Anal. Chem. 1993, 65, 29R-39R
Steel and Related Materials Thomas R. Dulski Carpenter Technology Corporation, R&D Center, P.O. Box 14662, Reading, Pennsylvania 19612-4662 ~
Review Contents Introduction General Considerations Atomic Absorption Optical Emission X-Ray Fluorescence Thermal Extraction Mass Spectrometry Chemical Methods Inclusions and Phases Surface Techniques Molten Metal Analysis Miscellaneous Methods Special Topics
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INTRODUCTION The message from the readers’ survey that followed the 1991edition of this review (1)was clear and unequivocal and has been heeded. Several significant changes in format and content have been incorporated. Areas that are included are listed in the Review Contents. The material is now organized exclusively along methodological lines, reflecting the heavy emphasis upon instrumental approaches in most ferrous metals industries. Classical “wet chemical” techniques now constitute but one heading among an array of analytical protocols and, by necessity, represent a concentrate of methods that are likely to find utility where alternatives are lacking or inadequate. This may disappoint some who share the belief that classical “definitive” analytical techniques are the anchor points on a roiling sea of comparative instrumental methods. But the undeniable reality is that few laboratories now maintain capabilities for classical chemistry, and some would argue that “standardless” instrumental approaches are on the near horizon, if not already here. The reader will also note that the number of review references has dropped dramatically, reflecting the survey consensus for a highly critical concentrate of only the new and most practical developments. While this very reasonable request will undoubtedly improve the readability and general utility of this review, it presents the danger that something important may be overlooked. For any such oversights, the reviewer assumes responsibility and offers apology. The material covered in this review was extracted from Chemical Abstracts beginningwith Vol. 113,No. 8and ending with Vol. 117, No. 4. Supplemental material was gleaned from Metals Abstracts between August 1990 and July 1992. As indicated, the material is largely organized by analytical methodology; however, several special topics, such as inclusions and phases, surface analysis, and molten metal analysis, incorporate a range of techniques and two final general categories encompass important miscellaneous information.
GENERAL CONSIDERATIONS The last two years have demonstrated that an economically beset industry must continue to wrestle with quality issues, often, like the Red Queen from Alice in Wonderland,running just to stay in place. Corporate customers have become more discriminating consumers. Tighter specifications and lower maximum limits for tramp elements have placed an increased burden on understaffed laboratories. Answers are being t In the references,an abbreviatedversion of citations in Chemical Abstracts and Metal Abstracts is given. If expanded, the correct citation would be (for example, in ref 1) Chem. Abstr. 1991,114(26), 258527~. 0003-2700/93/0365-0029R$12.00/0
Thomas R. Dulski is a Specialist in Analytical Chemistry at the R&D Center of Carpenter Technology Corporation, a producer of iron-, nickel-, and cobalt-base specialty alloys. I n the past 30 years he has been engaged in nearly all aspects of the application of analytical chemistry to both basic and specialtysteel production. He is the author of 13 technical publications, including a book chapter on classical wet analytical chemistry. He has been awarded the Certificate of Appreciation,the Lundell-Bright Award, and the title of Fellow by ASTM and has received the Pharmacia Prize. He is currently active in ASTM Committee E-1 and serves as chairman of Committee S-17.
sought with emerging technologies like glow discharge and inductively coupled plasma mass spectrometry. And total reflection X-ray fluorescence holds out some promise that the trace element realm may one day yield to a single nondestructive approach. Efficiencies in the production of steel commodities continue to press for increased analytical speed. Automated robot laboratories have been one response, while furnace-side molten metal monitors have been another. All of these nascent methodologiesare rapidly evolving, while elsewhereother workers continue to speed up and refine older techniques. Reviews of recent steel analysis trends in Germany (2)and Japan (3) have been published, as well as reviews of iron ore analysis (4) and of the use of lasers in steel analysis (5). A general review of the use of atomic spectroscopy for the analysis of industrial materials contains a good section on ferrous alloys (6). But if one subject shows a sharp peak of interest it is the field of trace element determination; no less than six review papers on this general topic have been published (7-12). One paper summarizes emission spectrochemical work (13) while another treats the use of statistics (14).
