Electrodeposition of corrosion-resistant Cr–P and Cr–P–W coatings from solutions based on compounds of trivalent chromium

Abstract

Corrosion-resistant Cr–P and Cr–P–W coatings were obtained by electrodeposition from aqueous and aqueous–organic (DMF/water) solutions. The inclusion of phosphorus in the deposits formed on the cathode was confirmed using the EDX and XPS techniques. X-ray diffraction data revealed that co-deposition of phosphorus with chromium resulted in amorphization of the obtained coatings. It was shown that the incorporation of phosphorus into the coatings leads to the disappearance of the region of potentials corresponding to active metal dissolution as compared to Cr coatings. As a result, the coatings demonstrate higher corrosion resistance. Cracking of Cr–P coatings leads to a decrease in their protective ability with respect to steel. The impossibility of obtaining thick Cr–P layers by electrodeposition from aqueous solutions limits the sphere of their possible application. This problem was solved by electrodeposition of Cr–P–W coatings from aqueous–organic media. At coating thicknesses above 10 μm, fully penetrating pores in Cr–P–W coatings practically disappeared. The addition of tungsten to the composition of cathodic deposits resulted in an additional increase in their corrosion resistance.