Anal. Chem. 1995, 67, 21R-32R
Thomas R. Dukki
R&D Center, Carpenter Technology Corporation, P.0. Box 14662, Reading, Pennsylvania 19612-4662 Review Contents General Considerations Atomic Absorption Flame Electrothermal Other Optical Emission Arc/Spark Inductively Coupled Plasma Glow Discharge Other X-Ray Fluorescence Hardware Applied Mathematics and Software Gross Composition Methods Surface Methods Thermal Extraction Carbon and Sulfur Oxygen, Nitrogen, and Hydrogen Mass Spectrometry Inductively Coupled Plasma Glow Discharge Other Chemical Methods Titrimetry Spectrophotometry Flow Injection Inclusions and Phases Surface Techniques Molten Metal Analysis Spectroscopic Methods Gaseous Extraction Methods Electrochemical Probes Miscellaneous Methods Special Topics Sampling Sample Preparation Alloy Sorting Standards
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This review continues with the new format initiated in the 1993 document (1)-that is, a methodological organization with great emphasis placed on instrumental approaches. In the past two years there has been little sign of renewed interest in classical “wet chemistry”, even where those techniques have proven to be of enduring value. Thus, once again, “chemical methods” must be relegated to a minor category. The material for this review was extracted from Chemical Abstracts beginning with Vol. 117, No. 6 and ending with Vol. 121. Additional material was found in Metals Abstracts between August 1992 and July 1994. This represents all of the new information available to the author as of November 3, 1994. Once again a sincere effort has been made to produce a highly concentrated, critical document which hopefully will find use as a practical 0003-2700/95/0367-0021$15.50/0 0 1995 American Chemical Society
reference resource, The author accepts responsibilityand offers apology for any oversights in this task. GENERAL CONSIDERATIONS The business of making steel is now clearly free of the illusion that domestic producers need only domestic markets in which to sell their commodities. The new global marketplace is an unfamiliar arena, with US. companies in the uncomfortable role of playing “catch-up”. Worldwide overcapacity and protectionist policies further suggest that in this heady realm many may survive, but few will prevail. Speed and quality play the same sine qua non role globally as they do in home markets. In fact, there are situations where only the on-time delivery of the irreplaceably best product will do. Analytical technology has a key function in this international competition: to ensure quality at unprecedented levels and to monitor production processes at unprecedented speeds. Thus, we have seen the growing acceptance of ICP and GD mass spectrometries for the routine determination of ultratrace contaminant levels and the emergence of “container”laboratoriesand molten metal probes to provide nearly instaneous furnace-side analyses. Determinators designed to quantify carbon and sulfur at part-per-million levels are becoming commonplace, while nitrogen by optical emission is emerging as a means of subtracting seconds in production control. Numerous reviews have been published that concentrate on broad areas of interest, such as trace analysis (2,3),determination of nonmetallic elements (4), on-line systems for production applications (5-3, quality assurance (8), analysis of the new ultraclean steels (9, IO),and the robotic containerlaboratory (11). The use of robotics, in particular, is growing rapidly, especially among the basic steel producers in developed countries. Container laboratories typically are designed with all aspects of sample preparation and instrumentcalibration under closed-loop computer control. X-ray fluorescence analysis is common in these systems (12, 13), optical emission and thermal evolution methods for carbon, nitrogen, and sulfur somewhat less so (14-17). The use of control charts for dynamic instrument control is another emerging concept (18, 19). Several product-specific summarytype papers have also been published covering analytical methodology for ferroalloys (2O), nuclear reactor core components (21),armor-plate (22),and high-purity iron (23). ATOMIC ABSORPTION
