Deposition mechanism and coating characterization for the trivalent chromium plating process
Different complexing agent contents were screened through Hull Cell experiments in the sulfate trivalent chromium electrodeposition system,and the best content ratio of complexing agent was obtained.The optimum content ratio was 80 g/L ammonium formate,20 g/L ammonium oxalate and 30 g/L urea.The optimal current density range was 5.11 A/dm2 to 20.68 A/dm2.Cyclic voltammetry and steady polarization were used to study the mechanism of Cr(Ⅲ)electrodeposition in this system,the trivalent chromium reduced through two steps.The first step was Cr3++e→Cr2+,which it was irrevers-ible,while the second step was Cr2++2e→Cr,which it was quasi-reversible.Ammonium oxalate increased cathodic polarization,while ammonium formate and urea reduced cathodic polarization.The chromium coating obtained by electrodeposited for 20 min was composed of Cr,Cr2O3 and Cr(OH)3 according to XPS analysis.The microstructure observation showed that with the increase of electrodepo-sition time,the morphology of the coating gradually changed from a flat surface to a nodular tructure.The plating layer exhibited a clear(110)preferred orientation.Electrochemical studies had shown that compared with 20 min chromium coating,5 min chromium coating had better corrosion resistance,the corrosion potential increases from-0.6377 V to-0.5633 V,and the corrosion current increases from 6.1030×10-6 A/cm2 increased to 5.4031×10-6 A/cm2.
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