C1100銅合金與6061鋁合金之摩擦攪拌焊接異質接合件腐蝕行為及耦合鋯化成機制研究

Abstract

本研究主要分為兩個部分，第一部分為探討6061鋁合金與C1100銅合金摩擦攪拌焊接異質接合件於3.5 wt. % NaCl 溶液之異質微結構、腐蝕機制；第二部份則著重6061鋁合金與C1100銅合金，單獨及耦合化成處理之微結構、腐蝕行為及化成機制。 由於良好的熱傳導性以及導電性，銅一直以來在車輛、電力工業扮演重要的角色。而鋁合金與銅相似地具備良好的導電性以及熱傳導性，為了達到成本降低的目的，具備良好的導電性以及熱傳導性的鋁合金廣泛地作為與銅進行異質材料接合的材料。近年來許多研究採用摩擦攪拌焊接(FSW)的製程，在低入熱量的條件下，盡量減少界金屬化合物(IMCs)的生成，來完成良好的鋁銅異質接頭，但過去探討鋁銅摩擦攪拌異質焊接的腐蝕行為的文獻非常有限。本研究以掃描式電子顯微鏡、穿透式電子顯微鏡、電化學量測等方式，研究了探討6061鋁合金與C1100銅合金摩擦攪拌焊接異質接合件異質微結構以及於3.5 wt. % NaCl 溶液之腐蝕機制。研究結果顯示，焊道區是高度異質的，包含了嵌入鋁基材中的銅顆粒、CuAl2及Cu9Al4等界金屬化合物。經浸泡實驗後，焊道區中的Al/CuAl2/Cu9Al4/Cu等相會引發微觀伽凡尼效應，而鋁銅之間的巨觀伽凡尼效應會加速焊道區的腐蝕，富鋁相（Al，CuAl2）相對於富銅相（Cu，Cu9Al4），將優先被腐蝕。 鋯化成處理是透過還原反應引起的局部pH升高而引發的，Al和Cu的標準電極電位分別為-1.676和0.340 VSHE，這顯示Al/H+的氧化還原是自發性的，而Cu/H+的氧化還原則不會發生，Cu/Al 耦合將會影響 ZrO2 化成膜層的均勻性。然而，銅和鋁在耦合條件下對化成膜的形成機制研究較少。因此，本研究第二部份則著重6061鋁合金與C1100銅合金，在pH 4 的15 g/L H2ZrF6 化成溶液中，於室溫下浸泡 5 分鐘進行單獨及耦合化成處理之微結構、腐蝕行為及化成機制。研究結果顯示，C1100銅合金無法於單獨化成的條件下進行化成處理，但在耦合化成的條件下，可於其表面沉積ZrO2化成膜，此膜層十分均勻，厚度僅約為20 nm左右，且具有陽極抑制效果。而6061鋁合金無論是單獨抑或是耦合化成，均可於其表面沉積ZrO2化成膜，且在兩種化成條件下，均有良好的陽極抑制效果，耦合化成的抑制效果要比單獨化成要來的更高，但在陰極部分則無顯著差異。耦合化成過程中，來自6061鋁合金的電子藉由導線傳導至C1100銅合金於其表面發生水的還原及析氫反應，使得C1100銅合金表面pH上升，並進一步促使ZrO2化成膜沉積於銅表面，而6061鋁合金則藉由鋁於酸性溶液中的溶解，驅動ZrO2化成膜的沉積。

This study is divided into two main parts. The first part investigates the heterogeneous microstructures and corrosion mechanisms of 6061 aluminum alloy/C1100 copper alloy friction stir welded joints in 3.5 wt. % NaCl solution. The second part focus on the microstructures, corrosion behavior, and formation mechanism of Zr-based conversion coating on 6061 Al and C1100 Cu galvanic couple. In light of its thermal and electrical conductivities, copper has been extensively used in the automobile and power industries, such as thermal dissipation substrate and power transmission wire. Aluminum, which has heat and electrical conductivities comparable to Cu. To reduce the use of Cu and Al, Cu composite structures have gained ever-increasing attention, especially for cost reduction while maintaining sufficient thermal and electrical conductivities. Recently, the processes of Al-Cu dissimilar FSW have been heavily studied. However, the corrosion studies of Al-Cu dissimilar FSW are scarce. In this study, the effect of heterogeneous microstructures of friction stir welding 6061 Al/C1100 Cu joints on the corrosion in 3.5 wt. % NaCl was studied, including macro- and micro-galvanic corrosions. The stir zone was highly heterogeneous, including Cu particles embedded in Al, CuAl2, and Cu9Al4 intermetallic phases. The Al/Cu macro-galvanic coupling accelerated the corrosion of the stir zone. The Al/CuAl2/Cu9Al4/Cu micro-galvanic coupling resulted in a uniform coverage of Al oxide/hydroxide on Al; conversely, the surface of CuAl2 was porous and rich in Cu due to dealloying of Al. The corrosion was not uniform, concentrating in Al-rich phases (Al, CuAl2). Hexafluorozirconate conversion reaction is triggered via pH rise resulting from the reduction reactions. The fact that the standard electrode potential of Al and Cu is -1.676 and 0.340 VSHE, respectively, indicates the redox of Al/H+ is spontaneous, but that of Cu/H+ is impossible. As a result, the Cu/Al galvanic coupling impacts the uniformity of ZrO2 conversion coating. However, the effect of Cu and Al galvanic coupling on the conversion coating formation is less studied. This study investigated the microstructure and corrosion resistance of ZrO2 conversion coatings on 6061 Al alloy and C1000 Cu alloy, which were immersed separately or coupled. Therefore, the second part of this study focuses on 6061 aluminum alloy and C1100 copper alloy, immersed in a 15 g/L H2ZrF6 solution at pH 4 at room temperature for 5 minutes separately or coupled. The formation mechanism of ZrO2 conversion coatings on Al/Cu couples was discussed and their corrosion protection capability was confirmed. Under the conditions of individual conversion treatment, C1100 copper alloy could not undergo conversion processing. However, under coupled conversion conditions, the ZrO2 conversion film was deposited on its surface. This film was very uniform with a thickness of approximately 10 nm and inhibited the anodic kinetics. For 6061 aluminum alloy, whether subjected to individual or coupled conversion treatments, a ZrO2 conversion film could be deposited on its surface. Under both treatment conditions, there was a good anodic inhibition effect. The inhibition effect was better under coupled conversion conditions compared to individual conversion, but there was no significant difference in the cathodic region. During coupled conversion, electrons from 6061 aluminum alloy were conducted through a wire to the surface of C1100 copper alloy, where water reduction and hydrogen evolution reactions occurred. This process caused an increase in pH on the surface of C1100 copper alloy, further promoting the deposition of ZrO2 conversion film. Meanwhile, 6061 aluminum alloy facilitated the deposition of ZrO2 conversion film by dissolving aluminum in the acidic solution.