Positive Material Identification (PMI) by Spectrochemical Analysis The essential tool for infrastructure integrity testing in the process industries

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1 Positive Material Identification (PMI) by Spectrochemical Analysis The essential tool for infrastructure integrity testing in the process industries Explosions, fires and other incidents at oil refineries, petrochemical plants and similar installations receive wide and often sensational publicity, particularly when there is loss of life or personal injury. Apart from the human cost, the financial losses to operators and insurers can run to many millions of dollars. Often described as accidents, these incidents are not infrequently traced to the use of piping, valves, and similar components made of inappropriate materials. Often the presence or absence of a particular alloying element in a steel component can be critical to its performance, but can be impossible to detect by a physical inspection of the item. Over the last 20 years Positive Material Identification (PMI) or Verification has become accepted practice in the process and equipment supply industries. Modern spectroscopybased analyzers are the answer to PMI in the demanding process plant environment. THE ROLE OF PMI A few examples of situations that can arise, sometimes with disastrous results, from the use of inappropriate materials will illustrate the need for PMI. Hydrogen gas is frequently encountered in petroleum and chemical processes, and at high temperatures and pressures can cause High Temperature Hydrogen Attack (HTHA) in steel components that if undetected over time can lead to component failure, with fire and explosion the likely result under these conditions. At elevated temperatures (above about 400 F (204 C)) hydrogen atoms can readily diffuse into carbon steels. The diffused hydrogen reacts with the carbon in the steel to form methane gas, that can accumulate at grain boundaries, weakening the steel and leading to cracks, fissures and ultimately to component failure. The use of steels containing alloy elements such as chromium, that have a stabilizing effect on the iron carbides, can effectively prevent HTHA. There are well documented cases where HTHA has led to catastrophic component failure due to the incorrect installation of a low alloy steel component. Ultra-pure water would not normally be thought of as particularly corrosive, but under certain conditions can be responsible for a mechanism known as Flow Accelerated Corrosion, or FAC. This has been experienced particularly in power generation, where ultra-pure water and steam are frequently encountered. The factors giving rise to FAC are complex, but essentially when hot water or steam with a low oxygen content flows along a carbon steel pipe, the usually passive layer of iron oxide that normally forms on the surface can be dissolved. Over time, metal is gradually eroded, leading to weakening of the pipe and ultimately to failure. FAC has been responsible for several high-profile accidents in the nuclear power industry. Like HTHA, it can be prevented by the use of steel alloyed with chromium. These two examples might suggest that chrome steels are the answer to many problems on the chemical plant, but the situation is more complex. Type 316 stainless steels are widely used in the construction of chemical plants, and all contain chromium. However their mechanical strength and durability depends also on the carbon content of the steel. 316 stainless steel contains up to 0.07%, whereas 316L contains maximum 0.03%. This small absolute difference is enough to give the alloys clearly different intergranular corrosion behavi-