The subject of on-line analysis and computer processingof analytical data in steel-making operations is reviewed in one paper (15), which cites 79 references. Specific experiences with the application of robotics and automation in steel production control laboratories are described in several papers. These analytical facilities vary in complexity and in the degree to which human intervention is required. Many incorporate automatic sample preparation; some recalibrate themselveswhen control chart limits are exceeded. Typically, some combination of optical emission, X-ray fluorescence, and thermal extraction methods for carbon and sulfur is involved. Work in Japan (16), Germany (17-21), The Netherlands (22), and the UK (23-25) has been published. An artificial intelligence program which optimizes instrumental analysis techniques for metallurgical needs has been developed using the symbolic language Prolog (26).
ATOMIC ABSORPTION Atomic absorption (AA) spectrophotometershave been in use in steel analysis laboratories since their commercial introduction in the 1960s. While too slow for most production control applications, the AA techniques (for now there are several) continue to play a vital role in finished product testing and other time-tolerant applications. One review paper 0 1993 American Chemical Society
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summarized the utility of atomic absorption in the analysis of iron and steel ( A I ) . Flame. The use of flame AA techniques in the ferrous metals industry was the subject of a review (A2). A series of schemes for trace element preconcentration using various solvent extraction group separations was published. Trace levels of nine elements were determined (A3). Eight elements were determined in high tungsten tool steel using a combination of flame AA and chemical techniques (A4). Among the interestin applications for specific elements, sodium and lithium were fetermined in steel and in nonmetallic inclusions separated from steel. In this work, the majority of the steel matrix elements were first removed by means of solvent extraction with methyl isobutyl ketone (MIBK) (A5). Boron was determined indirectly in iron and steel by formation of an associationcomplexbetween [Cu(l,lO-phenanthroline).J ?+ and BF4-and extraction with nitrobenzene. Copper was then determined in the organic phase by flame AA (A6). The interferences from aluminum and titanium in the nitrous oxide-acetylene flame AA determination of vanadium were studied, and means were found to overcome the problem ( A n . An indirect method for phosphorus in rare earth nodular cast iron involves reaction of the sample with a standard excess of molybdate, addition of diantipyrylmethane, and warming the solution for at least 2 h a t 50-80 “C. Unreacted molybdenum is then measured in the supernatent solution (AS). Electrothermal. While the supremacy of electrothermal atomic absorption as a practical trace element technique is no longer unchallenged, it remains today the only mature, reasonably rapid, approach for many applications. One review has summarized the utility of electrothermal AA for iron and steel analysis (A9). If materials are very homogeneous, it is sometimes possible to introduce solid metal chips into a graphite furnace for a rapid method that avoids sample dissolution. One group of investigators used this approach, combined with aqueous solution calibration, to determine tramp levels of thallium, bismuth, tellurium, selenium, and lead in nickel-based alloys (AIO). Similar methods for lead ( A l l )and bismuth (A12)have been reported by other workers. It is also possible to distinguish soluble and insoluble aluminum in iron and steel by a graphite furnace technique, as reported in two papers. The first describes a method in which the sample is dissolved in aqua regia (A13);the second employs a bromine-ester approach to isolate the aluminum nitride (A14). Spectral interference on the most sensitive bismuth resonance line when Zeeman background correction is employed was reported. These workers found that they could use the leas sensitive 306.8-nm line to analyze superalloys for bismuth if they measured peak height rather than peak area (A15). As with certain other elements, germanium determinations were found to be enhanced by the presence of nickel and iron nitrates, a mechanism which enhances the detection limits in nickel- and iron-based alloys (A16). In the preceding two years, direct solution-based methods were reported for antimony (A17),and tellurium (A181 and for tin (A19)and tin and lead (A20)using coated tubes. Among the methods reported in which the analyte was isolated prior to measurement, arsenic was