- 學位論文
FeCrNiCoMnx(x=1.0,0.6,0.3,0)高熵合金於0.5M硫酸中的抗蝕性質之研究
The Corrosion Behavior of FeCrNiCoMnx(x=1.0,0.6,0.3,0) High Entropy Alloys in 0.5M Sulfuric Acid Solution
摘要
高熵合金是一種2004年被提出的一種新型合金，近年相關研究也如雨後春筍般大量成長著。其中又以Cantor Alloy能形成穩定FCC單相且在低溫也優秀的機械特性，更是被廣為研究著，未來很有機會應用於工業界，因此本研究使用50公斤級大型鑄件去探討如果在嚴苛的工業環境下被當作結構用件應用著，是否能具有足夠的耐蝕性。而其中的Mn元素曾經有相關論文提出可能對抗蝕性有負面的影響，因此同時也使用低Mn材甚至是無Mn的Cantor Alloy作為對照，量化Mn元素對於Cantor Alloy抗蝕方面究竟有怎樣的影響。 經過SEM-EDX分析還有電化學實測後發現，鑄材的FeCrNiCoMnx(x=1.0, 0.6,0.3,0)高熵合金含有大量偏析，會使得電化學的再現性不佳，並且出現局部區域優先溶解的現象，所以為了減少偏析帶來的干擾，鑄材的FeCrNiCoMnx高熵合金會進行熱軋均質化的熱處理消除掉偏析。 經過熱軋均質化後的FeCrNiCoMnx高熵合金可以完全消除偏析並維持FCC單相結構，然而有許多在鑄造時留下的介在物並無法靠熱處理消除。這些介在物主要由熱力學最穩定的硫化物及氧化物依不同比例所構成可分成四大類，每種合金含有不盡相同種類的介在物，而其中Mn0.6(FeCrNiCoMn0.6)含有最高含量的介在物。介在物對電化學性質的影響透過極化曲線還有浸泡測試可以發現，介在物四周會容易因為電化學性質不同而有加凡尼腐蝕的可能，成為腐蝕的起始位置，導致所有FeCrNiCoMnx高熵合金的腐蝕速率的遠高於304L不鏽鋼。FeCrNiCoMnx高熵合金的鈍化性質則藉由EIS電化學測試和XPS分析鈍化層後得知Mn不但容易產生MnS型介在物，還會和Cr競爭氧化，導致FeCrNiCoMnx高熵合金中的Mn0.6、Mn0.3(FeCrNiCoMn0.3)即使底材有比304L不鏽鋼還多的Cr，鈍化層中最主要的鈍化物種Cr2O3含量不及於304L不鏽鋼，鈍化性質也是一樣的趨勢。 為了消除掉表面的介在物，本研究引入了不鏽鋼的酸洗配方，分別是氫氟酸和硝酸混合溶液的含氫氟酸配方和僅使用硝酸溶液的硝酸配方。雖然酸洗後無法直接達到陽極極化的鈍化效果，仍需要後續陽極極化處理，但仍然可以看到酸洗有一定程度上消除介在物的效果。含氫氟酸配方能夠消除掉表面大多數的介在物，而硝酸配方則只能消除掉硫化物的介在物，氧化物的清除效果有限，然而經由SEM影像可以發現氫氟酸雖可以清除掉大多數介在物，但同樣也會攻擊蝕刻晶界並讓介在物原本位置附近出現大量孔洞，可能會對後續鈍化有負面的影響。陽極極化後電化學EIS測試也發現使用含氫氟酸配方酸洗後試樣僅能小幅提升鈍化性質，對於部分合金甚至有負面的影響；使用硝酸配方酸洗後的試樣大多都可以大幅提升鈍化性質，僅Mn0.6可能因為酸洗後仍留有些許介在物存在而效果沒有那麼明顯，但酸洗對後續鈍化上依舊是帶來正面的效應。XPS的鈍化層分析中則發現，經含氫氟酸配方酸洗，表面會有大量F離子殘留，會導致鈍化層中富集的Cr2O3轉換成鈍化性質稍差一些的Cr(OH)3，且會和O競爭Cr，形成CrF後導致鈍化膜的融解，而在硝酸配方酸洗後卻沒有發現這樣的現象，依然是Cr2O3富集，甚至含有更多，因此，酸洗後鈍化性質上，含氫氟酸配方的酸洗效益遠遠不及使用硝酸配方進行酸洗所帶來的效益大。
並列摘要
High entropy alloy (HEA) is a novel material that was proposed by both Yeh and Cantor in 2004. Owing to its superb mechanical properties, such as high yield strength and high hardness, more and more relevant researches were carried out, and FeCrNiCoMn Cantor alloy and its family are one of them. From previous reports, Mn might have a negative effect on corrosion resistance, and for the possible industrial application in the future, so the corrosion properties of a 50-kg bulk ingot of industrial Cantor Alloy and its derivative alloy (FeCrNiCoMnx(x=1.0,0.6,0.3,0)) were studied to giving an in-depth insight into the influence of Mn. In the as-cast FeCrNiCoMnx alloy, the elements segregation, which would result in the poor reproducibility in electrochemistry test and the selective dissolution in a corrosive environment, between the dendritic and the inter-dendritic regions were found via the SEM-EDX analysis. So, the heat-treatment, such as hot-rolling and homogenization (H-HR), was needed to minimize the negative impact of element segregation. The element segregation was completely eliminated and the crystal phase was also in single FCC after hot-rolled and subsequently homogenized. However, some