2 General Corrosion (flow influenced) - Photo courtesy of the Energy Institute, London or. Welded seams formed in the low carbon 316L are more durable than with the higher carbon alloy. PMI has a role not only during the construction, operation and maintenance of process plant, but at all stages of the plant equipment supply chain. Incorrectly identified raw materials, poor record keeping or missing documentation could all result in in appropriate components being used. Even if the correct materials are used, the traceability and validation of the materials used in formulation and synthesis plantis a required part of GMP (Good Manufacturing Practice) in the very tightly regulated pharmaceutical and food industries. THE CHallenge The obvious way of achieving PMI is elemental analysis of the material. Many analytical techniques require a sample to be taken for laboratory analysis, which may cause delay or be difficult or impossible while a process plant is in operation - shutting down an oil refinery just to take a metal sample is hardly a realistic proposition. The ideal technique should be fast, simple to use on site and preferably non-destructive, so as not to compromise the integrity of the component under test. The ability to test components while the plant is operational, which may mean surface temperatures of several hundred degrees, would be an added bonus. Minimal sample preparation should be required. Small, hand-held X-ray Fluorescence (XRF) spectrometers can satisfy these requirements in most cases, and are often marke- Page 2/7 ted as a universal solution to PMI. It should be remembered however that they cannot handle every PMI task. For example, they do not measure carbon, so could not distinguish between the two types of stainless steel in the example above. For this and the measurement of some other elements like nitrogen, Optical Emission Spectrometry (OES) is the preferred technique. The new SPECTRO xsort hand held XRF spectrometer and the SPECTROTEST mobile OES metal analyzer, both from SPECTRO Analytical Instruments both employ the latest technology in their respective techniques and together provide a comprehensive solution for PMI. THE SPECTRO xsort The design, performance and simple operation of the SPECTRO xsort hand-held X-ray fluorescence spectrometer make it ideal for PMI in the petrochemical and process sectors. XRF spectrometry is a well proven technique for metals analysis, popular since its introduction in the 1950 s. The technique works by irradiating the surface of the sample with a beam of X-rays. This induces fluorescence in the atoms in the sample, which is then re-emitted as X-rays of a lower energy. Each element emits X-rays of a different and unique energy or wavelength, whose intensity is proportional to the concentration of that element in the sample. Detection systems have been developed that can discriminate between the energies emitted, measure their intensities and hence determine the concentration of the different elements in the sample. This technology is known as Energy Dispersive X-ray Fluorescence, or ED-XRF. The xsort has been optimized for fatigue-free on site analysis. Some relevant design considerations are: Weight, convenience and portability. Complete with battery pack, the xsort weighs less than 4 pounds (1.64 kg) and has an ergonomically designed handle and grip. All that is necessary is to bring the instrument in contact with the surface of the test sample and press the trigger. Usually little or no sample preparation is re- quired, but if samples are uneven or corroded, results may be improved by grinding the surface with a suitable abrasive. The user interface and results are displayed on an optimally positioned touchscreen. The instrument is robust, its housing made from shock- resistant ABS plastic. When not in use it can be carried in a handy holster. Analytical performance The two components that define the fundamental performance of an ED-XRF system are the X-ray source and the detector. The stability of the primary source of X-rays affects both the ultimate detection limit of the instrument and the precision of the analysis. Some early hand-held EDXRF instruments used radioactive isotopes as the source of primary X-rays, but these have associated safety and stability problems.

3 In the xsort a miniaturized low power X-ray tube, a close relative to the ones used in SPECTRO s high performance laboratory analyzers, ensures exactly defined excitation and hence good precision. The detector used in the xsort is an advanced Silicon Drift Detector, or SDD. Compare to the Si PIN diode detectors used in many other instruments, the SDD displays better resolution (ability to discriminate between elements) and can process information ten times faster, giving faster analysis. SPECTRO xsort delivers metal (alloy) grade identification and material verification in only 2 seconds for most alloys. This speed of analysis makes the xsort ideal for use in live plant situations, as it minimizes the time the unit (and the operator!) has to be near any hot surface, and reliable measurements can be obtained from surfaces as hot as 930 degrees F (500 C). The light elements like Mg, Al and Si, require a longer measurement time, the xsort just requires additional 10 seconds to identify various aluminum and magnesium alloys. The xsort can measure these light elements in air it does not require the path of the X-Ray beam to be flushed with Helium gas or even under vacuum during the measurement. Easy and Safe to Use It is not necessary to be a skilled analyst to carry out successful PMI with the xsort. The easy to understand graphical interface allows operations to be selected directly on the touchscreen using a finger or a stylus. Calibration of the instrument is automatic using stored calibrations combined with SPECTRO s ICAL (Intelligent Calibration Logic). With any XRF instrument stored calibrations must be periodically checked against a known standard. The xsort is fitted with an automatic shutter that closes between each measurement, partly to protect internal components and partly to protect the operator from possible exposure to X-rays. ICAL ingeniously uses the shutter itself (i.e. when it is closed) as the external standard and checks the calibration between measurements. No operator intervention is required as any necessary adjustments are made automatically. Similarly, it is not necessary to be an expert to interpret the results. xsort can automatically compare the results of the analysis with a stored library of alloy compositions and identify the alloy, or it can verify the alloy against a stored specification. Even easier for some applications, it can give a simple Pass/Fail message based on a reference sample. Of course, the full elemental analysis can be displayed if required, and all these results can be stored or sent to an external printer or PC via a wireless interface. The role of the automatic shutter in protecting the operator has already been mentioned. As an added safety feature, the SPECTRO xsort recognizes if no sample is present a fraction of a second after a measurement is started by the operator. If not, the shutter is closed immediately and the analysis aborted. There are also LEDs on the side of the instrument to tell the operator and others in the vicinity when the X-ray tube is activated and a measurement is in progress. Page 3/7

