An X-ray photoelectron spectroscopy investigation of chromium conversion coatings and chromium compounds

Hexavalent and trivalent chromium based conversion coatings on zinc electrodeposited steel have been investigated using X-ray photoelectron spectroscopy (XPS) with the aim of elucidating their film chemistry. Furthermore, a monochromatic Al Kα X-ray source was utilised and the spectra produced evaluated using curve fitting software to elucidate oxidation state information. In addition, a number of chromium compounds were investigated and used to complement the curve fitting analysis for the conversion coatings. High resolution Cr2p spectra from chromium compounds exhibited multiplet splitting for Cr2O3. Additional satellite emissions can also be observed for Cr2O3 and Cr(OH)3. Curve fitting of hexavalent chromium conversion coating (CCC) 2p3/2 spectra contained both Cr(VI) and Cr(III) species with the content of the former slightly higher when the X-ray beam take-off angle (TOA) was reduced to determine more surface specific information. The Cr(III) content was determined to be mainly composed of Cr(OH)3 with some Cr2O3. In comparison, trivalent CCCs were largely composed of Cr2O3 as opposed to Cr(OH)3. Survey scans of both coatings revealed that the trivalent CCCs had a higher relative zinc content.