電鍍鋅鎳/鉻複合鍍層於直接式熱沖壓鋼板高溫保護性質研究

電鍍鎘保護層在嚴苛海洋腐蝕環境底下具有良好的犧牲保護效果，加上其光亮的金屬光澤外觀，因此廣泛應用於船舶、航太工業的抗蝕鍍層。許多研究發現鎘會對於環境造成危害且有一定的致癌性，近年來則多以鋅或鋅合金鍍層取代之。鉻為不銹鋼(鉻含量大於10.5wt%)中主要添加的合金元素，在一般大氣環境下，鉻可以在表面形成緻密的Cr2O3氧化層，阻礙鐵離子經由氧化層擴散到表面與氧氣接觸，進而降低氧化的速率。在本研究當中，使用鋅鎳合金與鉻的雙電鍍層來解決在高強度鋼熱沖壓製程中傳統熱浸鍍鋅層與電鍍鋅層會遇到的液態金屬誘發脆化(LMIE)與高溫氧化的現象。液態金屬誘發脆化現象(LIME)為鋅鍍層在高溫下熔融成液態，並沿著鐵的晶界擴散進入，導致在沖壓時成為許多應力集中的小區域而造成沿晶破壞；高溫氧化則因鋅為化學活性高的金屬，高溫下會與氧反應形成大量的氧化鋅而使得有效的鍍層量減少並導致組成分布不均。利用掃描式電子顯微鏡(SEM)與穿透式電子顯微鏡(TEM)觀察鋅鎳與鋅鎳/鉻鍍層微結構，並藉由電子微探儀(EPMA)來分析鍍層與鋼底材界面間擴散的情況。高溫拉伸試驗模擬熱沖壓製程中試片在高溫下實際受到張應力的情形，由應力應變圖可以看出不同鍍層對於鋼底材的機械性質有不同的影響。高溫於表面產生的氧化物則利用X射線繞(XRD)射與X射線光電子能譜儀(XPS)進行分析，分析的結果可以更了解鍍層在高溫下氧化的情形。綜合上述結果及透過開路電位測試檢視鍍層是否具犧牲保護的效果，以及評估雙電鍍層(Zn-Ni/Cr)是否適用於實際熱沖壓製程。實驗結果發現，不同的電鍍溫度與溶液中鎳離子的比例會影響鍍層形貌、組成成分與陰極電流效率，且正常共電鍍的情形多發生於高鎳離子比與高溫(60℃以上)的情形下。EPMA的結果顯示鋅鎳鍍層在經過熱處理後會造成組成成分改變，導致此現象的原因與鍍層氧化以及鍍層與底材間的交互擴散有關。將鉻層電鍍於鋅鎳鍍層上方再經熱處理後發現鋅鎳鍍層的組成成分改變現象有明顯的降低。此外，鋅鎳合金層因其高熔點的性質，與純鋅層相比在高溫下與鐵基材的交互擴散較弱，具良好的高溫穩定性。鍍層中鎳的含量上升可提高鍍層的熱穩定性，電化學開路電位量測結果則顯示經熱處理後的鋅鎳鍍層中隨著鎳含量增加其陰極保護的效果隨之下降。在高溫拉伸試驗中，發現在較高的溫度與較慢的拉伸速率下，鋅鎳合金鍍層對鋼板的機械性質劣化較純鋅鍍層來得輕微。鋅在鉻中有相當大的固溶度，在高溫下會擴散至表面形成ZnO，而ZnO與Cr2O3在高溫下形成ZnCr2O4的尖晶石結構，而ZnCr2O4的結構也經由XRD與XPS鑑別出來，微結構也經由TEM進行細部觀察，結果顯示此結構能防止鋅鎳鍍層在高溫下劇烈氧化。從上述結果看來，鋅合金鍍層相較於電鍍純鋅層表現出較高的熱穩定性與較低的腐蝕速率，而鋅鎳/鉻複合鍍層則提供鋅合金鍍層所缺乏的抗氧化能力，可以減少鍍層因高溫下大量氧化所造成的損失。藉由調整適當的鋅鎳鍍層中的鎳含量來避免液態金屬誘發脆化並使鍍層提供足夠的陰極保護能力，鋅鎳/鉻複合鍍層可能會是適合熱沖壓製程的一個保護層。

關鍵字

熱沖壓鋼板、液態金屬誘發脆化；抗高溫氧化；陰極保護；電鍍複合層

並列摘要

Electrodeposited cadmium layer had provided excellent cathodic protection under serious marine conditions and expressed bright metallic color. Nowadays, the electrodeposited zinc and zinc-based coatings have been extensively applied in general auto industries as replacements for cadmium, which was found carcinogenic in many previous literatures. Chromium is the main alloy element to be added in stainless steel to form a compact chromium oxide above the surface, which retards Fe2+ ions diffusing through the oxide, thus slows down the oxidation rate. In this work, the electroplated bilayer (Zn-Ni/Cr) was used to prevent the Liquid Metal Induced Embrittlement (LMIE) phenomenon and serious oxidation that typically occur in hot stamping process degrading the mechanical properties and consuming the available coatings. The microstructure of Zn-Ni and Zn-Ni/Cr coatings was observed by SEM and TEM, and the thermal stability was examined by EPMA and Line scan. The mechanical properties of steels with and without zinc-based coating were illustrated by the strain-stress curve of high temperature tensile test and the broken specimens are examined by Metallographic analysis. Moreover, the surface oxides are characterized by X-ray Diffraction (XRD) and X-ray photoelectron spectroscopy (XPS), which help to further understand the oxidation situation in high temperature. By combining the results mentioned above with open circuit test and dynamic polarization curves, it is able to evaluate the performance of Zn-Ni/Cr bilayer coating in simulated hot stamping process. The results show that Zn-Ni coating’s morphology and current efficiency both strongly depend on the ion ratio in the bath and bath temperature. The normal co-deposition of Zn-Ni always occurs at high nickel ion ratio in the electrolyte and high deposition temperature. EPMA mapping results revealed that there’s a serious separation of compositions in the Zn-Ni coating after heat treatment due to the effect of oxidation and inter-diffusion with the substrate. With the addition of chromium layer on top the situation of separation of compositions in the Zn-Ni coating was apparently reduced. The thermal stability is becoming higher when nickel content increases in the Zn-Ni coating; however, the overall reduction potential will also move close to the substrate comparatively. In addition, spinel structure (ZnCr2O4) formed by ZnO and Cr2O3 at high temperatures was identified by XRD and XPS measurements, which indicated zinc diffused through the chromium layer to form zinc oxide at the surface. The phenomenon can be explained by the great solubility of zinc in chromium and high oxygen affinity of zinc. ZnCr2O4 layer was also observed in TEM for detailed information. In conclusion, the Zn-based coatings exhibit higher thermal stability and lower corrosion rate the electro-galvanized coating. On the other hand, Zn-Ni/Cr bilayer provides better oxidation resistance than Zn-Ni coating that reduces the loss of coating through serious oxidation at high temperatures. With the adjustment of suitable nickel contents in the Zn-Ni coating and achieve equilibrium between the prevention of LMIE and enough ability of cathodic protection, Zn-Ni/Cr bilayer might be a solution to meet the requirements of hot stamping process.

並列關鍵字

Hot stamping steel ； Liquid metal induced embrittlement ； High temperature oxidation resistance ； Cathodic protection ； Electrodeposited multi-layer

參考文獻

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國際替代計量

電鍍鋅鎳/鉻複合鍍層於直接式熱沖壓鋼板高溫保護性質研究

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