Positive Material Identification (PMI) may be applied to metals during the supply, fabrication & erection of those materials. The PMI is utilized for quality control and safety compliance and is an integral part of both production and asset integrity management across many industries including oil and gas, power generation, chemical, pharmaceutical, nuclear, aerospace, and metal fabrication.
PMI can be used to confirm that the chemical composition of the metallic parts have the correct percentage of key elements, thus ensuring that the material properties meet the purchase requirements and that such materials are properly analysed and identified to indicate the alloy. This practice of material identification is not usually required of materials already in service, however on some occasions it can be very useful.
The material testing can be performed on-site or on in-service process equipment and piping systems. PMI can be utilized on a variety of different part sizes, from large complex parts to small intricate items such as small fasteners.
The most common type of PMI is the application of portable X-ray fluorescence (XRF). XRF spectrometry is a comparative analytical method that employs low-energy (1keV to approx. 30 keV) X-rays or gamma rays to excite characteristic X-rays in the subject material. These X-rays emanate from the individual elements in the subject and may be analysed by either of the following means;
1) Qualitative (recognition of the elements by unique X-ray patterns) or
2) Quanitative (identification of characteristic X-rays and measurement of their intensities).
The technology behind these instruments is continually improving but XRF instruments typically are incapable of quantitative measurements for elements with an atomic number lower than 22 (titanium). In particular, the XRF method is incapable of measuring carbon (atomic number 6), which is a critical element in many of the alloys for which PMI is desired. When the measurement of carbon or other elements with an atomic number less that 22 is desired, the user may also apply optical emission spectrometers (OES). OES instruments produce an electrical arc between the instrument and work piece so the examined area shall be selected to minimise damage to critical surfaces.
The American Petroleum Institute recommended practice, API RP 578 – “Material Verification Program for New and Existing Alloy Piping Systems”, provides the guidelines for a quality assurance system to verify the alloy components.
Another widely used PMI guide is ASTM E1476 is a Standard Guide for Metals Identification, Grade Verification, and Sorting. Both of these documents cover the use of the handheld XRF and other applicable techniques.
- Ensuring that products and components have been manufactured using the correct alloy
- Finding potentially incorrect alloys
- Ensuring material conforms to the correct standard and specification
- Ensuring welded components have used the correct filler material
- Failure Analysis PMI
(Example of PMI carried out to confirm the correct steel piping grade)
Source/Credit: Bruker PMI
Author: James Murphy, Operations Manger of IPEC Inspection Ltd.