With the increasing regulations in safety, reliability, traceability, and regulatory compliance, material verification has become an essential component in a safety and reliability program throughout the manufacturing and energy markets.
While material specifications used in industry are becoming increasingly more specific, the need for various PMI testing in the field has been steadily increasing. The handheld XRF, OES, and portable handheld LIBS analyzers are all common technologies used to determine fast, accurate, and reliable analysis. Each one of the technologies offers its own unique advantages and provides the user with qualitative and quantitative analysis of the material composition. The common term used in the industry for performing any of these techniques is known as Positive Material Identification (PMI). All three technologies are used in a variety of ways to ensure compliance with a quality management program. Some examples are:
- Incoming material verification to ensure that the products or components are the correct alloy
- Fabrication to ensure the welded component and their filler metals are the correct alloy, meeting the specified material requirements
- Sorting and identifying unknown or mismarked materials ensuring the right material is used for the specified process
- Retro-PMI in-process applications for verifying material currently in process for compliance with the required specifications
Understanding the limitations and differences in each of these techniques is critical when performing material analysis. The analyzer can be used on a wide range of components across many industries. They are consistently used in both production and asset integrity management programs in petrochemical, refining, power generation, fabrication, pharmaceutical, nuclear, and aerospace. These verification programs help eliminate costly mix-up of rogue materials, identify unknown materials, improve product quality, and help prevent injuries and loss of life. Each verification program will have different requirements when it comes to identifying the material to be analyzed. Each alloy or material will have various elemental requirements and restrictions. Understanding each technique will help determine which technology to use to verify the material.
X-ray Fluorescence (XRF) is the most commonly used Non-Destructive Test (NDT) method offering the user a portable handheld analyzer that delivers fast, accurate results. Handheld XRF analyzers use an x-ray tube to emit an x-ray beam into a sample, exciting the electrons and displacing them from the inner shell. The vacancy from the inner shell then gets replaced with an electron from an outer shell. As this electron fills the vacancy of the inner shell it releases energy in the form of a secondary X-ray. This release of secondary energy is known as fluorescence. Each element present will emit unique energy characteristics of itself. By measuring the unique characteristic of released energy emitted from the sample it is possible to determine which elements are present. This is called qualitative analysis. Then, by measuring the intensities of the unique energy and applying correction factors, it is then possible to measure how much of each element is present in the sample. This is called quantitative analysis.
Some XRF analyzers are capable of measuring light elements in low concentrations such as Silicon, Phosphorus, Sulfur, Aluminum, and Magnesium. XRF has limitations on the elements that can be measured. Elements lighter than Magnesium cannot be measured using XRF. This limitation of XRF makes it impossible to grade materials such as low carbon stainless steels, carbon steel, and low alloy materials because Carbon cannot be measured utilizing XRF analyzers. For example, XRF can measure the elements required to identify 316 stainless steel, but cannot measure the Carbon required to identify whether that same 316 material is L or H grade. Carbon is the essential element required to verify the different grades of stainless steels ie; 316L or 316H.
Optical Emission Spectroscopy
Optical Emission Spectroscopy (OES) is an optical method that can be used to detect almost all types of elements, including carbon and light elements in a variety of different matrixes including stainless steel, nickel, and carbon steel, etc. The OES technique is used to grade material by measuring the element carbon, to identify alloys such as low carbon or high carbon stainless steels, low alloys such as 41xx series, 86xx series, and 10xx series carbon steels, to name a few.
Although OES is considered a portable technique, it would be better classified as a transportable technique. OES instruments vary in weight and size depending on the manufacturer but can weigh upwards of 45-60 lbs. and requires an argon tank that, depending on the size of tank utilized, would also weigh about 20 lbs. The instrument and argon tank are typically transported on a cart to help make the instrument mobile. Because of the weight and size of this field mobile OES, working in elevated work areas could require mechanical assistance to lift an OES instrument to higher elevation platforms. Also, when performing an analysis with OES each sample to be analyzed will require sample preparation utilizing a grinder that uses a zirconium aluminum oxide sanding disc to prepare the surface. Sample preparation is a critical step in any OES analysis and a sample that is not properly prepared will yield undesirable and inaccurate results.
In the OES technique, atoms are also excited; however, the excitation energy comes from a spark formed between the sample and instrument electrode. Unlike XRF that utilizes an x-ray tube to irradiate the sample, OES uses the energy of a spark that causes the electrons in the sample to emit light, which is converted into a spectral pattern. Each element produces a unique color of optical light. By measuring the intensity of the peaks in this optical light of the spectrum, the OES analyzer can produce qualitative and quantitative analysis of the material composition. Even though OES is considered a nondestructive testing method, the sample needs to be prepared with a mechanical sanding device and the spark does leave a small burn on the sample surface that would need to be removed after analysis.
Laser Induced Breakdown Spectroscopy
Laser Induced Breakdown Spectroscopy (LIBS) has been around for many years and is a technique used primarily in laboratory equipment. With recent advances in technology the technique has now been developed into a portable handheld analyzer capable of measuring carbon in the field for material identification and grading of materials. Like OES, argon is still needed to analyze carbon in a LIBS handheld analyzer. Instead of an external argon tank, regulator, and hose connection to the OES unit, the LIBS analyzer uses a consumable argon cartridge integrated into the instrument; and with the analyzer’s battery the instrument weighs less than 6.5lbs.
Sample preparation is still required for analysis but the handheld size of the instrument, the grinder, and sanding discs can all be contained in a small case and transported to elevated working platforms, pipeline ditches, and hard to access areas with minimal effort giving the user true field portability in a handheld carbon analyzer. A proper sample preparation is a critical step in the analysis of the sample. Poor sample preparation will yield undesirable results. With proper sample preparations the user can achieve fast, reliable, and accurate results. The LIBS analyzer can be used to measure light elements in low concentrations such as carbon, silicon, and aluminum. The capability of the LIBS analyzer gives the user the ability to easily grade L and H grade stainless steels, low alloys, and carbon steels. The instrument can also perform carbon equivalent (CE) or residual element (RE) calculations programmed by the user in an easy-to-use and intuitive interface.
The LIBS technique utilizes a pulsed laser to ablate the surface of the sample creating a plasma. As the plasma cools, the electrons from the cooling plasma are excited, causing the plasma to emit light. Each element of the periodic table produces a LIBS spectral peak unique to itself. By using a detector to measure the unique characteristics of light emitted, it is possible to detect what elements are present within the sample. By measuring the peaks of light and their intensities in the sample, the chemical composition can be rapidly determined and quantified in weighted percent concentrations (%), or parts per million (PPM).
There are several accurate and viable technologies to achieve chemical analysis in the field. XRF, OES and LIBS technologies give the user a portable option to verify or grade material. Understanding the differences and limitations of each of these technologies will help the user choose the instrument to meet the specified requirements of the material being analyzed.
Still not sure which elemental analyzer is right for your requirements? Check out this Handheld XRF and LIBS Analyzer Selection Guide.
Editor’s Note: To learn more about the differences between XRF, OES, and LIBS technology, visit Thermo Fisher Scientific at the 2019 Fair Latin American Foundry Fair (FENAF), from September 17-20, booth J16.