Failure analysis is akin to detective work. It requires an arsenal of tools to find exact clues that can lead to the root cause. In the following article, we take a closer look at one of the most important tools used by our in-house Sherlock Holmes team: the SEM-EDX.
.jpg "Intergranular corrosion under SEM-EDX")
SEM-EDX is short for Scanning Electron Microscopy - Energy Dispersive X-ray spectroscopy or Raster Electron Microscopy with Energy Dispersive X-ray Spectroscopy. That is quite a mouthful for a technique that allows high-resolution images to be taken, combined with chemical analysis of the surface.
SEM captures different types of samples
Using SEM, several types of samples can be examined at a detailed visual level:
- on a fractured part, study of the fracture surface using SEM can provide the type of fracture
- structure and thickness determination of coating layers
- the surface condition of e.g. seats can be visualised
- the presence of fine cracks in e.g. bolts.
Electron microscopy: the benefits
The major advantage of SEM over light microscopy is the excellent depth of field and the higher magnifications that can be achieved. Magnifications up to 10000x are attainable although even at low magnifications details emerge that are not visible optically. Metallographic specimens can also be examined, if an electrically conductive coating is applied.
Modes of SEM
Electron microscopy imaging can be done in several modes, each with a specific purpose:
- In thesecondary-electron mode, the provided images are easy to be interpreted to e.g. evaluate surface condition or detect cracks.
- The backscattered-electron mode can visualise composition differences at a glance: dense phases will light up brighter than less dense areas.
Chemical analyses with EDX
Besides imaging, an electron microscope can also perform chemical analysis. All chemical elements starting from carbon (Z≥6) can be semiquantitatively detected. This allows determining the composition of different types of materials.
Some typical examples are:
- analyses of corrosion products to detect any harsh components such as chlorides
- characterisation of deposits found in pipes/reactors
- determination of the type of (stainless) steel: e.g. 304 vs 316
Using EDX, targeted point analyses can also be performed to determine e.g. the composition of precipitates or intermetallic compounds on a metallographic sample.
Strictly speaking, EDX cannot determine chemical compounds but only the chemical elements present. Nevertheless, an experienced operator can use the measured spectra to obtain a good estimate of the composition of the sample under examination.
The possibility of combining imaging by SEM with chemical analysis by EDX thus makes an electron microscope an indispensable aid in all types of materials research.
Would you like more information regarding failure analyses and other material testing, or would you like to carry out an SEM-EDX analysis? Be sure to contact us at info@metalogic.be and we will be happy to help.
