31. 01. 2024


How to get useful information on the nature and thickness of coatings?

In previous newsletters we have already discussed the salt spray test according to EN ISO 9227:2023 (for more info, read here) and the condensation test according to EN ISO 6270-2:2018 (for more info, read here) and their practical applications.
The methods provide an excellent indication of the quality of the coatings and how well they retain their function in such severe environments as those of the test. The application of EN ISO 9227:2023 and EN ISO 6270-2:2018 are therefore suitable for analysing any discontinuities, pores and damage to the coating, which are likely to be the points where corrosion will be activated leading to degradation of the product.
At this point, in the case of metallic coatings, a control combined with testing that can provide useful indications about the coating, its nature and thickness is XRF (X-Ray Fluorescence) analysis according to ISO 3497:2000.
By irradiating the sample with high-energy X-rays and quantitatively evaluating the emitted (or absorbed) fluorescence radiation, a detailed picture of the composition of metal alloys, both qualitative and quantitative analysis of the metal content in solid samples and calculation of the thickness of metal coatings, can be obtained.
The fluorescence intensity is a function of the atomic number of the elements and their quantity. In a coated sample, the surface layer, the intermediate layer (if present) and the substrate will generate characteristic fluorescence. A suitable detector can be set to select one or more specific energy bands, making the instrument capable of measuring the thickness and/or composition of a surface coating and those beneath it simultaneously and almost immediately.
As an example, at Catas, an analysis was conducted to differentiate between two externally identical painted samples. While one showed widespread signs of corrosion, the other did not. Although the salt spray test was useful in distinguishing between the samples, XRF analysis revealed that the defect was not solely due to the painting process but also to the absence of an intermediate zinc layer in the defective sample. The paint itself, being non-metallic of course,  is invisible to XRF and therefore does not affect the measurements.

In conclusion, while the salt spray test according to UNI EN ISO 9227:2023 and the condensation test according to EN ISO 6270-2:2018 are essential for evaluating the resistance of metallic components in both indoor and outdoor environments, XRF analysis can provide more detailed information on the object under examination and can be a valuable aid.

Cristian Dorigo

For info:
Tommaso Pascolini
+39 0432 747257