Analysis of coating weight and iron content for iron-zinc alloy coatings by using X-ray fluorescence with multiple regression

Abstract

Zinc-iron alloy coatings are produced by annealing of zinc-coated steels, in which the iron and the zinc are interdiffued to form iron-zinc alloy. The analysis of the coating weight and the iron content within the coating is very important since they are closely related to the corrosion properties. However, it is difficult to measure these alloy coatings by using the conventional X-ray fluorescence (XRF) method because iron is contained both in the coatings and the substrate. Furthermore, there is some concern that the non-uniform concentration of iron within the coatings may result in significant errors in the XRF analysis. The difference in probing depth for Zn Kalpha and Zn La lines can be used to separate the iron contained in the coating from that in the steel. The critical coating weights for the Zn Kalpha and the Zn Lalpha lines, were determined by fitting the line intensities of zinc electroplated coatings, and the coating weights of commonly produced zinc-iron alloy coatings were found to be higher than the critical coating weight, for the Zn Lalpha line and lower than that for, the Zn Kalpha line. A multiple regression method was successfully applied with the Zn Kalpha and the Zn La line intensities for the determination of the coating weight. The iron contents in the-coatings were determined by using the Zn Lalpha line only. With the proposed method, a coating weight accuracy to 0.57 g/m(2) and an iron content to 0.57 % was achieved for a number of commercially produced zinc-iron alloy coatings.