Pt 31.311 Pd 30.546 Rh 147.089 Au 91.95 Ag 1.022

“Surface Segregation” in Metals: Its Impact on Value Assessment and XRF Testing Accuracy

What is Surface Segregation?

“Surface segregation” is a phenomenon where certain atoms in a metal migrate toward the surface, resulting in a surface composition that differs from the alloy’s bulk composition. In other words, while the internal material may be homogeneous, the surface layer may exhibit differences in the concentrations of various elements.

This phenomenon can occur in a range of metal alloys, especially in metals with diverse chemical and physical properties. Surface segregation often results from energy differences between the material’s surface and interior, environmental factors, or heating and cooling processes the materials have undergone. For precious metal recyclers and dealers, understanding surface segregation is especially important, as it affects the accuracy of composition measurements taken with specific tools, such as XRF devices.

How Does This Phenomenon Affect XRF Testing?

XRF (X-ray Fluorescence) testing is based on exposing the material’s surface to X-rays and analyzing the emitted photons to identify the elements present on the surface. However, when the material’s surface composition differs from its internal composition, the test may not accurately reflect the entire mass’s composition.

In the case of cast blocks containing precious metals (such as scrap precious metals), this phenomenon can lead to incorrect value assessment. For instance, if valuable atoms like silver or gold migrate to the surface and create a higher concentration, the XRF test might indicate a higher value than the overall block’s average. Conversely, if non-valuable elements migrate outward, the XRF test might underestimate the value.

Recommendations for Dealers and Recyclers

To ensure as accurate an assessment as possible, it’s advisable to perform the XRF test on multiple points across the block’s surface rather than relying on a single measurement point. Such sampling allows for assessing whether the block’s composition is uniform and homogeneous. Additionally, when feasible, it’s recommended to conduct complementary tests on samples drilled from the block’s interior to obtain a comprehensive analysis that includes all material layers.

By adopting a careful approach and understanding the phenomenon of surface segregation, dealers and recyclers can improve the accuracy of their metal composition assessments and prevent errors in material valuation.