三價鉻電鍍:鍍層裂痕控制與材料特性分析

Electroplating of trivalent hard chromium deposits: crack control and material characterization

林彤雨(Lin, Randy)

指導教授 ： 胡啟章

國立清華大學/工學院/化學工程學系/碩士(2012年)

摘要

因環境及人體健康的考量，歐盟規定於2006年7月開始減量使用六價鉻電鍍之電子組裝相關產品。因此，三價鉻製程的研究在近期引起了高度的重視。第三章主要著重於三價鉻製程之電鍍機制與鍍液中硼酸成分對於電鍍之影響。在35 °C, 40 A / dm2下進行電鍍分別改變硼酸的濃度以觀察其對鍍層的影響。藉由線性掃瞄伏安法可得知電極表面之電化學行為。在鍍層分析方面分別使用掃描式電子顯微鏡(SEM)、電子微探測儀(EPMA)、維式硬度機(Vicker’s microhardness indenter)及 X光晶格繞射(XRD)測量鉻鍍層表面形態、組成、硬度及結晶性。其結果顯示鍍層裂痕受到硼酸濃度的影響，增加鍍液中硼酸的濃度可抑制氫氣的產生使的鍍層裂痕減少，以改善鍍層的抗腐蝕性。除此，也可發現改變鍍液中硼酸含量不會影響鍍層之硬度及組成。
第四章為使用部分因素實驗設計法與最陡上升實驗於三價鉻電鍍之研究。並以直流電鍍製備鉻鍍層，將其裂痕量化加以控制。考慮到鍍層裂痕的多寡會影響其抗腐蝕性質，所以選用鍍層裂痕作為其應答。經由比較各個電鍍因素(硼酸濃度、鋁鹽濃度、鉻鹽濃度、pH值和電鍍溫度)於鍍層裂痕之影響，其結果顯示電鍍溫度為主要決定鍍層裂痕的因素。此外，無裂痕的鍍層可在45°C下電鍍而得，最後由表面與截面影像再加以確定鉻鍍層之裂痕。
在第五章中將以最陡上升實驗所得到的鉻鍍層使用電化學方法與電化學阻抗頻譜分析其抗腐蝕性質，並由電化學阻抗頻譜、X光電子能譜儀、掃描式電子顯微鏡及能量散佈光譜分析其鉻鍍層的鈍化行為。其實驗結果顯示在40°C下所製備出的鍍層具有最低的腐蝕速率，且其阻抗會隨著陽極極化而增加。此現象亦可在掃描式電子顯微鏡及能量散佈光譜之結果得到印證。

關鍵字

硬鉻 ； 電鍍

並列摘要

On July 1st 2006, European Union (EU) mandated a switch to reduce the chromium production by hexavalent chromium in many assembly electronic processes because of environmental and health considerations. Accordingly, the studies of trivalent chromium process have attracted much attention recently. In chapter 3, this study is to investigate the electrodeposition process of trivalent chromium, including its deposition mechanism and the effects of boric acid. The chromium depositions are developed in the baths of different concentrations with B(OH)3 at a current density of 40 A / dm2 at 35 °C. The electroplating behavior of the deposits are investigated by the linear sweeps voltammetric (LSV) analyses. The chromium layers are characterized by scanning electron microscopic (SEM) and electron probe X-ray micro analyzer (EPMA), Vicker’s microhardness indenter and X-ray diffraction (XRD). In the experimental results indicate that cracks are significantly affected by boric acid concentration. Furthermore, the addition of boric acid can reduce the evolution of hydrogen and improve the surface morphology. And the reduction of cracks also could enhance the ability of anti-corrosion. Moreover, it could find that the addition of boric acid wouldn’t affect the hardness and composition of the deposit.
Trivalent chromium electrodeposition have been studied by the applications of the experimental strategy of fractional factorial design (FFD) and path of the steepest ascent (PSA) in the chapter 4. The crack length on chromium deposits which are electroplated under a direct-current (DC) mode is quantified and could be precisely controlled and predicted. The crack length of chromium deposits is used as the response variable since the number of the crack was proportional to the anti-corrosion ability. Compared with the effects of process parameters (the concentration of H3BO3, the concentration of AlCl3 ˙6H2O, the concentration of CrCl3˙6H2O, pH and the plating temperature) on the crack length of chromium deposits, the results reveal that the crack length is determined by the plating temperature. Moreover, chromium deposits with no cracks could be prepared at 45°C. Besides, the top-view and cross-section images have the same results with the crack length.
In chapter 5, the anti-corrosion abilities of chromium deposits obtained at the PSA are investigated by the electrochemical methods and EIS spectra. Furthermore, the passive film of chromium deposits is also measured by XPS, SEM and EDS analyses. According to the testing results, the chromium deposit obtained at 40°C have a better corrosion resistance. And the corrosion resistance increases with the positive shift of the polarization potential, due to the formation of chromium hydroxide. This phenomenon can be confirmed in SEM and EDS analyses.

並列關鍵字

hard chromium ； electroplating

參考文獻

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