SEM EDX Analysis of Sub-Micron Thin Oxide Scale on 316L Stainless Steel

Stainless steel is a promising substrate for the synthesis of Carbon Nanotubes (CNTs).An important first step in growing CNTs is a heat treatment in air to oxidize the stainless steel, forming a thin layer of oxide scale (Figure 1A) which later catalyzes CNT growth [1].Understanding the composition of this oxide layer across various preparation times and temperatures allows us to better model CNT growth and choose the ideal parameters for growing CNTs on stainless steel.Popular methods to analyze the composition of thin films, such as Transmission Electron Microscopy (TEM), are timeconsuming, expensive, and often result in sample destruction.Energy Dispersive X-Ray (EDX) analysis of thin films using a Scanning Electron Microscope (SEM) can be difficult due to the large size of the e-beam interaction volume, compared to the small thickness of the region of interest; X-Ray emission in these conditions tends to be dominated by the substrate material, drowning out any usable data from the thin film.However, we have demonstrated that usable comparative EDX data concerning the composition of thin oxide scale (50-200 nm) can be gathered using largely non-destructive SEM techniques.316L stainless steel samples were cut from a larger sheet into 1 cm 2 tiles.These samples were placed in a Chemical Vapor Deposition (CVD) furnace and heated to temperatures ranging from 600C to 900C, while exposed to air flowing at 371 sccm for times ranging from 0.5 to 4 minutes.This annealing process forms a thin layer (50-200 nm) of metal oxide scale on the steel surface (Figure 1A).The samples were studied in a Helios Nanolab 600 SEM equipped with EDX analysis and Focused Ion Beam (FIB) capabilities to collect compositional data.A high-angle (80tilt) was used to increase exposure of the surface of the sample to the electron beam, thus increasing X-ray emission from the region of interest (Figure 1B).EDX spectra obtained from samples with different anneal parameters (Figure 2) showed varying ratios between X-ray peaks for different elements.For example, the height of the Cr K peak (normalized to the Fe K peak) increased with preparation time, indicating that relative concentrations of metals in their oxide scale layers change based on anneal parameters.Further, the FIB was used to mill away the scale from the top layers of samples to gather EDX control data from the bulk steel substrate (Figure 2, inset).Analysis of data, collected from a wide range of oxidation temperatures and times, identified several trends regarding how the concentrations of metals in the oxide scale layer change with anneal parameters: (1) the Cr/Fe ratio increases with time and temperature; (2) the Ni/Fe ratio decreases with time and temperature; (3) the Cr/Mn ratio decreases with time and temperature; (4) Apparent relative surface oxidation rates of elements of interest: Mn > Cr > Fe > Ni.Further, a trend in the relationship between Cr/Fe ratios and anneal parameters becomes evident by plotting extracted (Cr K )/(Fe K ) ratios from EDX spectra and generating a contour plot (Figure 3).While this method does not provide quantitative elemental composition of the surface oxide layer, it does provide nondestructive comparative data regarding the surface characteristics that can be later correlated with CNT growth [5].