Atomic absorption spectrophotometryhas been challenged on a number of fronts. ICP-OES is more flexible than flame AA, and ICPMS is both more flexible and more sensitive than electrothermal AA. And yet there is life in this relic of the 1960s, because of its comparative freedom from interferences and its relative simplicity of application. Atomic absorption is slow by today’s standards, and so it must be relegated for final check certification Analytical Chemistry, Vol. 67, No. 72,June 15, 7995 21R
work. And yet it consistently has found niches where it proves itself indispensable. Flame. Traces of molybdenum in cast iron have been determined by flame AA following extraction from 60%(v/v) HCl with ethyl N-laurylthioacetateinto butyl acetate and back extraction with 1%(v/v) ammonium hydroxide ( A I ) . Total rare earths in steel were determined by an indirect method based on the reversal of the phosphate suppression of magnesium absorption intensity (A2). Cobalt has been determined following preconcentration as a water-insoluble organic complex on a column of naphthalene (A3). Titanium has been preconcentrated by extraction from 0.1 M HCV0.1 M HF with a solution of Amberlite LA-2 in benzene, followed by back extraction into the aqueous phase with 1.2 M HCV0.5 M H F however, molybdenum, vanadium, and zirconium interfere (A4). Metallic iron was determined in ores and slags by selective dissolution with bromine/methanol. The excess bromine was reduced with potassium nitrite in a 30% methanol/l%HCl aqueous solution (A$. The use of less sensitive spectral resonance lines has been described for the determination of high concentrations of chromium, nickel, manganese, molyb denum, cobalt, and copper in steels and inclusion isolates (As). Electrothermal. One review of the application of graphite furnace atomic absorption spectrophotometryto the determination of trace impurities in steels, heat-resistingnickel alloys, and highmelting alloys has been published (A7). Another paper describes the optimization of dissolution, furnace parameters, and matrix modifiers for the determination of bismuth, lead, selenium, tellurium, and thallium in high-temperature alloys using Zeemancorrected GFAA (AS). High-pressure microwave dissolution was applied in the Zeeman GFAA determination of arsenic, bismuth, lead, antimony, selenium, tin, and tellurium in steels and nickel alloys (A9). A study of phase diagrams has revealed the source of double-peak phenomena in the direct atomization of metal samples @IO). Very low levels of the hydride-formingelements (arsenic, bismuth, germanium, antimony, selenium, tellurium, tin) were collected as gaseous hydrides on a palladiumcoated L'vov platform held at a low temperature. They were then atomized in the graphite furnace. The method has been applied to tin in steel (All). In another paper, citric acid was proposed as a matrix modifier for tin determination in iron and steel (AIL'). Other studies have shown ammonium dihydrogen phosphate to be effective in this role for the determination of lead in nickel alloys and steels ( A I 3 ) , while palladium and nickel ( A I 4 or palladium and ascorbic acid (AI$ are used for the determination of selenium. Palladium has also been used as a matrix modifier for silicon in low-alloy steels ( A I @ . Cadmium can be determined in stainless steels over the range 1-10 ppm without matrix modifiers or prior separations (AI 7). A thionalid precipitation procedure has been utilized to decrease the blank and increase the precision and accuracy of the determination of trace bismuth in superalloys 6418).