4 examples of pmi with SPECTRO xsort The following results illustrate the One of the most popular steels for ability of the SPECTRO xsort to differentiate between different alloys. 304 stainless steel, the standard aus- chemical plant construction is Type In each case, the average values tenitic chrome nickel steel alloy. This quoted are based on three 2-second alloy is considered to be indestructible and is characterized by high cor- measurements, but a single measurement would suffice to identify rosion resistance, making it especially the alloy. SPECTRO xsort calculates suitable for vessels and piping that and displays the calculated error on will contain aggressive liquids at high the result automatically if required. It temperature and pressure. However is not necessary to display all the analytical results, as the SPECTRO xsort acetic acid causes accelerated cor- it is not the universal solution: hot can identify the alloy automatically: rosion of Type 304 steel. Type 316, As has been mentioned above, minor with a higher molybdenum content, differences in composition can significantly affect the performance of dif- SPECTRO xsort can differentiate be- is more resistant to this corrosion. ferent alloys. tween these alloys easily. Corrosion by Acidic Production Chemical (Scale inhibitor) - Photo courtesy of the Energy Institute, London The difference in the molybdenum content can be clearly seen. Mn Cr Mo Ni Cu AlSI 304 Certified Value [%] Average Value [%] 2 * SD [%] AlSI 316 Mn Cr Mo Ni Cu [Note: The concentration values in the tables below are expressed as percentages, and the precision of the measurement is expressed by SD or 2*SD, also in percent concentration. SD is the Standard Deviation of a number of measurements and indicates the spread of the data about the average (or mean) result. The smaller the SD, the better the precision of the measurement. 2*SD, also known as two sigma is twice the standard deviation and indicates that 95% of readings will fall within this range. In the examples given these values are calculated on the basis of three separate measurements, but the SPECTRO xsort can also use the data collected during a single measurement to calculate an SD. Clearly any concentration reported that is of the same magnitude as the measurement error is going to be unreliable, which leads to the concept of Limit Of Detection or LOD, the lowest concentration that can be reliably measured. This is conventionally three times the SD. xsort can be set up to either display the < symbol, or not to report a result, if it detects a concentration lower than the LOD.] Page 4/7

5 Most chlorine environments require alloys with a higher content of chrome and molybdenum. A high level of tungsten, as in alloys C-22 and C-276 results in good pitting and crevice corrosion resistance. Alloy C-22 Cr Mo W Fe Co Ni Alloy C-276 Cr Mo W Fe Co Ni The significant elements in these alloys are heavy elements, easily determined by XRF. The lighter elements are more difficult to measure, and it is here that the superior performance of the SPECTRO xsort becomes apparent. AISI AlSI 321 Mn Cr Mo Ni Ti V Cu is a general purpose austenitic stainless steel with added titanium to reduce intergranular corrosion. With its high signal throughput, SPECTRO xsort can reliably determine titanium and differentiate between 321 and 304 in only 2 seconds. 304 steel, while offering good corrosion resistance, is difficult to machine. 303 austenitic chrome nickel steel is excellent for machining has a lower corrosion resistance. The cause of this can be found in the different sulfur contents. 304 chrome AlSI 303 Si* Mn S* Cr Mo Ni V Cu *Note that only grade identification is possible with S and Si. If more than grade is required using S and Si then other instrumentation is necessary. nickel steel contains a maximum 0.03% sulfur, whereas 303 chrome nickel steel contains a minimum 0.15%. The measurement of sulfur is even more of a challenge, but with a slightly longer measuring time, xsort can identify the higher sulfur content and separate the 303 and 304 grades. Page 5/7

