Analysis of Stainless Steel by Dual View Inductively Coupled Plasma Spectrometry

Transcription

1 Prodigy ICP Application Note: # 1046 Analysis of Stainless Steel by Dual View Inductively Coupled Plasma Spectrometry Introduction Stainless steels are a corrosion resistant family of iron alloys that have a minimum of 10.5% Chromium. Their corrosion resistance is largely due to the formation of a passive chromium (III) oxide (Cr 2 O 3 ) layer, approximately 1 to 5 nanometers (nm) thick, on the surface of the steel. If this layer is damaged by cutting, scratching or abrasion, it will regenerate, provided sufficient oxygen is available. By contrast, Stainless steels have poor corrosion resistance in low oxygen environments since the oxide layer cannot be repaired quickly enough. In addition to chromium, Nickel, molybdenum and niobium are also alloyed to improve corrosion characteristics. There are three main types of stainless steels: austenitic, ferritic and martensitic. These three types of steels are identified by their microstructure or predominant crystal phase. Austenitic steels have austenite (γ-iron) as their primary phase. These are alloys containing chromium and nickel (sometimes manganese and nitrogen), structured around the Type 302 composition of iron, 18% chromium, and 8% nickel. They are normally non-magnetic. Type 304 surgical stainless steel is the most widely used stainless steel and contains 18-20% chromium and 8-10% nickel. Ferritic steels have ferrite (α-iron) as their main phase. These steels contain iron and chromium, based on the Type 430 composition of 17% chromium. Ferritic steels are magnetic and are less ductile than austenitic steels. Typical uses are in automotive exhaust systems, catalytic converters and chimney liners. Martensitic steels are low carbon steels built around the Type 410 composition of iron, 12% chromium, and 0.15% carbon. They have great strength and are magnetic. Typical uses include cutlery, springs, screen and strainers. Table 1 shows acceptable ranges and maximum concentrations for elements in some stainless steels. AISI Number Type C Mn Si Cr Ni P S Wt % 302 Austenitic Austenitic Ferritic Martensitic Table 1 Composition of Various Stainless Steels 6 Wentworth Drive Hudson, NH U.S.A. Tel: Fax:

2 This application note will demonstrate the ability of the Teledyne Leeman Labs Prodigy High Dispersion ICP to analyze stainless steels. Prodigy s dual viewing option will be used to permit measurement of the high concentration elements in radial mode, and the low concentration elements (phosphorus (P) and sulfur (S)) in axial mode. Instrumentation A Prodigy High Dispersion Inductively Coupled Plasma (ICP) Spectrometer equipped with a dual view torch and an 88-position autosampler was used to generate the data for this application note. The Prodigy is a compact bench-top simultaneous ICP-OES system featuring an 800 mm focal length Echelle optical system coupled with a mega-pixel Large Format Programmable Array Detector (L-PAD). At 28 x 28 mm, the active area of the L-PAD is significantly larger than any other solid-state detector currently used for ICP-OES. This combination allows Prodigy to achieve significantly higher optical resolution than other solid-state detector based ICP systems. The detector also provides continuous wavelength coverage from 165 to 1100 nm permitting measurement over the entire ICP spectrum in a single reading without sacrificing wavelength range or resolution. This detector design is inherently anti-blooming and is capable of random access, non destructive readout that results in a dynamic range of more than 6 orders of magnitude. For applications that require the measurement of chlorine, bromine or iodine an optional halogen detection system is available. The Prodigy uses a MHz free running, water-cooled oscillator, allowing it to handle the most difficult sample matrices. A high sensitivity sample introduction system ensures that sufficient and steady emission signals are transmitted to the spectrometer. The torch and sample introduction system are uniquely integrated into the optical system through Prodigy s innovative Image Stabilization system, which treats the torch as an optical component by rigidly attaching it to the spectrometer. This configuration yields unprecedented long-term stability. The sample introduction system for this work consisted of a four-channel peristaltic pump, HF resistant Cyclonic spray chamber, demountable quartz torch with an alumina injector and a Concentric Nebulizer. Operating Parameters are shown in Table 2. Parameter Setting Part Number RF Power 1.2 kw Torch Demountable Coolant Flow 20 l/m Auxiliary Flow 0.0 l/m Nebulizer Pressure 50 psi Nebulizer HF Resistant Concentric Spray Chamber HF Resistant Cyclonic Sample Uptake Rate 1.4 ml/min Optical Purge Flow Low Axial Integration Time 30 sec Radial Integration Time 15 sec Injector Alumina Table 2. Prodigy Operating Conditions

