Cr-Electrodepostion influenced by cations and anions added in the Cr(III)-containing bath

• Main Authors: Nguyen Van Tai, 阮帆泰
• Other Authors: Jing-Chie, Lin
• Format: Others
• Language: en_US
• Published: 2016
• Online Access: http://ndltd.ncl.edu.tw/handle/j32d6c

碩士 === 國立中央大學 === 機械工程學系 === 104 === Bath effect on the electrochemical deposition of Cr-C was investigated in this work. Different cations (i.e., Na+, Mg2+ and Al3+) and anions (i.e., NO3-, Cl-, SO42- and PO43-) were added in the Cr (III)-containing bath resulting in ionic strength increase from 7.00 to 10.75 to explore their effect on the deposits. The surface morphology of the deposits was found to become smooth and dense in the baths where Na+, Mg2+ and Al3+ cations were added, especially in the baths containing 0.19 - 0.94 M Mg2+ in response to the ionic strength from 7.75 to 10.75. With respect to anion effect, we found that better appearance was the deposits come from baths containing SO42- rather than other anions like NO3-, Cl- and PO43-. Analysis with x-ray diffractometer (XRD), all the deposits belong to amorphous structure. Examination by X-ray photoelectron spectrometer (XPS), the deposits revealed a composition of metallic Cr, Cr-C, and Cr(III) oxides in different amounts depending the baths chosen. The current efficiency increased with increasing the concentration of Na+ and Mg2+ but decreased with the concentration of Al3+ in the bath. The results of direct current cathodic polarization and alternating current electrochemical impedance spectroscopy (EIS) provided comprehensive interpretation for this effect. In the bath with added Na+, Mg2+ and Al3+, the current density for hydrogen reduction was pertinently depressed; however, the reduction of Cr(III)-complex and its further reduction were enhanced. Due to this fact, the deposit come from the baths added with cations inhibited the formation of hydrogen bubbles and thus avoiding the formation of pores. Under a constant ionic strength of 10.75, the polarization resistance (Rp, in Ω) of Cr-reduction increased in the order: 0.044 (SNa_5) < 0.065 (SMg_5) < 0.123 (Sf) < 0.130 (SAl_5). As a result, Cr-reduction rate increased in the bath added with 1.25 M Na+ and 0.94 M Mg2+ but decreased with that added with 0.25 M Al3+. EIS data confirmed this inference, the charge transfer resistance (Rct in Ω) for Cr-reduction increased in the order: 0.759 (SNa_5) < 0.977 (SMg_5) < 1.107 (Sf) < 1.255 (SAl_5) and the diffusion coefficient (in m2s-1) of the reactive species decreased in the order: 9.443 x 10-12 (SNa_5) > 5.219 x 10-12 (SMg_5) > 4.766x10-12 (Sf) > 2.687x10-12 (SAl_5). A schematic diagram is proposed for understanding the bath effect on the Cr(III)-electroplating. Addition of 1.25 M sodium sulfate (SNa_5) or 0.94 M magnesium sulfate (SMg_5) to the Cr(III)-containing bath led to smooth and dense deposit which revealing the highest mechanical hardness (i.e., 950 Hv) and best corrosion resistance to 3.5 wt. % NaCl solution.