Comparing Two PMI Test Methods
With Positive Material Identification (PMI) the alloy composition, and thus, the identity of materials can be determined. If a material certificate is missing or/and you need to be certain about the type of material used, PMI as an NDT method is the best solution. Positive Material Identification is particularly used for high quality metals like stainless steel and high alloy metals.
Two types of PMI
Because specifications for materials used in industry are increasingly more specific, the need for PMI testing has been steadily increasing. XRF and OES types of PMI are available, and both analysis techniques offer advantages and disadvantages.
X-ray Fluorescence (XRF) instruments work by exposing a sample to be measured 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 the intensity and characteristic energy of the emitted X-rays, the analyzer can provide qualitative and quantitative information regarding the composition of the material being tested.
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 In the latest generation of portable XRF analyzers, isotopes have been replaced by small X-ray tubes requiring much less documentation.
In the Optical Emission Spectroscopy (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.
OES instruments are larger in size and use argon gas to improve accuracy. Sample preparation plays an important role; however, there are almost no limitations to the instruments' ability to analyze elements typically used in metals. One of the key reasons OES technology is chosen instead of XRF is OES's 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 materials like stainless steel, 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 from degraded accuracy and precision or repeatability.