inclusions could not be removed by heat treatment still remaining in the alloy. These inclusions composed of sulfide and oxide could be divided into four main types by the composition difference. All of them may be the initiation of corrosion in the corrosive solution due to the galvanic corrosion between themselves and their surrounding areas, so it caused that the corrosion rate of FeCrNiCoMnx alloy was much more serious than that of 304L SS after the immersion tests. Moreover, the passive properties of FeCrNiCoMnx alloy were researched via the EIS tests and XPS analyses. It showed that Mn not only produced MnS type inclusion that may decrease the corrosion resistance but also compete with Cr for oxidation. Therefore, even both of the Mn0.6、Mn0.3 alloys had more Cr in the base metal, the content of Cr2O3 in the passive film of them was lower than that of 304L SS, which leading to a worse passive property for them. To remove the inclusions, two different pickling processes for stainless steel, which is hydrofluoric acid (HF) pickling and nitric aid (HNO3) pickling, were utilized on FeCrNiCoMnx alloy in this study, and both of them could eliminate all the sulfide type inclusions. On the other hand, for the oxide type inclusions, the HF pickling process could remove most of them clearly, while the HNO3 pickling process could only remove some of them. It seems that the HF might be a better choice to improve the corrosion resistance property due to a better surface inclusions cleaning ability. However, via the SEM pictures, lots of big black holes could be easily observed on the surface where had been pickled by HF, and the grain boundaries were also etched. These phenomena may make a negative impact on the passive property. EIS tests for passive films that formed after anodic polarization also showed that the samples pickled by HF would increase their passive resistance slightly, or even decrease it, while the samples pickled by HNO3 would improve their passive properties obviously, even more than one order of magnitude. Moreover, XPS results showed that some fluoride ions were remaining on the surface of the samples after pickled by HF. Literature suggested that the remaining fluoride ions would make the main composition of the passive transforming from Cr2O3 to Cr(OH)3, which had a slightly worse passive property than Cr2O3, and compete with O for Cr then form a soluble CrF, causing the break down of the passive film. In contrast, the sample pickled by HNO3 would still maintain a better passive film mainly composed of Cr2O3 rather than Cr(OH)3. Therefore, the HNO3 pickling process might be a better choice to remove the inclusions remaining on the surface of the FeCrNiCoMnx alloy than the HF pickling one.