Positive Material Identification (PMI) refers to the identification and analysis of various metal alloys based on their chemical composition in nondestructive testing (NDT). Measurement results are shown in the form of elemental concentration in percentage or by specific alloy name such as SS316L or Inconel 625. PMI is a field-testing method made possible by the portability of most PMI analyzers. These instruments also can be used in the laboratory.
Because specifications for materials used in industry are increasingly more specific, the need for PMI testing has been steadily increasing for the past several years. Periodic plant maintenance shutdowns are less frequent and consequently the materials used in the plant are in use longer. A wider variety of alloys that are indistinguishable to the eye are being used in individual plants. When facility staff replace components, they must be able to guarantee that the new part matches engineering specifications. Recent industrial accidents have cost the lives of workers and heightened the awareness of the need for accurate and comprehensive PMI inspections.
Today, many major oil companies require that every pipe, flange, connector, valve and welding seam in critical parts of the plant be measured to verify that materials match engineering specifications. Finally, global sourcing is another factor driving the need for PMI. Currently, metal buyers and suppliers are operating globally, and buyers need to verify the quality of the product they are receiving.
PMI MethodsThe two main technologies used for alloy identification in PMI are X-ray fluorescence (XRF) and optical emission spectroscopy (OES). XRF instruments work by exposing a sample to a beam of X-rays. The atoms of the sample absorb energy from the X-rays, become temporarily excited and then emit secondary X-rays. Each chemical element emits X-rays at a unique energy. By measuring intensity and characteristic energy of the emitted X-rays, an XRF analyzer can provide qualitative and quantitative analysis regarding the composition of the material being tested.
In the OES technique, atoms also are excited; however, the excitation energy comes from a spark formed between sample and electrode. In this case, the energy of the spark causes the electrons in the sample to emit light, which is converted into a spectral pattern. By measuring the intensity of the peaks in this spectrum, the OES analyzer can produce qualitative and quantitative analysis of the material composition. Although OES is considered a nondestructive testing method, the spark does leave a small burn on the sample surface.
Limitations and AdvantagesBoth analysis techniques offer advantages and disadvantages. XRF analyzers are easy to use, the units are light and small in size, and the sample to be measured does not require much preparation. But, there are limitations on the number of elements that XRF units can measure. Also, traditional methods of generating X-rays have used radioactive isotopes, the use of which requires much documentation. In the latest generation of portable XRF analyzers, isotopes have been replaced by small X-ray tubes requiring much less documentation.
OES instruments are larger in size and use argon gas to improve accuracy. Sample preparation plays an important role, but on the other hand, there is practically no limitation on the instruments’ ability to analyze elements typically used in metals. One of the key reasons why OES technology is chosen instead of XRF is because of its superiority in the measurement of light elements in metals, such as carbon and aluminum. OES is the only reliable way to measure carbon outside of the laboratory, which commonly needs to be measured in samples of stainless steels, magnesium and silicon. The technology also is employed in the measurement of aluminum in aluminum alloys. OES measurements can be attained without an argon atmosphere, but will suffer degraded accuracy and precision or repeatability.
PMI Operators and InnovationsTypical operators of PMI analyzers are:
- Nuclear and conventional power plants
- Refineries or chemical plants.
- Metal fabricators that produce pressure vessels, valves and other components.
- Manufactures of aerospace components.
- Inspection companies performing PMI testing for their customers.
- Manufacturers of medical equipment.
Modern software packages are available for easy data transfer and printing, and results are shown on color displays and units can communicate wirelessly. Analysis speed and accuracy are improving all the time, and on some latest XRF units, Aluminum, Magnesium and Silicon can be detected, although with accuracies not comparable to OES units. Some of the latest generation portable OES analyzers perform measurements approaching the accuracy of benchtop instruments, but with the advantage of portability.