are great benefits of FIB imaging.

RESULTS OF EDX ANALYSIS

Figure3 shows typical EDX element maps taken at the cross–section of the coating. The interface between the steel substrate and the coating is evident. Moreover, it is possible to study distribution of elements in the coating, the dependence on depth for each element and the presence of elements on the surface of the coating.

Figure3 for example shows the lack of uniformity of Zn in the volume of the coating and its agglomeration around coating defects. Alis present on the coating surface and its concentration decreases rapidly with increasing depth in the coating.

CONCLUSION

Cross–sections were successfully prepared in protective coating of steel samples using LYRA XMU system. FIB part of LYRA was used for milling and polishing of cross–sections, SEM and EDX were used for structural and elemental analysis of coating. High resolution FIB imaging with high surface sensitivity gave excellent sharpness of cross–section edges and revealed a great variety of surface details. The possibility of rapid milling using high ion current as well as high resolution imaging utilizing strengths of both beams are worthwhile features of Lyra system.

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