雙相高熵合金鈰酸鹽化成皮膜之抗蝕研究

Abstract

Fe50Mn30Co10Cr10為一種雙相高熵合金，根據文獻說明雙相高熵合金的機械性質優於一般高熵合金，當鐵原子百分比從20%提升50%、錳原子百分比從20%提升30%時，並經過熱處理後，Fe50Mn30Co10Cr10的極限拉伸強度850MPa、斷裂率70%，其優異的機械性質應用在工程上相當廣泛，但也由於錳含量的關係，其氧化物不穩定，使其抗蝕能力無法與高熵合金媲美。為了改善Fe50Mn30Co10Cr10抗蝕性，本研究使用化成技術，此項製程技術不但快速，又能保有原始基材之特性，除此之外，硝酸亞鈰與雙氧水是本研究所使用的化成溶液配方，不僅改善鉻酸化成溶液的環保問題，在式樣的美觀上也兼具特色，綜合上述，本研究首次在Fe50Mn30Co10Cr10進行鈰酸化成表面處理，藉此達成抑制腐蝕之目的，在視覺上也擁有商品化的雛形。 本研究第一部份為監測化成溶液之參數，熵合金目前鮮少人進行化成研究，測試過繁多類型之溶液後，皆較難驅動基材反應，最終選定鈰酸鹽作為化成溶液主體，並初步以鈰酸溶液進行OCP監控以確認皮膜生長情況；第二部分，透過動電位極化曲線和交流阻抗頻譜進行電化學分析，結果顯示經過鈰酸化成的雙相高熵合金在含氯離子之腐蝕測試液中其腐蝕電位較未化成之試片明顯上升，腐蝕電流降低兩個次方量，表示其在此測試中能延緩腐蝕發生；在交流阻抗結果也顯示其皮膜電阻優於基材，此外，本實驗也進行長期監測皮膜浸泡在氯離子環境下的狀況；第三部分，為了深入了解表面形貌與化成機制，利用掃描式電子顯微鏡和X射線光電子能譜儀器分析皮膜結構與成份，再搭配穿透式電子顯微鏡觀察剖面形貌，量測出膜厚約17.91 ± 2.4 nm，同時以原子力顯微鏡之三維表面形貌來確定成核成長之表面。由於化成機制較為繁瑣，最後結合高解析X射線光電子能譜探討金黃色皮膜之化成機制，說明基材與化成溶液之關係以及溫度對pH值的變化。

It was found that Fe50Mn30Co10Cr10 dual-phase high-entropy alloy (DP-HEAs) had superior mechanical properties higher than those of CoCrFeMnNi high entropy alloys (HEAs). However, poor corrosion due to the higher manganese content is one of the main challenges restricting the further application of DP-HEAs. In order to improve the corrosion resistance of DP-HEAs, a new cerium chemical conversion coating based on a mixed solution of cerium(Ⅲ) nitrate hexahydrate (Ce(NO3)3‧6 H2O) and hydrogen peroxide (H2O2) has been first proposed in this study. The electrochemical behavior of bare and coated samples was evaluated by using OCP test, potentiodynamic polarization, and electrochemical impedance spectroscopy (EIS). The results showed that, from the potentiodynamic curves, the corrosion potential increases and the corrosion current density decreases by two order of magnitude for Ce-coated samples. The chemical state of the elements in the coatings was investigated by scanning electron microscope (SEM) with energy dispersive spectrometer (EDS) and X-ray photoelectron spectroscopy (XPS). The transmission electron microscope (TEM) result revealed that the thickness of cerium conversion film formed on the surface of DP-HEAs are around 17.91 ± 2.4 nm. Also, the mechanisms of golden yellow-colored cerium conversion coatings formation are discussed.