Stainless steels are steels with a high degree of corrosion resistance to a wide range of aggressive chemicals. The corrosion resistance is mainly due to their high chromium content which is normally more than 12 wt%. Chromium makes the surface passive by forming a surface oxide film which protects the underlying metal from corrosion. Nickel added to stainless steel improves corrosion resistance in neutral or weakly oxidizing environments. Molybdenum added to stainless steel improves corrosion resistance in the presence of chloride ions.
Analysis of samples for melting control of stainless steel requires the fastest possible turnaround of results. This minimizes errors due to melting losses and maximizes production. At the same time close specifications and tight control of expensive alloy additions demand the highest possible accuracy and precision.
Speed, precision, stability, sensitivity and accuracy are important criteria applied by the stainless steel manufacturer in using X-ray fluorescence spectrometry as method of analysis.
Axios FAST is a fully integrated simultaneous wavelength dispersive XRF spectrometer, complete with X-Y sample handler and state-of-the-art software. It is engineered for excellence in terms of both analytical and operational performance.
X-ray tube: Rhodium anode 4 kW SST-mAX tube
Tube power: 60 kV/ 66 mA
Fixed channels: C, Si, P, S, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, As, Mo
Shielding plate: 27 mm Rh plated
Sample holder: Stainless steel 27 mm aperture
Flow counter window: 0.6 μm for carbon analysis
Sample preparation and measurement details
14 standards (SS 461/1 – SS 468/1 and SS 469 – SS 474) were used to set up the calibration. The standards were freshly surfaced prior to measurement with a Herzog grinding machine using 150 grit Al2O3 sandpaper. The X-ray intensities for 15 elements were simultaneously measured for 30 s at the peak. The calibration was performed using the Fundamental Parameters (FP) regression model incorporated in the SuperQ software package. During calibration line overlap corrections were automatically determined through multiple regression by the software. No other corrections were applied.
The analysis speed is usually limited by the element with the lowest sensitivity (carbon in this case) and/or the lowest concentration. With the Axios FAST excellent results are obtained for all elements, including carbon.
Table 1 gives an overview of the elements with their concentration range, their respective values for the ‘root mean square’ (RMS) error, K factor and the lower limit of detection (LLD) based on a 100 s counting time.
Table 1 – Values for ‘root mean square’ (RMS), K factor, and lower limits of detection (LLD)
RMS (absolute error) and K (relative error) represent the accuracies of the calibration. The K factor is a weighted residual error and is a measure of the differences between the measured concentration and the supplied, certified chemical concentration. As a general rule, K factors < 0.05 indicate accurate calibrations. Using the K factor it is possible to estimate the accuracy of a result, accounting for both instrumental error and errors associated with the certified concentrations of the reference materials defining the calibration. Figures 1 and 2 show the calibration graphs for Cr and Ni.
Figure 1. Calibration graph for chrome
Figure 2. Calibration graph for nickel
Precision and instrument stability
The precision, repeatability and reproducibility of Axios FAST is outstanding, not only for short-term measurements (20 consecutive measurements), but also for longer-term measurements (measurements carried out over a period of 10 days), see Table 2 and Figure 3. For comparison, the counting statistical error (CSE), which is the theoretical minimal possible error, is also shown in Table 2. No drift correction was applied.
Table 2. Analytical precision for Si, Mn, P, S, Cr, Mo, Ni, Cu and Fe (measured on SS 461/1)
Figure 3. Short- and long-term stability measurements of Cr and Ni in SS 461/1 stainless steel standard
The Malvern Panalytical Axios FAST is capable of analyzing stainless steel precisely and accurately. A total analysis time of only 30 seconds is required to analyze 15 elements, including carbon.