The galvanic zinc-nickel coatings are considered among the most popular and the most effective sacrificial coatings for protection of steel elements. They are commonly used in many branches of industry, e.g. automotive or aerospace. In spite of the excellent anticorrosion properties of the Zn-Ni coatings, they do not fulfill the requirements of some applications and an additional treatment improving their corrosion resistance is often necessary. These coatings, similarly to other metallic materials are usually additionally protected by the use of conversion coatings. The most popular conversion coating is the chromate coating which is ideal for the task. Nevertheless in the classic method of its formation toxic and carcinogenic hexavalent chromium compounds are used. So currently new technologies based on trivalent chromium are sought after. The chromate coatings obtained in such a way are not as excellent like in the case of the classic coatings and there is the possibility of oxidation of trivalent chromium to its toxic hexavalent form when exposed to the oxidizing environment. There are premises that in the nearest future the consecutive regulations prohibiting every chromium compounds will appear. The perspective of total chromium compound elimination forces the combined scientific and industrial effort to carry out further investigations on alternative conversion coatings formation.
The undertaken research concerning new approach for surface layer modification of galvanic Zn-Ni alloy coatings consisting in formation of chromium-free conversion coatings by anodic oxidation method. The anodizing process was conducted in selected alcoholic solutions. The received coatings were characterized in terms of their morphology (OM, SEM), chemical composition (EDX, XRF, XPS, GDOES) and phase composition (XRD). The corrosion resistance measurements of the coatings were carried out by the potentiodynamic method as well as by electrochemical impedance spectroscopy (EIS). The anodic oxidation process led to the colorizing of the Zn-Ni coatings and UV-Vis spectrophotometry studies were carried out to quantify the color change.
Acknowledgments: This work was supported by The National Centre for Research and Development under research project no. LIDER/009/433/L-4/12/NCBR/2013