XPS Analysis of Stainless Steel Surfaces
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XPS Analysis of Stainless Steel Surfaces Tim Nunney, Thermo Fisher Scientific, East Grinstead, West Sussex, UK The Thermo Scientific K-Alpha XPS was used to analyze the surface and layer composition of stainless steel surfaces to identify the successfulness of the passivation process. Introduction The surface chemistry of stainless steel is strongly influenced by the chemical or mechanical processing of that surface. Preventing corrosion is a major ambition of any steel manufacturing process and this is often achieved through passivation. This typically involves the formation of a chromium-rich ‘passive’ layer on the top surface (Figure 1) and there is a number of different methods employed to encourage the growth of such a layer. The key metrics which can be used to identify properly passivated steel are the total Cr/Fe atomic ratio, the Cr oxide/Fe oxide ratio, and the total oxide thickness. Experimental and Results Passivated and unpassivated steel samples were analyzed using Thermo Scientific K-Alpha XPS. The iron chemistry of surfaces was investigated using high energy resolution XPS. Metallic and oxide states were observed in both cases, with significantly reduced oxidation on the passivated sample. Figure 2 shows the chemistry of the top 5 nm of the surface. Peak fitting of the two spectra allows us to quantify the different chemical states. The broader oxide peak shape of the passivated sample is due to a mixture of iron (III) and iron (II) states. These two oxide states are present in almost equal quantities. In contrast, the unpassivated sample only has iron (III). Passivation significantly changes the ratio of iron oxide and metallic states. Chromium data from the unpassivated and passivated samples, in contrast, showed only a minor difference in the oxide-to-metal ratio between the two samples (Table 1). Combining the iron and chromium datasets, we can derive the useful parameters which are then used to assess the quality of the passivation (Figure 3). The goal of passivation is to maximize the chromium- containing layer on the surface. Key Words • Passivation • Stainless Steel • Surface Analysis • XPS Application Note: 52108 Figure 1: Passivated stainless steel samples and schematic of layer composition Passivated Sample Unpassivated Sample Figure 2: High resolution Fe spectra from passivated and unpassivated stainless steel surfaces Non-destructive Surface Measurement Passivated Unpassivated Cr Total /Fe Total 1.56 0.77 Cr-O/Fe-O 2.85 0.76 Fe-O/Fe metal 0.77 4.26 Cr-O/Cr metal 5.33 5.38 Oxide Thickness 1.2 nm 3 nm Table 1: Useful steel parameters measured non-destructively with K-Alpha
Transcript of XPS Analysis of Stainless Steel Surfaces
XPS Analysis of Stainless Steel SurfacesTim Nunney, Thermo Fisher Scientific, East Grinstead, West Sussex, UK
The Thermo Scientific K-Alpha XPS was used to analyzethe surface and layer composition of stainless steel surfacesto identify the successfulness of the passivation process.
Introduction
The surface chemistry of stainless steel is strongly influencedby the chemical or mechanical processing of that surface.Preventing corrosion is a major ambition of any steelmanufacturing process and this is often achieved throughpassivation. This typically involves the formation of achromium-rich ‘passive’ layer on the top surface (Figure 1)and there is a number of different methods employed toencourage the growth of such a layer. The key metrics whichcan be used to identify properly passivated steel are thetotal Cr/Fe atomic ratio, the Cr oxide/Fe oxide ratio, andthe total oxide thickness.
Experimental and Results
Passivated and unpassivated steel samples were analyzedusing Thermo Scientific K-Alpha XPS. The iron chemistry ofsurfaces was investigated using high energy resolution XPS.Metallic and oxide states were observed in both cases, withsignificantly reduced oxidation on the passivated sample.Figure 2 shows the chemistry of the top 5 nm of the surface.Peak fitting of the two spectra allows us to quantify thedifferent chemical states. The broader oxide peak shape of the passivated sample is due to a mixture of iron (III)and iron (II) states. These two oxide states are present inalmost equal quantities. In contrast, the unpassivated sample only has iron (III).
Passivation significantly changes the ratio of iron oxideand metallic states. Chromium data from the unpassivatedand passivated samples, in contrast, showed only a minordifference in the oxide-to-metal ratio between the twosamples (Table 1). Combining the iron and chromiumdatasets, we can derive the useful parameters which arethen used to assess the quality of the passivation (Figure 3).The goal of passivation is to maximize the chromium-containing layer on the surface.
Key Words
• Passivation
• Stainless Steel
• Surface Analysis
• XPS
ApplicationNote: 52108
Figure 1: Passivated stainless steel samples and schematic of layer composition
Passivated Sample Unpassivated Sample
Figure 2: High resolution Fe spectra from passivated and unpassivated stainless steel surfaces
Non-destructive Surface Measurement
Passivated Unpassivated
CrTotal/FeTotal 1.56 0.77
Cr-O/Fe-O 2.85 0.76
Fe-O/Femetal 0.77 4.26
Cr-O/Crmetal 5.33 5.38
Oxide Thickness 1.2 nm 3 nm
Table 1: Useful steel parameters measured non-destructively with K-Alpha
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These parameters describe the elemental and chemicalproperties of the steel between 0 and 5 nanometers. Whenthe results of these measurements indicate a deviation fromrequired passivation standards, a sputter profile experimentcan be used to gain a better understanding of the problemsinvolved. Profiles of the passivated and unpassivated samples(Figure 4) confirm the significant reduction in oxide thicknesson the steel. The passivation prevents much of the iron oxidation observed on the unpassivated sample. However,the chromium oxide thickness is also reduced on the passivated sample. This reduction in total oxide thicknessmeans that more of the iron-rich metallic substrate is seenby the surface XPS measurement.
Summary
K-Alpha analysis indicated significant reduction in ironoxidation as a result of passivation. Quantification of elemental and chemical states using XPS showed that thepassivation quality fell short of requirements. Sputter depthprofiling revealed the cause of the passivation problem to bea significant reduction in the level of iron oxidation at thesurface. Quantitative measurement of a number of usefulparameters, however, demonstrated that the passivationmethod had not produced a protective layer of the requiredquality. Sputter profiling of the surface revealed that thepassivating chromium oxide layer was too thin.
Acknowledgements
Thermo Fisher Scientific would like to acknowledge Brian Strohmeier and RJ Lee Group for all of their help with this work.
Figure 4: Sputter depth profiles of unpassivated and passivated steel surfacesFigure 3: Comparison of Cr and Fe chemical states of passivated and unpassivated steel surfaces
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Unpassivated
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