6 SPECTROTEST As these results show, the xsort will be able to handle most of the PMI tasks encountered in the chemical process industry. However as was mentioned above, in some cases elements outside the scope of the xsort, like carbon or nitrogen, will need to be measured for a complete PMI. The SPECTROTEST is a mobile metal analyzer using the principle of Optical Emission Spectrometry (OES). In this technique, the atoms in the sample are excited using an electric spark or arc, where each element emits light of characteristic wavelengths, but this time in the ultra-violet and visible regions of the spectrum. This light is separated into its different wavelengths using a diffraction grating, and again individual intensities are measured with a suitable detector. The SPECTROTEST detector is also stateof-the-art: highly sensitive and fast multiple CCD s give fast analysis and generate high quality data that enables the same sophisticated approach to data handling. Like SPECTRO xsort, the SPECTROTEST uses ICAL procedures and can identify and verify alloys automatically in a few seconds. Alloys can be differentiated not only by their metal content but on the basis of elements like carbon, nitrogen and boron, which are impossible to measure by handheld XRF.The physical requirements of the optical system mean that SPECTROTEST is larger, weighing 64 lbs (29 Kg), and less portable, although the flexibly coupled up to 8 m long sample probe still allows access to confined spaces. The nature of the excitation process (arc or spark) also means that extra care may be needed when flammable vapors or liquids are likely to be present. Examples of pmi with SPECTROTEST AlSI 316 Certified Value [%) Average Value [%] 2 * SD [%] C Si Mn P S Cr Mo Ni V Cu Stress Corrosion Cracking of a 316L SS Pipework - Photo courtesy of the Energy Institute, London As mentioned at the beginning, the only difference between steel alloys 316 and 316L is their carbon content: the 316 alloy contains up to 0.07% carbon; the 316L maximum 0.03%. SPECTROTEST can easily differentiate between these two alloys. AlSI 316L Certified Value [%) Average Value [%] 2 * SD [%] C Si Mn P S Cr Mo Ni V Cu Page 6/7

7 Using SPECTROTEST, it is also possible to conduct onsite identification of austenitic steels alloyed with nitrogen or so-called duplex steels (those whose structure is approximately half austenite and half ferrite). As an alloying element Duplex Alloy (X2CrNiMoN22-5-3) C Si Mn P S <0.002 Cr Mo Ni V N in austenitic steels, nitrogen stabilizes the austenitic structure without decreasing the durability of the steel. In high alloy, chemically resistant steels, it also increases their resistance to corrosion to a certain extent, especially pitting corrosion. Among the duplex steels, the X2CrNiMoN (1.4462) grade has found wide acceptance. The nitrogen content in duplex steels usually ranges from %. Conclusion PMI has become an essential part of good practice and risk reduction in the process industries. The SPECTRO xsort hand-held ED-XRF analyzer and SPECTROTEST mobile OES analyzer provide a complete solution for PMI in this demanding sector. Page 7/7 GERMANY SPECTRO Analytical Instruments GmbH Boschstr Kleve Tel.: Fax: spectro.sales@ametek.com U.S.A. SPECTRO Analytical Instruments Inc. 91 McKee Drive Mahwah, NJ Tel.: Fax: spectro-usa.sales@ametek.com Hong Kong (Asia-Pacific) SPECTRO Analytical Instruments (Asia-Pacific) Ltd. Unit 1603, 16/F., Tower III Enterprise Square No. 9 Sheung Yuet Road Kowloon Bay, Kowloon Tel.: Fax: spectro-ap.sales@ametek.com Subsidiaries: CHINA: Tel , Fax , spectro-china.sales@ametek.com, FRANCE: Tel , Fax , spectro-france.sales@ametek.com, GREAT BRITAIN: Tel +44.(0) , Fax +44.(0) , spectro-uk.sales@ametek.com, INDIA: Tel , Fax , sales.spectroindia@ametek.com, ITALY: Tel , Fax , spectro-italy.sales@ametek.com, JAPAN: Tel , Fax , spectro-japan.info@ametek.co.jp, SOUTH AFRICA: Tel , Fax , spectro-za.sales@ametek.com, SWEDEN: Tel , Fax , spectro-nordic.info@ametek.com. SPECTRO operates worldwide and is present in more than 50 countries. Please contact our headquarters for your local representative. SPECTRO 2009, Subject to technical modifications J-11, Rev 1, Photos: SPECTRO, Getty Images, Corbis, Photos of damaged pipes courtesy of the Energy Institute, London