Electrochemical nucleation and growth of black and white chromium deposits onto stainless steel surfaces

Chromium electrodeposition onto a stainless steel substrate from 0.62 M CrO3 aqueous solution containing, 0.05 M BaCO3 and 0.01 M H2SiF6 was evaluated in the absence (i) and presence (ii) of 0.02 M KNO3. Chromium electrodeposited from the electrolytic bath (i) was white-gray colored, whereas deposition from bath (ii) formed a black-colored surface. SEM-WDX and XPS analysis of the steel surfaces covered with these two deposits revealed different features for black and white chromium films. While white-Cr film was mainly composed by Cr(OH)3, the black-Cr film comprised Cr2O3. The white-Cr deposit appeared as a smooth film while the black deposit consisted of numerous dispersed, nano-sized clusters. Analysis of potentiostatic current density transients (j–t curves) recorded in these systems according with the theoretical model proposed by M. Palomar–Pardavé, B.R. Scharifker, E.M. Arce, M. Romero–Romo, Electrochim. Acta, 50 (2005) 4736, indicates that formation of white-Cr deposit was carried out by multiple nucleation of Cr(OH)3 centers with three-dimensional growth limited by mass transfer of (Cr2O7)2-(Cr2O7)2- ions. On the other hand, black-Cr deposit electrocrystallization involved the formation of Cr2O3 nuclei growing at a rate limited by the mass transfer of (Cr2O7)2-(Cr2O7)2- ions, with simultaneous reduction of nitrates on their surfaces.