coprecipitated with manganese dioxide (A21)and also extracted into isobutyl methyl ketone as the arsenomolybdate complex (AZZ),vanadium was isolated from the steel matrix by a series of solvent extraction steps (A23),and gallium was separated by solvent extraction with 8-quinolinol-chloroform (A24). Other. Glow discharge atomic absorption (GDAA)is now commercially available; it allows the direct analysis of conductive solids, such as steels; and it extends the technique to analytes such as boron, cerium, and zirconium, which show poor sensitivity by other AA approaches. The advantages of glow discharge sputtering for AA are summarized in one paper ( A S ) . Another paper describes the deleterious effect of water vapor in the argon used for GDAA. For exam le, at 40-50 mA and 4-6 Torr Ar, 140 ppm water vapor red3uced the nickel absorbance for a steel sample by 12%. For other matrices and for lower sputtering currents, the reduction effect was much reater (A26). Other investigators have employed a piezoefectric microbalance to monitor sputtering rate for the direct determination of nickel and aluminum in alloy steels and of aluminum in nickel-based alloys (A27). In one study, statistical evaluation led to optimized parameters. The BOR
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optimized GDAA was applied to the analysis of carbon and low-alloysteels, austenitic stainless steels, and a cobalt-based alloy (A28). Hydride generation and other “tube-in-flame” atomic absorption techniques have gained only a moderate level of acceptance over the years. Their use ersists in specialized niches, however. Arsenic in steel was Atermined by generation of the volatile trichloride, which was concentrated in a cold trap and then measured by “tube-in-flame” AA (A29). Bismuth in nickel-base hi h-temperature alloys (A30)and tin in steel (A311 have been fetermined by hydride generation atomic absorption.
OPTICAL EMISSION Despite advances in other techniques, many metals industries, including steel, continue to depend heavily on optical emission spectrometry in its traditional forms. The field of optical emission is far from stagnant,however, with important developments in excitation sources, detection devices, and software control. Glow discharge, after a long pubescence, may be finally reaching maturity as a practical technique for both gross analysis and depth profile studies. Those who lament the passing of photographic plate techniques for their versatility now greet the birth of pixel-based detectors and the era of electronic snapshot spectra. A r d S p a r k . Time-resolved spark excitation, in which line intensities are sampled only during a preselected portion of the discharge cycle,has been shown to yield superior precision. This new approach has been applied to the determination of carbon, phosphorus, and sulfur in steels. When these three elements were present at the 100 pm level, a conventional spark technique yielded standard Ieviations of f8.7 ppm for carbon, f4.2 ppm for phosphorus, and *2.4 ppm for sulfur. When the time-resolved technique was applied, the standard deviations dropped to f2.3 ppm for carbon, f0.8 ppm for phosphorus, and f0.5 for sulfur (B1, B2). A spark source combined with an echelle spectrometer and a detection system based on a charge injection device KID) has been evaluated for the analysis of steels and other metals (B3). Such a system allows the use of multiple lines to calibrate each element. Also, the complete spectrum from each analysis may be stored on magnetic tape for future reference or further quantification in a manner analogous to the storage of a photographic plate spectrum. Many papers describe interference effects from steel matrix elements, sample history,and sample preparation. Common remedies include extending the length and/or the energy of the preburn, varying the nature of the excitation conditions, changing the abrasive medium used to olish the sample surface, and selecting alternate analytical [ne, (B4B7). The determination of acid-soluble aluminum by a peak integration optical emission method has been tested and found to yield accurate results if metallic aluminum and aluminum oxide are homogeneously dispersed in the steel. However, production samples are seldom homogenous enough (B8). The reliability of carbon and sulfur determination of cold rolled steel was improved by removal of a carbon-depleted surface layer and a change in excitation conditions (B9). The reproducibility of results from carbon and medium-alloy steel was shown to improve with the use of a combined low-voltage arc and spark source (BIO).The effects of various boroncontainingphases on the spark source optical emissionanalysis of iron-base