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

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题名:	Cr-Electrodepostion influenced by cations and anions added in the Cr(III)-containing bath
作者:	阮帆泰;Tai,Nguyen Van
贡献者:	機械工程學系
关键词:	陰離子;陽離子;電沉積;三價鉻;腐蝕特性;電化學交流阻抗;Cations;Anions;Electrodepsition;Trivalent chromium;Corrosion behavior;EIS
日期:	2016-07-22
上传时间:	2016-10-13 14:52:28 (UTC+8)
出版者:	國立中央大學
摘要:	本研究中，探討鍍浴對電化學沉積碳化鉻鍍層之影響。在三價鉻的鍍浴中，添加不同的陽離子（Na+, Mg2+ and Al3+）以及陰離子(NO3-, Cl-, SO42- and PO43-) 提升離子強度，由7.00增加至10.75，進而探討添加之陰、陽離子對鍍層的影響。在鈉離子、鎂離子及鋁離子添加在鍍浴後，電鍍後的鍍層之表面形貌變為更平滑緻密，特別是添加0.19 M~0.94 M之Mg2+離子，使鍍浴之離子強度由7.75提升至10.75。而陰離子添加在鍍浴中，我們發現含有硫酸根離子（SO42-）之鍍液電鍍出之鍍層，相較於添加其他陰離子（NO3-，Cl-及PO43-）有更佳的形貌。X光結晶繞射分析，所有的鍍層屬於非晶結構，藉由X光光電子能譜儀分析，不同鍍浴所電鍍之鍍層中，分別含有三種不同的化學組成鍍層，如金屬態之鉻、碳化鉻及三價的氧化鉻。鍍浴中鈉離子及鎂離子濃度的增加將提升電流效率，但添加鋁離子則會導致下降。直流的陰極極化曲線以及電化學交流阻抗分析之結果，可提供全面性的詮釋電鍍時陰、陽離子添加所產生之現象。添加Na+、Mg2+及Al3+的鍍液，將有效抑制氫還原的電流密度，然而亦更進一步增加三價鉻錯合物的生成。基於上述之分析結果，添加陽離子至鍍浴中將避免氫氣氣泡在鍍層表面的形成孔洞。在固定的離子強度為10.75下，鉻還原的極化阻抗依序增加：0.044 (SNa_5) < 0.065 (SMg_5) < 0.123 (Sf) < 0.130 (SAl_5)。因此，在鍍浴中添加1.25 M Na+ 及 0.94 M Mg2+將促使鉻還原的速率增加，但添加0.25 M Al3+將使其下降。電化學交流阻抗結果證實此推測正確，鉻還原之電荷轉移阻抗(Ω)將下列順序增加：0.759 (SNa_5) < 0.977 (SMg_5) < 1.107 (Sf) < 1.255 (SAl_5)，而反應物種之擴散係數(m2s-1)將隨下列順序減少：9.443 x 10-12 (SNa_5) > 5.219 x 10-12 (SMg_5) > 4.766x10-12 (Sf) > 2.687x10-12 (SAl_5)。在本論文亦提出三價鉻電鍍之示意圖以利瞭解鍍浴對電鍍之影響。添加1.25 M之硫酸鈉或0.94 M之硫酸鎂將使得三價鉻之鍍層有更緻密及平滑之效果，亦得到鍍層最高之硬度(~ 950 Hv)，以及在3.5 wt. % 氯化鈉溶液中有最佳的腐蝕抑制效果。關鍵字：陰離子，陽離子，電沉積，三價鉻，腐蝕特性，電化學交流阻抗 ;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.
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