Other. Here we consider first the hydride techniques which utilize a flame or a quartz tube heated in a flame. Microwave dissolution of nickel alloys in HF/HN03 was utilized in the determination of trace arsenic (AI9). L-Cysteine was found to be an acceptable reducing and releasing agent in the hydride determination of arsenic and antimony (AZO). It has been demonstrated that the double peaks that are sometimes observed in the hydride determination of tin are due to tin that has diffused to the wall of the silica tube. The problem is eliminated if argon 22R
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containing 10%hydrogen is used as the purge gas (A21). The determination of tin was significantly improved in a different study by adding oxygen to the pure argon purge gas and integrating the absorbance signal (&Z). Methods for bismuth in steels and nickel alloys (A23)and for bismuth and arsenic in electrical steel and in superalloys (A24 have also been published. Glow discharge solid sampling approaches in atomic absorption spectrophotometry continue to be utilized. Papers have been published on applications for nickel-base superalloys (A25), steels, cast irons, and other metals (A26,A27). The technique has a distinct sensitivityadvantage over conventional atomic absorption for certain refractory oxide-forming elements. OPTICAL EMISSION This field has been characterized by a series of technological advances over the last decade that promises to keep it vital to steel and other metal industries. New sources like the hot, stable plasmas and waveform-gatedelectrical discharges and new detectors like the charge injection- and charge-coupled solid state devices have made optical emission a resurgent methodology. Always dominant at minor and trace analyte levels, it now rises at times to challenge X-ray fluorescence in the determination of major amounts. Arc/Spark. The use of a vacuum W emission line for oxygen (1302 nm) with the 396.1 nm aluminum line allows the accurate determination of soluble aluminum, but insoluble aluminum still proves to be dacult (BI-B3). Work continues in many areas related to the refinement of the electrical discharge. An advanced form of high-energy prespark was applied to 20 analytes, including carbon and nitrogen in steels and cast irons (B4).Pulse distribution analysis was applied to the determination of calcium, aluminum, nitrogen, and oxygen in steels (Ba. One paper provided an overview of the potential of digital techniques to control both the excitation waveform and the sampling of the measured signal. It also described how digital processing can be used to flag samples with inclusions, holes, and other faults, eliminating the need for the judgment of skilled operators (B@. The high-speed steel matrix in composites with tungsten carbide could be selectively excited with an electric spark discharge. The high-temperaturetungsten carbide eutectic (mp 2670 "C) was not eroded under the experimental conditions (B7).Other studies have shown that a high-repetition-rate condensed arc produces an analytical signal that represents the central region of the sample crater (BS). Impregnating electrodes with chloride salt solutions was found to increase dc arc sensitivity for titanium, zirconium, hafnium, vanadium, niobium, tantalum, molybdenum, and tungsten (B9).A 1:0.02 potassium pyrosulfate/gallium oxide mixture was employed as an excitation buffer in the determination of lead, bismuth, tin, antimony, arsenic, and copper in iron-nickel alloys (BlO). A cooperative study was undertaken by the American Iron and Steel Institute (AISI) member laboratories to identifyemission interferences on 37 spectral lines representing 20 elements ( B I I ) . Low-weight/low-volumesamples present special problems. One paper describes a copper support disk with small holes into which the steel samples are pressed (BIZ); another describes the optimization of ac arc excitation conditions for small samples (Bl3). Carbon has been successfully determined (along with an array of other elements) using a fiber-optic cable with high W transparency to conduct the light from a spark discharge into a mobile spectrometer (BI4). Special sample preparation and
calibration techniques for the analysis of wire samples have been described (B15). Inductively Coupled Plasma. A two-part review of the application of this technique to the alloy addition and tramp concentration ranges in steels and ferroalloys was published (B16, B17). Another review was confined to the subject of ultratrace analysis and the establishment of a lower limit of quantification when direct sample insertion and electrothermal vaporization were used (B18). A collaborative study by the British Steel/BISPA Chemists’ Committee ICP Spectrometry Working Party utilized acid dissolution of unalloyed and low-alloy steels and fusion of the insoluble residue. Ten elements were determined, and only aluminum and silicon showed notable variation (B19).A hybrid system that combines conventional sample introduction with hydride generation was used to determine arsenic, selenium, and antimony