alloys of the Fe-B-C-A1 and Fe-B-C-Cr alloy systems were reported in one paper (BI1). Rare earths were determined in steel following preconcentration on a cation exchanger (B12). Rare earths were determined in ferroalloys by a rapid method in which the powdered sample was mixed with graphite and sodium chloride and excited directly (B13). The effect of sample temperature and the means to its compensation were described (B14). Inductively Coupled Plasma. The high temperature and stability of the ICP torch and the attendant linearity of calibrations with ICP spectrometers have made this the preferred methodology for many steel industry applications. The establishment of industry standard methods was discussed in one paper (B15), while another reviewed the field (B16). Most of the schemes for directly analyzing solids involve decoupling of the sampling and excitation processes. Thus, a laser beam or a spark is used to generate an aerosol, which is transported to the plasma by a stream of inert gas. One group of workers investigated the laser ablation process
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by comparing the aerosol particles produced by six different types of laser ulses. Their objective was to characterize the problem of secctive vaporation (B17).The selective vaporization effect is minimized by using the highest eak power for the laser pulse. By using a special desi I8P torch, it is possible to calibrate with aqueous-basedso ution standards to analyze laser-ablated low-alloy steels, if iron is used as the internal standard (B18).Another group of workers employed a specially designed cell in the aerosol path in which light loss due to the ablated material produced a correction signal for the ICP analysis. This normalization approachwas employed with ar on and helium microwave-induced plasmas as well (B19).8park ablation/ICP optical emission was successfully employed to determine aluminum, titanium, niobium, and vanadium in low-alloyed steels. It was found to show precision similar to solution-basedICP and detection limits similar to spark optical emission (B20, B21). Free-cutting steels required a preliminary etch with hydrochloric acid to partially remove insulatin inclusions prior to initiation of the presparking. For &ese materials, spark ablation/ICP yielded element precisions between 2 and 5 % of the amount present (B22). Less popular are techniques in which the solid sample is directly introduced into the plasma. Bismuth, antimony, lead, selenium, and tellurium have been determined in iron by this means (B23). Silicon,calcium,magnesium, aluminum, and titanium were determined in iron ores and sl s using additions of sodium fluoride, poly(viny1chloride),an? Teflon with a sample elevator approach to introduce the sample into the plasma (B24). Another technique involves introduction of slurry suspensions of the finely ground sample into the plasma. One study involved dispersing 0.1 g of ground slag in 100 mL of 0.35 % ammonium hydroxide solution. Silicon, calcium, magnesium, aluminum, iron, man anese, titanium, sodium, and potassium were determined in t is manner, using a standard reference slag prepared identically to calibrate the instrument. While the precision was inferior to that from a total dissolution approach, it was deemed adequate for control purposes (B25). Some investigators are attempting to extend the detection limits of conventional ICP/OES by employing electrothermal atomization. Vanadium and titanium (B26)and both total and metallic (Le.,noncombined) calcium (B27)in steels were determined using this approach. Other workers have linked an ion chromatograph to the ICP s ectrometer to circumvent spectral interferences. Molybenum, manganese, and nickel were determined in nitric acid dissolved steel samples by this method. Chromium results were poor, however (B28). Volatilization of analyte element compounds followed by direct introduction of the volatile s ecies into the plasma is another popular means to increase tf e sensitivity and decrease the interference effects in ICP/ OES. Arsenic was determined in steel by first passing the acid-dissolved sample through a strongly acidic anion exchange column to absorb iron and nickel and then introducing the element into a hydride generator linked to an ICP spectrometer (B29). In another approach, arsenic from the sample solution is extracted on-line as As13 into a xylene solution; the xylene extract is continuously reacted with sodium borohydride in dimethylformamide and acetic acid to generate arsine gas which is introduced into the plasma (B30). Sulfur was determined in low-alloy steel down to 20 ppb by mixing an alkaline solution of the sample with 6 M