Preparation And Corrosion Resistance Of A Cr-Pd Double Coating On Carbon Steel

The thick dense trivalent chromium coating and palladium plating were electrodeposited on carbon steel substrate.First the trivalent chromium coating was successfully electrodeposited on carbon steel and then palladium plating was electrodeposited over the surface of chromium coating.Both the metallic coatings were electrodeposited by direct current power supply technique.Two different metallic layers were carried out on carbon steel to show their favorable influence of passivation.Coating produced on steel substrate was subjected to various surface preparatory steps including mechanical and then electropolishing was performed.The double coatings were predominantly based on Cr and Pd having adequate adhesive strength between the films,which showed synergetic effect on passivation.Potentiodynamic polarization&electrochemical impedance spectroscopy were conducted to study the corrosion behavior of Cr-Pd films.Excellent corrosion resistance on Cr-Pd double layer was observed in hot 20 wt.%sulfuric acid and sea water solutions solution at 80?.Further,mechanical properties were analyzed by erosion corrosion test in aggressive solution at boiling temperature.Trivalent chromium coating and Pd coating significantly improved the corrosion resistance in aggressive H2SO4 and sea water solutions.The electroplating time,temperature,pH and current densities for both trivalent chromium coating and palladium plating had the important effects on microstructure,average thickness,coating uniformity and corrosion resistance of the final product films.The average thickness of both the films were 13 ?m which varied according to the current densities and time of the electroplating for the palladium coating.The surface morphology of the finally deposited Pd film was compact with small grains,although in the first layer of trivalent chromium coating some cracks were observed but the Pd layer covered all the cracked morphology with its adequate small and compacted grains.Surface morphologies of Cr-Pd films varied according to the current densities and time of deposition.The X-ray diffraction(XRD)results of the Cr-Pd multi-layer films showed the crystallite structure.The average grain size decreases with increasing the current densities of the films.X-ray photoelectron spectroscopy(XPS)showed that Cr-Pd multilayer coating surface is composed of mainly of palladium,oxygen,and carbon(carbon was added for calibration of the spectrum).Anodic polarization curves revealed that the Cr-Pd multilayer coating with different current densities has higher corrosion resistance than that of pure chromium coating.The EIS results revealed that the pure chromium coating on carbon steel has very low impedance value than that of Cr-Pd multilayer coatings.The addition of second layer over the surface of Cr coating increased the passivation and significantly improved the corrosion resistance.The Phenomena of the improved corrosion resistance of the double coatings were the development of a Solid and dense anticorrosive film of Pd that revealed a physical barrier to the origination and growth of corrosion in contrast with Cr(III)coating.Additionally,the mechanical properties were analyzed by the erosion corrosion test in hot sulfuric acid with different stirring speeds.Results showed the good anti-erosion properties with all the stirring speeds.