Although handheld X-ray Fluorescence (HH XRF) analyzers are a boon to alloy analysis, there may be quality control problems if they are not used with properly surfaced samples. It’s not always as simple as “point and shoot.” Technicians and their supervisors must be aware of possible quality issues with unsuitable sample surfaces. For a clean, flat, smooth metal sample surface which completely covers the measurement window; HH XRF can always provide an accurate alloy grade ID and chemical analyses worthy of laboratory instrumentation. The penetration depth of X-rays in steel is typically less than about 100 microns (ìm), depending on the X-ray energy. These incoming X-rays and the characteristic X-rays emitted by each element may be absorbed by any intervening matter, such as paint, coatings, and corrosive layers. Such absorption or attenuation of the X-rays can seriously affect the analytical performance of the analyzer. The reduction in analytical integrity (i.e., precision and accuracy of analysis) will be especially severe for the lighter elements (i.e., lower than Ti in the Periodic Table) due to the very low energy of these X-rays. The user would be well advised to carry a portable grinder. Typical grinding disks use 80 or 120 grit aluminum oxide. A light grind for 5 to 10 seconds will usually be sufficient for the specific problem areas discussed below. 1) Paint: If possible, choose an unpainted portion of the sample. If not, perform a light grind to provide a clean spot for the analyzer. 2) Coatings: There are many types of coatings associated with metal samples, but oil is probably the most common. Wipe the sample to remove the bulk of the oil. If you are measuring light elements oil is a much greater problem, in this case if possible, use a bit of acetone to properly clean the surface. 3) Plating & Corrosion: A common plating is the zinc coating on steel to produce “galvanized” steel. The analyzer will detect a lot of zinc and some of the underlying iron and fail to match any alloy in the library. Fortunately, such surface conditions are generally evident from visual inspection and a light grind should quickly provide a sample surface adequate for alloy analysis. 4) Surface Roughness: A flat, smooth sample surface is optimal. If initial analyzer results are ambiguous (i.e., uncertain alloy grade ID), then use the grinder. Alloys of aluminum and magnesium, light metal alloys, are particularly susceptible to poor sample surface condition due to the scatter which will be caused by uneven surfaces.
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