hydrochloric acid and passing the generated hydrogen sulfide directly into the plasma (B31). Other means of dealing with sensitivity and interference problems are available. Analytes can be isolated and concentrated by solvent extraction, for example. Lead, arsenic, antimony, bismuth, and tin were isolated from a dilute sulfuric acid solution of iron and steel samples in the presence of ammonium iodide by extraction with methyl isobutyl ketone. The analytes were backextracted into an aqueous phase with 4 M nitric acid, which was then reduced to 10 mL and measured by ICP/OES. The bismuth results were corrected for the presence of copper (B32). Boron has also been determined down to 1ppm using a solvent extraction approach (B33,B34). Analytes can also be isolated and concentrated by ion exchange, as in work with the determination of boron in iron disilicide and highurity iron (B35). Sometimesanalytes are best concentrated y a precipitation technique-for example, rare earth as fluorides, using yttrium as an added coprecipitant (B36). Direct analysis of acid-dissolved samples remains the standard
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approach for ICP/OES, of course. On-line electrolytic dissolution of solid samples has been studied to increase productivity (B37). Conventional dissolution and manual sample introduction are the mainstays of the technique, however. Line selection and interelementcorrection continue as the conventional answers to most roblems. Pa ers on methods for chromium, manganese, mo ybdenum,nic el, and boron in low alloys (B38), trace elements in ferroalloys (B39), boron in boron-alloyed steels (B40)and in steel-makingslags (B41), and lead in free-cutting steel (B42) were published. Glow Discharge. While the idea of a glow discharge lam as an emission source for optical spectrometry is quite olcf it is only in recent years that the technique has entered a period of rapid rowth. In many ways, this low-pressure dc discharge woul% appear to be the ideal source for optical emission work in the steel industry; however, until recently, expense, limited commercial availability, and sample requirements slowed its acceptance. And only recently have al orithms been developed which allow the full potential of GbiOES for quantitative depth profile to be realized. One review of the technique which covers both gross analysis and depth profile work has appeared (B43). A lamp was built based on a modification of the Grimm design and used to analyze a series of low-alloy steels. Also, a model which predicts the sputtering rates of various metals was proposed (B44). A hollow cathode lamp operated in a microwaveboosted mode was used to determine chromium in steel up to 1.72%. The microwave-boosted source showed increased sensitivityover a conventional hollow cathode (B45).Another hollow cathode lamp design employs three electrodes. Variation im osed on a negative voltage applied between an intermeJate electrode and the sample cathode roduces modulated emission intensities from the sam le. 8 n c e the argon emission does not vary, selective demoddtion removes argon line interferences (B46). The dual-cathodelamp design was applied to the determination of manganese in steel 0347) and of vanadium in tool steel (B48).A study has shown that powdered samples can be determined by pelletizing with an excess of co per metal. To prevent atmospheric contamination of t i e glow discharge chamber, a special device maintains low ressure at the rear surface of the pellet (B49). The spectra o!tained from steel samples using an echelle gratin /photodiode array spectrometer are lower in backouni but much more complex than those obtained with an KP. Customized optical masks are one practical approach to uantify analytes from a bewildering plethora of lines (B50). low discharge spectra were also analyzed by a high-resolution fast Fourier transform spectrometer. With such a system, molybdenum could be detected in steel down to 30 ppm (B51). A glow discharge photodiode array spectrometer was used in conjunction with multivariate calibration of seven elements to test a neural network approach to alloy identification for iron and nickel-base materials (B52). Com ared to conventional spark excitation for highl alloyed anffor heat-treated steels, glow discharge is relative&insensitive to matrix effects (B53). However, for steels and cast irons GD/OES is not free of the effects of microstructure (B54),and with high alloys, the metallographic structure effect on carbon intensity was reduced by raising the sample temperature (B55). With a vacuum spectrometer of appro riate design it is possible to measure oxygen, nitro en, anzhydrogen using lines in the extreme