3 The analytical viewing zone for both the axial and radial views was set by using a 10 ppm Mn standard. The optimum viewing position is automatically selected by the Prodigy s software. Method Sample Preparation Four Stainless Steel reference materials NIST SRMs 121b and 123c (AISI 348), BCS CRM 467/1 and Euronorm ZRM were used in this study. Approximately 1 gram of each material was placed in a Teflon beakers, covered with a minimum of deionized water (DIW) and placed on a hot plate. The samples were dissolved using 10 ml of aqua regia (HCl/HNO 3, 3:1) and 1 ml hydrofluoric acid (HF) while gently heating. Once the dissolution was complete, the samples were diluted to 100ml with DIW. Calibration Standards Calibration standards were made from single element Teledyne Leeman Labs Plasma Pure ICP standards. In addition, the standards were matrix-matched to the Fe concentration of the reference material by using Plasma Pure Fe concentrate. The final acid concentration in the standards was 5% Aqua Regia/1% HF. The concentrations in the calibration standards are listed in Table 3. Std1 Std2 Std3 Std4 Wavelength Parameters Mn P, S, Cu, Mo, Co Si, Nb Ni, Cr Ti, V Table 3 Calibration Concentrations, ppm For each wavelength, the Prodigy uses a 3 x 15 pixel subarray, which is centered on the wavelength of interest. Background correction points and the analytical peak have both position and width values within the subarray. In the Table 4 below, the position value is designated by x in the column header, while w indicates the width. The default position for the analytical peak is 7 with a width of 3 pixels. Where possible, two wavelengths were used for each element. The letter r after the wavelength indicates analysis using the radial view. All data in the subarrays are collected simultaneously. In addition, all pixel data are saved, permitting recalculation of results at a later time. 3

4 LBX LBW Peak X Peak W RBX RBW Line Left Right Background Left Background Peak Right Background Background Position Width Position Peak Width Position Width Mn r Mn r P P S S S Si r Si r Si r Cu r Cu r Ni r Ni r Ni r Cr r Cr r Mo r Mo r Mo r Nb r Nb r Co r V r Ti r Ti r Table 4 Wavelength Parameters As an example, Figure 1 illustrates the elements parameters for the P nm line. In Figure 1, the left and right background regions begin at pixel positions 1 and 10, respectively, with widths of 1 pixel. The analytical region of interest, where the P peak is found, begins at pixel position 7 and has a width of 3 pixels. The thin line running under the peak shows the calculated background correction based on the two correction regions. Figure 1. P 177 nm Element Parameter Example 4

5 Figure 2 illustrates a calibration curve showing typical precision and linearity for the concentration range used. Figure 2. Typical Calibration Curve Results After igniting the plasma and allowing a 15 minute warm-up period, the Prodigy was calibrated using the autosampler. Once the calibration was complete, a 1 ppm QC Standard was analyzed with an acceptance criteria of ±10%. Upon successful completion of the QC Standard analysis, the reference samples were analyzed. After the sample analysis, the QC Standard was re-analyzed. (The Prodigy s software allows the entire sequence to be run unattended. Should a QC Standard be out of specification, Prodigy allows for an variety of actions including automatically recalibrating and rerun the QC Standard and any samples that were analyzed since the last successful QC Standard was run.) The analysis results are shown in Tables 5-6. All concentrations are in %. The values measured by the Prodigy are contained in the column labeled Measured % while the certified values are in the column labeled Certified %. (The certified value listed is not expected to deviate from the true value by more than ±1 in the last significant figure reported; for a subscript figure, the deviation is not expected to be more than ±5). The agreement between the measured and certified values is quite good. The Prodigy software is also capable of calculating the average concentration for an element when multiple analysis lines are used. The column labeled Average % displays the results from the multiple lines used. 5

6 NIST 123c NIST 121b Measured, % Average, % Certified, % Measured, % Average, % Certified, % 0 Mn r Mn r P P S S Si r Si r Si r Cu r Cu r Ni r Ni r Ni r Cr r Cr r Mo r Mo r Mo r Nb r Nb r Co r V r Ti r Ti r Table 5. NIST 123c and NIST 121b Results, % 6

7 Euronorm ZRM BCS CRM 467/1 Measured, % Average, % Certified, % Measured, % Average, % Certified, % Mn r ± 0.03 Mn r P ± P S ± S Si r Si r Si r Cu r Cu r Ni r Ni r ± Ni r Cr r ± 0.08 Cr r Mo r Mo r ± Mo r Nb r Nb r Co r ± V r Ti r Ti r Table 6. Euronorm ZRM and BCS CRM 467/1 Results, % Discussion A comparison of the measured and certified values of the elements determined in the four stainless steel reference samples is shown in Tables 5 and 6. The agreement between the measured and certified value is very good, except for the S in the ZRM 286-1, which is low. The ZRM reference material was re-prepped and reanalyzed two additional times without any significant change in the measured S value. Post digestion spike recoveries on both of these preps yielded recoveries close to 100%. The most likely cause is volatization of the sulfur during the sample dissolution procedure. Use of a close vessel digestion procedure would prevent such losses. Conclusion The analysis of alloying elements in stainless steels has been carried out for 13 elements using a dual view Teledyne Leeman Labs Prodigy High Dispersion ICP. Accurate results were obtained by carefully matrix matching the base iron concentration of the calibration standards to the samples. 7

8 The HF sample introduction system performed without any clogging of the torch or nebulizer and did not require the use of an argon humidifier. The image stabilized plasma and the simultaneous data collection of both peak and background data combine to provide exceptionally precise and stable results. 8