at trace levels as well as other analytes in steels and other matrices (B20).The effect of the addition of various surfactants on steel solution transport into the plasma was studied (B21). Flow injection electrodissolutionhas been applied to the rapid analysis of ferritic stainless steels (B22). Trace sulfur in high-purity iron was determined by a flow injection technique in which the sulfur is preconcentrated on an alumina microcolumn (B23). Trace sulfur was also determined as H2S after a flow injection conversion with a mixture of sodium iodide, hydroiodic acid, and hypophosphorous acid. The partially heated flow channel consists of a Teflon reaction tube and a gadliquid separator (B24). Solvent extraction with 2-ethyl-1, 3-hexanediol in xylene was used to preconcentrate boron for ICP determinations at sub-ppm levels (B25). Other published procedures utilized pyrohydrolysis (B26, B27) and methyl borate distillation (B28) to isolate the boron analyte. A comparison of spark ablation and the nebulization of microwave-digested acid-dissolved samples showed a lower detection limit for boron with the spark ablation process (B29).Trace silver in superalloys was preconcentrated on sulfhydryl cotton (B30). Phosphorus in steel was isolated by ion exchange chromatography (B31),as were the rare earths in neodymium-iron-boron permanent magnet alloys (B32). Ferrovanadium was analyzed using a spark ablation/ICP-OES a p proach. The samples were remelted using iron as a dilutant prior to analysis (B33). Arsenic, bismuth, and antimony at tramp levels in steels and nickel alloys were determined by a hydride approach utilizing a Teflon phase separator. The interference of nickel in nickel alloys was reduced by the addition of suitable amounts of Fe (III) and acid (B34). Conventional, direct acid dissolution approaches were successfully applied to the analysis of high-silicon ferroalloys (B35),low-silicon-contentalloys (B36),and iron ores (B3T). Methods for ferroboron (B38), casting and charge powders, feldspar,md feldspar slags (B39), and barium-siliconiron alloy (B40)were also published. Glow Discharge. GD-OES publications continue to be divided between gross compositional techniques and surface depth profile methods. Among the bulk composition procedures are methods for pure iron, low-alloy steels, stainless steels, tool steels (B41) and cast irons (B42).Nitrogen was determined in steel with a detection limit of 5 ppm using the sensitive vacuum UV line at 1199.55 A (B43). In steels and nickel-base alloys, lead was detected at 0.3 ppm, bismuth at 0.06 ppm, and both calcium and magnesium at 3 ppm (B44). Soluble, insoluble, and total aluminum, and soluble, insoluble, and total boron, as well as 17 other elements, have been determined in steel using a GD-OES
approach (B45). The influence of alloy morphology and surface condition, as well as composition, on a GD-OES analysis was examined in one paper (B46).In another paper it was reported that steel sputtering rates increase as their measured hardness decreases. It was also reported that by increasing sample temperature to 400-500 “C the effect of microstructure on the 1657.0 A carbon line was significantly diminished (B47). Depth profile analysis of surfaces appears to be a growing application of GD-OES now that reliable quantitative methods are available. The relationship between sputtering rate and composition is complicated, however. Various quantitative approaches have been applied to oxide layers on nickel-chromium high-temperature alloys (B48, B49) and to metal coatings on steels (B50). By applying a high-frequencyvoltage to the sample cathode, it was feasible to obtain depth profiles of titanium, zinc, iron, and carbon from a zinc electroplated steel surface coated with a nonconducting titanium oxide/polyester layer (B51). The composition and coating weight of iron-zinc alloy coatings on steel were determined (B52-B54), and the composition of electroless-plated nickel layers on steel was studied before and after heat treatment (B55). A new glow discharge source design has been applied to steel analysis (B56). Other. Steel samples were directly introduced into a helium/ hydrogen mixed-gas capacitively coupled microwave plasma. Detection limits of 5 ppm tin and 0.08 ppm lead were obtained, and calibration curves were linear (B57). Direct laser excitation continues to be applied to steel samples, usually in connection with time-resolved spectroscopic measurement. Methods have been described for carbon (B58),chromium (B59),aluminum, and manganese (B60). Laser excitation combined with a fiberoptic link to a spectrometer was designed as an inspection tool in a production environment (B61). Trace elements in steel have been determined using a laser ablation/microwave-inducedplasma instrument which employs an echelle spectrometer (B62). XmRAY FLUORESCENCE
