Underfilm corrosion of steel sheets observed by confocal 3D-XRF technique

A confocal micro-X-ray fluorescence (XRF) method with a vacuum chamber was used for investigation of underfilm corrosion that occurs on a painted steel sheet used for automotive applications. A painted steel sheet was prepared by applying electrodeposition coating onto an electro-galvanized steel sheet with a zinc phosphate conversion coating. The spatial resolution of the confocal micro-XRF was about 10.9 ?m at an energy of 17.4 keV. The phosphorus signal from the zinc phosphate layer was able to be observed nondestructively by measuring under the vacuum condition. Each layer of the painted steel sheet was observed from the depth profile analysis of the characteristic elements of each layer. The painted steel sheet was scratched to expose the substrate surface, and then the sheet was then immersed in a NaCl solution for 10 days. The in-depth elemental map of Ti of the corroded steel sheet revealed blister-type corrosion around the scratch. The in-depth elemental maps of Zn and P suggested that the zinc phosphate layer dissolved from the scratched part into the NaCl solution.