Oxidising alternative species to chromium VI in zinc galvanised steel surface treatment. Part 1 - A morphological and chemical study

Surface treatment of zinc galvanised steel, using chromate based baths, was extensively used in order to both improve the corrosion resistance and enhance the chemical adhesion of organic coatings. Although the detailed formation mechanism of chromium conversion coatings has remained a subject of wide interest for 30 years, it still remains to be completely clarified. Meanwhile, as a consequence of recent environmental legislation (US Environmental Protection Agency identified the Metal Finishing Industry as one of the most significant contributors to environmental pollution), a large amount of effort is being made to study alternatives to Cr(VI) baths. In the first part of this work, the authors present the main results and conclusions of an experimental and systematic study on zinc conversion layers (ZCLs) based on molybdates, permanganates, vanadates and tungstates, using a good Cr(VI) bath as reference. Considering the alternative ZCLs, molybdate-based ones, they showed the best anticorrosive behaviour in salt spray. For this reason, they were characterised by scanning electron microscopy with energy X-ray dispersive spectrometry associated (SEM/EDS), X-ray diffraction (XRD) and X-ray photon spectroscopy (XPS), in order to understand their protective mechanisms. In a second part of this work, electrochemical a.c. measurements will be involved in order to complete the study. (C) 1998 Elsevier Science S.A.