OXIDIZING ALTERNATIVE SPECIES TO CHROMIUM-VI IN ZINC GALVANIZED STEELSURFACE-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 resistan ce and enhance the chemical adhesion of organic coatings. Although the detailed formation mechanism of chromium conversion coatings has rema ined a subject of wide interest for 30 years, it still remains to be c ompletely clarified. Meanwhile, as a consequence of recent environment al legislation (US Environmental Protection Agency identified the Meta l Finishing Industry as one of the most significant contributors to en vironmental pollution), a large amount of effort is being made to stud y alternatives to Cr(VI) baths. In the first part of this work, the au thors 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, the y showed the best anticorrosive behaviour in salt spray. For this reas on, they were characterised by scanning electron microscopy with energ y X-ray dispersive spectrometry associated (SEM/EDS), X-ray diffractio n (XRD) and X-ray photon spectroscopy (XPS), in order to understand th eir protective mechanisms. In a second part of this work, electrochemi cal a.c. measurements will be involved in order to complete the study. (C) 1998 Elsevier Science S.A.