Simplifying Complex Failures with EDX Images

Introduction

Failure analysis can be perceived as a mystery box for personnel that are not involved: they send in a failed component and need to know why it failed, if the problem could exist elsewhere, how to inspect for it, and ultimately how to prevent recurrence. One of the greatest challenges for a failure analyst is to present complex information clearly for an audience that may not have the same depth of knowledge on specialized subjects. Sometimes, a failure mechanism can be summarized with an image that can tell the whole story – a “Eureka” Image. The old adage, “a picture is worth a thousand words,” is particularly relevant.

This article summarizes three examples where a technique called Energy Dispersive X-ray (EDX) mapping was used to clearly illustrate the failure mechanism, producing a “Eureka” image. The first example is about how an interesting, layered scale in carbon steel piping summarized how the cycling conditions affected the failure mechanism. The second example shows how a region missing scale in a carbon steel crude heater tube summarized the active corrosion mechanism. The last example is about how a missing alloying element led to a diagnosis of dealloying in a nickel aluminum bronze valve.  First, a brief explanation of the EDX technique is provided so that the Eureka images can be interpreted.

EDX Technique Explained

The Scanning Electron Microscope (SEM) is a valuable tool for a failure analyst to examine samples at high magnifications. In brief, an SEM functions by bombarding a sample with electrons in a vacuum chamber. The incident electrons interact with the near surface atoms in a sample. The incident energy causes many interactions within the sample. Some of the useful interactions used during a failure analysis include: 

• Secondary Electron (SE) imaging, useful for examining topographical features, 
• Backscatter Electron Detection (BSD), useful for examining compositional differences, and 
• Energy Dispersive X-ray (EDX) analysis, useful for determining and quantifying the elements present in a sample. 

EDX analysis is the most relevant technique for the following three case studies discussed. EDX forms the basis of chemical composition analysis by a process called X-ray Fluorescence (XRF). In brief, the incident electrons excite the electrons in a sample and produce X-rays characteristic to the elements present. The resultant X-rays are detected and converted into information about what elements are present.