ultraviolet. T%eseelements and others were determined in depth profile studies of zineiron electroplated sheet steel; nitro en, chromium, and iron were determined in a depth prof!le study of nitrogen-implanted low-alloy steel (B56). Refinements to an em irical model for depth profile quantifications have been pubfished. The work is illustrated with quantitative depth profiles for stainless steel and white iron, among other metala (B57).A rapid analysis of the surface layer of zinc-nickel lated steel can be obtained by first cleaning the absorbel contaminants from the material in a s ecial cleaning step using a gas interrupt and a preliminary ischarge (B58).Glow discharge/optical emission has been employed to perform depth profile measurements of oxide films on steels (B59), copper-nickel and iron-chromium binary alloys ( B O ) ,and nickel-based alloys (B61), decarburization depth in steel (B62),and zinc-nickel coatings on steel (B63). Other. Direct spectral analysis of the light from the interaction of a laser beam with a solid sample has received
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some attention. Transient laser-induced plasmas can be analyzed by a time-resolution technique to maximize signalto-background ratios. Such an approach has been applied to the determination of iron, silicon, magnesium, calcium, and titanium in iron ores (B64-B66). With steel samples (among other sample materials), one group of workers used a second laser beam to increase the intensities of the analyte lines (B67). Besides conventional optical emission, laser-induced fluorescence can also be measured and has proved analytically useful for steel and iron samples (B68,B69). Laser ablation has been coupled to a microwave-inducedplasma and applied to steel and other materials (B70).
X-RAY FLUORESCENCE It is difficult to imagine a modern laboratory which analyzes steel in any quantity without an X-ray fluorescence spectrometer. The technique is not without its weaknesses, however-low atomic number elements are insensitive or impossible, trace elements are difficult, sample size and shape requirements create problems, and despite great strides with fundamental parameters approaches, standards which match the sample matrix continue to be required. Reviews which cover steel ( C I ) ,iron (C2),and iron ores, concentrates, slags, sinters, and limestone (C3) were published. Total Reflection. While applications for steel analysis remain tentative, the field is excited by the potential of total reflection X-ray fluorescence. A major journal has devoted an entire issue to this emerging technology (C4). This new approach offers the potential for trace element quantification as well as wide dynamic range and surface characterization studies. One paper dealt with trace analysis of high-purity iron. The dissolved sample was extracted with methyl isobutyl ketone to remove the matrix, and trace levels of titanium, vanadium, chromium, manganese, nickel, copper, lead, and bismuth were measured by total reflection X-ray fluorescence (C5). Niobium has been measured in steel using a total reflection approach (C6). Other studies of surface layers (C71 and the quantification of trace quantities of various elements (C8) may prove useful for future work with steels. In related work for trace analysis, detection limits for various energydispersive techniques were compared (C9). Another paper described a correction equation which improved accuracy for traces of tin, antimony, zirconium, and lead in high-alloy steel when conventional X-ray fluorescence spectrometry was employed (CIO). Applied Mathematics a n d Software. By use of a rhodium target tube, it has been shown that the intensity and distribution of background X-rays for different types of samples can be predicted from theory. A specific background correction formula for the determination of trace elements in steel has been devised ( C I 1 , CI2). A partial least squares calibration model for nickel alloys was found to give superior results to the conventional multiple linear regression Calibration (CI3). Fundamental parameter correction algorithms have been applied to steels and stainless steels (C14) and have been combined with peak deconvolution and background correction routines (C15). For the calibration of energydispersive instruments an inverse Monte Carlo method (C16) and fundamental parameters approaches (CI 7-C19) have been applied to ferrous alloy analysis. Gross Composition Methods. The nickel-base alloys Inconel 713LC and Inconel 617 were analyzed for aluminum, titanium, chromium, manganese, iron, cobalt, zirconium, niobium, molybdenum, and tungsten using fundamental parameters derived from a disk standard, prepared