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 CrO(3) aqueous solution containing, 0.05 M BaCO(3) and 0.01 M H(2)SiF(6) was evaluated in the absence (i) and presence (ii) of 0.02 M KNO(3). 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 Cr(2)O(3). 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-Pardave, 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 (Cr(2)O(7))(2-) ions. On the other hand, black-Cr deposit electrocrystallization involved the formation of Cr(2)O(3) nuclei growing at a rate limited by the mass transfer of (Cr(2)O(7))(2-) ions, with simultaneous reduction of nitrates on their surfaces. (C) 2010 Elsevier B.V. All rights reserved.