Preparation of Crystalline Chromium Coating on Cu Substrate Directly by DC Electrodepositing from Wholly Environmentally Acceptable Cr(III) Electrolyte
Introduction
Presently, almost all of the chromium coatings that are electrodeposited using Cr(III) electrolytes are amorphous [1], [2], [3], [4], [5], with their X-ray diffraction (XRD) patterns showing a characteristic broad bread-like peak. G. Saravanan et al. prepared a nanocrystalline chromium coating using a pulsed electrodeposition method of Cr(III) electrolytes containing a large amount of dimethylformamide (DMF) [6]. This process for the production of a chromium coating is destined to be rejected by industries because DMF has been introduced as a new source of toxic pollution, although the harm of Cr(VI) was avoided. In addition, F. I. Danilov et al. also claim to have prepared a nanocrystalline, hard chromium coating from Cr(III) electrolytes containing urea and formic acid, but the XRD pattern they provided revealed that the coating possessed a main phase that was amorphous, which is not distinctly different from the phase structures of previously reported chromium coatings [7].
As is well known, the chromium coating produced by a traditional Cr(VI) electrodeposition process is nanocrystalline, which has been widely used in industry due to its excellent toughness, hardness, and comprehensive performance. The properties of the coatings are dependent on their microstructures, and compared with the conventional chromium coatings obtained from Cr(III) electrolytes, the amorphous chromium coatings using Cr(III) electrolytes show obvious flaws, such as insufficient toughness, high brittleness and internal stress, and a poor binding force between the coating and the substrate [8], [9].
The goal of this study was to prepare crystalline chromium coatings by the direct current electrodeposition of Cr(III) electrolytes without the use of a subsequent heat treatment. At the same time, we aimed to avoid the introduction of other harmful substances, which is the most significant contribution of trivalent chromium electrodeposition. For this aim, we attempted to use direct current (DC) in the present research.
Section snippets
Experimental section
Two types of Cr(III) electrolytes were employed in this research. One is a chloride system electrolyte, and the other is a sulfate system electrolyte. The chloride electrolyte has the following main components: 0.4 mol/L CrCl3·6H2O, 0.6 mol/L HCOOH, 2.6 mol/L NH4Cl, 0.6 mol/L H3BO3, 1.0 mol/L KCl, and 0.2 mol/L CH3COOH. In this system, the current densities used for electrodeposition were between 5 A/dm2 and 30 A/dm2. The electrolyte temperature was controlled in the range of 15–55 °C, and the pH was
Results and discussion
There are many electrolyte systems used for Cr(III) electrodeposition [7], [10], [11], [12], [13], but for those systems based solely on chromium salts, Cr(III) electrolytes can be simply classified into two groups: chloride electrolytes and sulfate electrolytes. A large number of studies are focused on the electrodeposition of a chromium coating using chloride Cr(III) electrolytes. To produce a crystalline chromium coating by the direct electrodeposition of Cr(III) electrolytes, we first chose
Conclusions
A crystalline chromium coating with a good appearance can be electrodeposited using a wholly environmentally acceptable sulfate Cr(III) electrolyte. A significant preferred orientation of bcc Cr crystals occurs in the crystalline chromium coatings using the Cr(III) electrolytes. The color and luster of the electrodeposited chromium coating do not appear to be relevant to its main phase but are significantly influenced by the current density used. Generally, the larger the current density is,
Acknowledgments
The authors would like to thank the National Natural Science Foundation of China (grant No. 51305434) and the Natural Science Foundation of Gansu Province (grant No. 096RJZA127).
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