PMI as Quality Assurance Tool for Production
PMI (Positive Material Identification) means verifying mainly metallic material based on the chemical composition. PMI is used mainly when field measurements are needed and in the laboratory people still speak about complete elemental analysis. A new method called LIBS is also briefly discussed in this article.
Analyser shows the full analysis of the main alloying elements.
Sometimes only the trade name (grade) of a metal is required, but sometimes the concentration of important elements like carbon is also needed. The main technologies used for PMI are XRF and OES. Detailed technical discussion of PMI is beyond the scope of this article. We will however discuss the pros and cons of the various techniques.
The two main reasons to use PMI are global sourcing of material and the quality assurance requirements for products of most customers. The sourcing of components and subassemblies is very global-driven by the need for cost reduction. A typical example is a cast component. The buyer wants to make sure the cast component (imported from overseas) is manufactured from the material specified in the attached material certificate. There are, unfortunately three possibilities here: the certificate tells the true story, the certificate tells the true story of a different cast component, or the certificate has been “further processed”.
Experience shows that the more global sourcing a fabricator uses, the more problems they are likely to experience due to materials getting mixed-up. It is also common that the material verification is taken seriously only after an incident has already occurred. PMI done on a purchased component before it is used in the production can save a lot of money and production capacity.
Typical user application is the receipt inspection of material.
The Traceable Quality Assurance of fabricators’ products is required more and more by their customers and authorities, especially for critical components like pressurized vessels (production or repair). Local regulations stipulate certain testing and documentation of the materials used. There are a growing number of industries where material mix-up cannot be tolerated at all with classical examples bring petrochemical and nuclear plants.
For global petrochemical companies it is practically impossible to accept components or process equipment for critical parts of the process unless PMI has been done. PMI is carried out many times, along with Ultrasonic testing of the welding seams. Sometimes only 10 percent of the products are tested but in critical processes 100 percent PMI is required. This means that every pipe, valve, flange, bend, bolt, nut and welding seam is verified and verification is documented. In certain cases the verification of the alloy grade is enough; in other cases however, a full analysis of the main alloying elements is required.
One of the trends during the last decade has been the PMI of old refineries and chemical plants. In plants built before 1980, PMI was seldom done during construction. Consequently, this means that many of these plants are currently having PMI carried out - while plant is in full operation. This creates specific challenges for the PMI because the measurements have to be made on surfaces that are operating at several hundred degrees. Normally only XRF can be applied in these cases.
Depending on the policy of the company, PMI can be done by the company itself or it can be contracted to an outside inspection company. The trend is to outsource these activities, but this all depends on how frequently and urgently PMI is needed.
The Standards to be Followed
It is worth mentioning here that even though Foundries and Metal manufacturing plants use XRF and OES in their laboratories as standard instruments, this is not normally regarded as belonging to the PMI area; PMI is for confirmation and (big) laboratory analysers are used for verification.
Most of the NDT methods used in the industry are standardized. In PMI no real standards have been created. However, one of the front-runners in PMI, American Petrochemical Institute, has created their own recommended practice API (American Petroleum Institute) RP – 578 (2nd edition). It is not an actual standard but a recommended practice and is a widely used comprehensive documentation on the subject.
Of course the biggest Petrochemical companies (Exxon, BP, Indian engineers) have their own internal recommended practices as well. These standards tend to specify equipment for PMI on a performance basis rather than specifying a certain brand to be used.
Portable Equipment
The first PMI units (portable elemental analysers with software able to tell the alloy grade name) came in the early 1980s. The units were quite bulky; the weight was close to 10 kg and used radioactive isotopes, which limited their use in some locations. Also the analysis of light alloying elements was impossible or quite clumsy. But despite this, they were portable and could analyse a large object without the need to take a sample, thus destroying the component.
The real step forward in portability and usability came when the miniaturized X-ray tubes and solid-state X-ray detectors came to the market. This allowed handheld XRF (HHXRF) analysers to be built as compact pistol-like units. The next step forward happened some 6 years ago when detector technology allowed the convenient analysis of light (Mg Al Si, S, P) alloying elements. This made the analysis of many Ti alloys and Al/Si bronzes possible, and also helped to expand the use of the technology to aluminium alloys, especially in the aerospace industry. At the same time the software has developed so that the documentation of the measurements is done more or less automatically in tamper-proof files. This also greatly improved the ruggedness of the analyser.
Mobile OES & LIBS Analysers
Typical mobile OES analyser.
The role of mobile OES technology is much smaller than that of portable XRF, but the real benefit of OES (compared to XRF) is that low levels of carbon can be detected and separated. In essence this means that low carbon stainless steels can be separated from standard grade (304/304L, 316/316L). The actual use of OES analysers is more demanding under field conditions, but in return higher accuracy can be achieved for some elements.
In the past year LIBS (Laser Induced Breakdown Spectroscopy) has been offered as a handheld instrument for PMI analysis. This technique offers relatively fast analysis when light elements are required to determine the grade name. While the technique offers relatively simple operation, the analytical performance has not yet been proven. In addition, LIBS potentially offers the analysis of very light elements like carbon and possibly beryllium. However, these elements require special consideration and may not be analysed without a shield gas. Prior to using this technique the user should analyse known samples to determine that the analyser is capable of providing the necessary precision and accuracy.
PMI Applications
It is worth mentioning that coatings can be measured with an HHXRF unit as well. The coating thickness (normally in the range of one to several tens of micrometers) can be measured as well as the concentration of different elements in a specific coating depending on the calibration. The limitation is that the analytical task gets a bit difficult if the same element is in the coating and in the base material, or if the coating is too thick.
Some of the more recent applications of HHXRF include FAC (Flow Accelerated Corrosion) and Sulfidation (Sulfidic) Corrosion Failure analysis (as defined in API 939C). The FAC requires the analysis of 500 ppm Cr levels and the Sulfidic Corrosion analysis requires analysis of 1000 ppm levels of Si in steels. Both measurements are possible with the latest generation analysers.
Thus, it appears that more and more PMI analysis will be required in the future as new materials are being developed. The awareness of the unexpected costs caused by material mix-up is encouraging fabricators and users to pay close attention to the material being used in all applications and to have the ability to prevent mix-ups from happening.
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