In the past two years, the published papers in this field have been about equally divided between hardware innovations and software development. The use of synthetic layered crystals for light element determinations is becoming an important area of work. Total reflection instrumentation is the state of the art in the trace realm. Fundamental parameter correction algorithms have matured from curosities into working tools. And X-ray fluorescence spectrometers are being integrated into robot “container”laboratories. Hardware. Ultrapure metals can be analyzed by total reflection X-ray fluorescence once the matrix has been removed. The new technique is also applicable to microvolumes of aqueous solutions and acids (C1). A conventional wavelength-dispersive instrument that combines fixed channels with a scanning goniometer provides great flexibility for iron and steel analyses (C2). X-ray fluxes of 7.0 and 20.5 keV from a synchrotron source have been used to determine elements from titanium to molybdenum (2 = 22-42) in power-generatingturbine assemblies (C3). An instrument has been designed for multipoint analysis of small samples utilizing an X-ray beam size of approximately 50 pm (C4). Syntheticlayered crystals were utilized as the dispersive medium in the microanalysis of light elements. The technique has been applied in the identification of a nitride inclusion on the surface of a stainless steel sample (C5). Analytical Chemistry, Vol. 67, No. 12, June 15, 1995
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Applied Mathematics and Software. Artificial neural networks were compared to the Rasberry-Heinrich approach and a linear learning machine approach for modeling corrections for the chromium-nickel-iron system. Over 100 steel samples varying widely in chemical composition were utilized. It was found that the artificial neural networks generally out-performed the other two methods (C6). The accuracy of steel analyses were compared by use of algorithms that difEer in the means by which they relate a coefficients to sample composition (C7). The deJongh theoretical a correction approach was applied to the analysis of fused iron ores (C8). The accuracy of the fundamental parameter calibration was assessed at 1%,relative, for bulk composition and of several percent, relative, for multilayer depth profile work (C9). The corrections for absorption and secondary fluorescence were treated separately in one study that employed steel samples (C10). A calculation for the intensity of a convergent X-ray beam has been published. The equations were applied to microbeam analyses of chromium-iron and nickel-iron alloys (C11). The particle size of a-iron, ferromanganese, and other powder samples was determined from the intensity of two measured element lines (C12). Gross Composition Methods. Three methods of sample preparation for ferroalloys were compared (1) remelting and dilution with pure iron, (2) pelletization of the ground sample, and (3) oxidation and alkaline salt fusion. For both major and minor components, preparation 3 showed the best relative standard deviations (C13, C14. Contradictory evidence was published by another team of investigators, who also compared the three techniques for the preparation of ferroalloys. They found the pelletization approach most satisfactory (C15). Remelting and dilution with iron were used to prepare samples of ferrochromium for a comparison of chromium results with a spark ablation/ICPOES approach. Differences between the two measurement techniques were attributed to random error (C16). The determination of carbon in steels depends on such factors as the amount of graphite, the diameter of the spherolites, and the graphite/cementite ratio. In one paper, the sample microstructure was examined by metallographic techniques and the results were related to X-ray fluorescence count rates (C17,C18). Procedures for the analysis of nickel-base alloys were published (C19), as were molten salt fusion procedures for iron ore (CZO). The effect of microstructural differences on the relative intensity of X-ray fluorescence l i e s was investigated (C21). Surface Methods. A three-dimensionalX-ray fluorescence analysis was proved feasible by comparing lateral coating thickness measurements of zinc layers on steel with cross-sectional measurements on a scanning electron microscope (C22). MonochromaticX-ray energy with variable incidence and takeoff angles from the sample allows depth profiling of zinc layers on steel. Copper Ka radiation was used for the excitation of iron Ka from the steel substrate, while germanium Ka was used for excitation of zinc Ka from the coating (C23). THERMAL EXTRACTION
Spectrometric methods continue to improve, but the edge must still be given to thermal evolution methods for accuracy and reliability in the determination of carbon, sulfur, nitrogen, oxygen, and hydrogen. The trace realm has now become a practical working range for these techniques. The central issue in steelworks that must deal with the new superclean steels is 24R
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sampling-how does one present a sample to the laboratory without contaminating it? One general overview of the thermal extraction approach was published (01). Carbon and Sulfur. Reviews of thermal extraction methods (02) and of the general subject of trace (
