Determining the exact chemical composition of alloys is extremely important due to a number of reasons, for example it may be necessary to verify that a critical component is made from the correct alloy when a mill test certificate is unavailable or the validity of said certificate is in question.
There are hundreds of different alloy compositions, each with their own set of specific properties. Certain alloys of the same base metal composition can often have very different sets of properties. One example of this is the resistance of stainless-steel alloys to corrosion via acid; some steel alloys are highly resistant to certain acids while others are not. Choosing the incorrect grade can result in sudden and unpredictable failures.
The method of correctly identifying an alloy is referred to as positive material identification (PMI). This is a blanket term for various technologies and techniques used to determine the composition of an alloy. PMI can determine both the elemental composition (quantitative) and the grade of the alloy (qualitative). There are many different techniques used to determine alloy composition, but the two main techniques used in the PMI industry, XRF and OES, are discussed below.
X-ray fluorescence spectroscopy, or XRF, is a method of PMI that uses low-energy X-rays to scan the chemical composition of alloys. A handheld instrument is used and can determine the composition of the alloy within seconds.
The X-rays excite the atoms in the sample, which then fluoresce, producing secondary X-rays that are reflected to the detector. The energy (or wavelength) of these reflected X-rays can be used to precisely determine which elements are contained in the sample. The composition of the alloy can thus be determined by the device.
It must be noted that, due to the high scattering of the X-rays with the metal atoms, the X-rays only reach a depth of around 100 microns into light alloys. This depth decreases as the alloys become denser. It is therefore critical that the surface of the material is representative of the bulk material. Any kind of coating on the surface such as galvanised coatings, paint or surface contamination will dramatically change the result of the scan.
Table 1 - XRF Advantages/Disadvantages
The unit is light and easy to use
Can only measure a few hundred microns into the surface of the sample for light alloys and a few tens of microns for heavier alloys
Very little surface preparation is required on the sample
Not all elements can be detected with this technique
Can sample small pieces of material such as wire
Optical emission spectroscopy, or OES, is a method of PMI that creates a spark on the sample in the presence of argon gas. The spark excites the atoms within the sample. These excited atoms emit light at specific frequencies which are then used to precisely determine the composition of the alloy. Measurements can be taken without the use of argon gas at the expense of accuracy in the result.
One of the major advantages of OES is its ability to measure the light elements that are not detectable by XRF. As such, OES is a very versatile method to determine the chemical composition of alloys.
Table 1 - OES Advantages/Disadvantages
Can detect light alloying elements
The system is bulky and requires argon gas for accurate results
A burn mark is left on the material
Significant surface preparation is required
XRF can identify up to 90 % of the elements on the periodic table, i.e. elements heavier than magnesium. Some of the typical alloys that can be identified by PMI are indicated below.
XRF is not able to identify the exact composition of the alloys containing elements lighter than magnesium (including lithium, beryllium, boron, carbon, nitrogen), such as the following:
It should be noted that despite XRF being incapable of detecting these elements, the alloy can sometimes still be identified by identifying the other alloying elements.
OES can identify all the above, including alloys containing light elements such as carbon, lithium, boron and beryllium.
The two most common methods used to determine the chemical composition of an alloy (or any metal) are X-ray fluorescence spectroscopy (XRF) and optical emission spectroscopy (OES). XRF is non-destructive, however, OES is more sensitive than XRF to lighter elements.
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