by vacuum arc melting, and wet chemically characterized (C20). Monel alloys were dissolved in acids and deposited on filter paper (to control matrix effects) prior to X-ray fluorescence analysis (C21). Blast furnace and converter slags were finely ground and then fused with a sodium tetraborate-lithium tetraborate flux with an automatic fluxer prior to X-ray fluorescence determination of seven elements ((222). A method for vanadium, zinc, and titanium in blast furnace slags was also developed (C23). A combination of fundamental parameters and empirical coefficients was used for the complete compositional characterization of iron ores (C24). The concentrations of nickel and zinc ions in electroplating baths for steel sheet are determined with an on-line X-ray fluorescence unit. Iron and sulfur intensities are also measured for 32R
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interelement corrections (C2.5). An X-ray fluorescence bias for titanium was discovered in NationalInstitute of Standards and Technology (NIST) standard reference materials 1165 and 1264 (low-alloy steels). The bias was traced to microinhomogeneity of titanium inclusions (C26). Coarse carbides have a marked effect on phosphorus determination in steel. It has been recommended that the samples be annealed or remelted and rapidly cooled prior to X-ray fluorescence analysis (C27). Verification of alloy steel grades was conducted in less than 30 s with a portable X-ray fluorescence analyzer, which employs a high-resolution mercury(I1) iodide detector (C28). Iron, chromium, manganese, andnickel were determined in stainless steels by applying the partial least squares calibration model with a portable radioisotope X-ray spectrometer (C29). Surface Met hods. Nickel-zinc alloy coatings on steel sheet have been analyzed on-line for nickel content and coating weight by employing an iron filter and a nickel filter and two ionization chamber detectors (C25). An X-ray fluorescence technique which utilizes a fundamental parameters approach was applied to single- and multiple-layer thin-film coatings. Accuracy was found to be fl % for composition and k 3 % for coating weight (30). A method based on the theoretical intensities of K-series lines from the sample elements at two different reflection angles when plated steel plate is irradiated at two different incident angles yields plating weight and composition (C31). A related scheme is employed for two layer coatings on steel strip (C32).
THERMAL EXTRACTION This category of instrumental procedures, more commonly (albeit misleadingly) known as "combustion methods" includes determinations of carbon, sulfur, oxygen, nitrogen, and hydrogen in which the analyte species is evolved as a gaseous molecule by heating the solid sample in the presence of a reactive or an inert gas. Measurement is typically by infrared absorption or thermal conductivity measurement, although other techniques may be involved. These techniques have survived challenges by spectrometric methods because they are rapid, reliable, and highly accurate. Carbon a n d Sulfur. Trace levels of these metallurgically critical analytes represent a new regime for the analytical technology. Steel laboratories are now faced with demands for rapid determination of levels below 10 ppm. One paper describes the determination of trace sulfur levels in highpurity iron using a coulometric titration approach @ I ) . A solid electrolyte electrochemical cell which employs potassium carbonate has been used to determine low levels of carbon ( 0 2 ) . An important development is the commercial availability of trace carbon and sulfur determinators by instrument manufacturers such as Leco Corp. One publication describes some of the special problems, such as contamination control, which are encountered when one attempts to determine carbon below 50 ppm ( 0 3 ) . The speciation of forms of carbon in cast iron and in carbides isolated from steel was demonstrated using temperature programming in oxygen and in inert gas ( 0 4 ) . Many carbon and sulfur determinators are adversely affected by the presence of fluoride in the sample. One paper described a coulometric approach for sulfur in weld fluxes and electroslag remelting fluxes in which fluoride from the sample is absorbed by silica gel (D5). An interesting paper from Germany describes an experimental approach which permits the determination of carbon, sulfur, nitrogen, oxygen, and hydrogen by combustion of the sample at 1200 "C; nitrogen and hydrogen are measured with a thermal conductivity detector and COz, CO, HaO, and SO2 are measured by nondispersive infrared photometric detectors (D6). Oxygen, Nitrogen, and Hydrogen. Studies aimed at reducing the blank values for trace oxygen ( < 5 ppm) determinations of high-purity iron were published. An outer graphite crucible was degassed at approximately 2373 K with silicon metal, an inner graphite crucible was inserted, and the combination was degassed at approximately 2573 K. An apparatus blank due to adsorbed moisture was removed by use of a ribbon heater. The sample was placed in the inner crucible with tin metal flux and extracted for more than 90 s. The relative standard deviation at 1.2 ppm oxygen was 8.35 (D;3. When aluminum is high, it can exert a gettering
