Determination of Lead, Cadmium and Total Chromium in Coating of Prepainted Steel Sheet and Strip by X-Ray Fluorescence Spectrometry

Twelve calibration samples, with their coating thickness and amounts of lead, cadmium and total chromium in the coating distributed in definite gradient, were self-prepared. Thickness of the coatings were determined by the DJH method given in GB standard of GB/T 13448-2019, and the values of contents of lead, cadmium and total chromium in the coatings were determined by ICP-AES with sample pretreated by ultrasonication, as described in IEC 62321-5. A method of X-ray fluorescence spectrometry (XRF) for direct determination of lead, cadmium and total chromium in coating of prepainted steel sheets and strip was proposed by using this set of calibration samples. Base material of the prepainted steel sheets and strip was cold-rolled steel with a thin layer of Zn-plating, both of which might contain the same elements having contents at close concentration lavels to those in the coating or more strictly the organic layer which was the object of analysis, and thus serious interference would be encounted. Furthermore, the matrix elements, titanium and silicon, in the organic coating were also a source of interference. However, all these interferences were overcome by using the film analysis software and the software of fundamental parameter method which were attached to the XRF intrument. As shown by the testing results, values of correlation coefficients obtained for lead, cadmium and chromium attained to 0.9, and detection limits (3s) found for the 3 elements were 5×10^-4% for lead and chromium, and 3×10^-4% for cadmium. Three calibration samples were taken at random and analyzed by the present method, giving results of the 3 elements in consistency with the values determined by the standard method given in IED 62321-5, and values of RSDs (n=6) found by this method were 8.5% for lead, 9.2% for cadmium and 11% chromium, which were found smaller than the RSDs (n=6) found by the same standard method mentioned above, 29% for lead, 14% for cadmium and 17% for chromium. The present method showed its merits of rapidity and accuracy, and was suitable to be used for quality examination of the coating of prepainted steel sheet and strip as required by RoHS.