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effect which results in low oxygen values. A solution to the problem was describedin which the double-graphitecrucibles were employed with the use of a 4 1 tin:sample ratio. The samples were 10-1575 aluminum-iron alloys, which were introduced enclosed in nickel capsules (08).Most papers on hydrogen determination have been confined to the separate field of molten metal analysis (see below); however, one of these also describes a new instrument for oxygen, nitrogen, and hydrogen in solid steel specimens (09).Also, some of the new solid electrolyte sensor technology develo ed for molten steel is applicable to solid steel specimens (b10).A furnace-site diffusible hydrogen analyzer which employs a semiconductor detector is designed to be used with a quartz molten steel sampler. Hydro en diffusing from the cooling steel collects in the sampler, w ich is inserted in the analyzer and mechanically crushed to release the hydrogen. The measurement re uires less than 1 min (011). The hydrogen embrittlement high-strength steels has keen studied by employing a variety of analytical techniques, includin vacuum fusion, electron-spin resonance spectrometry, an secondary ion mass spectrometry (012).
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MASS SPECTROMETRY Inorganic mass spectrometry is currently undergoing a renaissance which may soon lace it in the forefront of analytical techniques employezby metals industries. Broad elemental coverage and extreme sensitivity are confident predictions for the newer instrumental designs. At the same time, cost, speed, ruggedness, and the ability to handle a wide variety of complex matrices remain only partially resolved questions. Clearly, the next few years should determine whether the promise of this technology will finally be fulfilled. Inductively Coupled Plasma. Low-alloy steels ( E l )and irons (E21have been analyzed b ICP/MS. Acid-dissolved samples showed a relative staniard deviation of less than 10% for low levels of vanadium, chromium, cobalt, nickel, cop er, gallium, arsenic, molybdenum, tin, antimony, and l e a l A matrix-matched reference sample is required for calibration. In a reconcentrationapproach,high-purityiron samples were 8ssolved and extracted with 4-methyl-2pentanone prior to measurement (E3). Bismuth was determined in iron and steel by means of a graphite furnace interfaced to an ICP/MS. Detection limits were 20 times lower than for continuously nebulized solution sam les. Although the presence of iron from the matrix de resse the sensitivity, 0.01 pm bismuth could be detected 84).A very interesting deveyopment is the couphng of a spark ablation sampling device to an ICP/MS instrument. Cheaper and more reproducible than the more prevalent laser ablation accessories, the chief disadvantage to this approach appears to be ita limitation to electrically conductive samples. One group of investigators applied this hybrid technique to steel analysis. They found simplified spectra, detection limits below 0.1 pb, and relative sensitivity factors similar to those produced!y glow discharge mass spectrometerswith the same magnetic sector and detector (E5). Selective vaporizationof elemental analytes is a concern with laser ablation ICP/MS. One aper compared six types of laser pulses for this effect by cotecting the ablated aerospl particles, dissolving them in acid, and analyzing the solutions by ICP/OES or ICP/MS (E6).Another pa er treated the optimization of laser parameters for bot[ gross composition analysis and depth profiling (E7). The use of iron as an internal standard im roves the accuracy and recision in the laser ablation ICF/MS analysis of steels. !he best results were obtained for boron, arsenic, niobium, and tin; the poorest results were obtained for silicon, aluminum, and phosphorus (E8).For depth profilin tin-plated steel sheet, factors which affect the de th resofution were studied. The laser ablation rates were getermined by measurin the resultant crater cross sections after given intervals Glow Discharge. Carbon, nitro en, and oxygen were determined at trace (
