6061鋁合金之鋯酸化成處理與腐蝕行為研究

The Corrosion Behavior of Hexafluorozirconic Acid Conversion Coating on 6061 Aluminum Alloys

王家宣(Chia-Hsuan Wang)

指導教授 ： 林招松

國立臺灣大學/工學院/材料科學與工程學研究所/碩士(2020年)

https://doi.org/10.6342/NTU202002564

摘要

鋁合金具有低密度、高比強度的優點，因此為現在主要的輕量化材料，雖然鋁合金的原生氧化層即具有抗蝕能力，但是在實際應用上，仍需要化成處理提升抗蝕性及塗裝性，由於傳統六價鉻的致癌性，目前許多研究以鋯化合物化成(以下稱鋯化成)取代。
本研究在探討6061鋁合金在鹼洗酸洗前處理及鋯化成處理後的膜層性質， 其中鋯化成主要分為三個部分做討論：一、測試Chemetall公司的Gardobond X4707商用藥水，並參考X4707藥水的ICP-OES元素檢測結果，以六氟鋯酸配置出模擬X4707的化成液組成，簡稱為FZA化成系統；為了更進一步提升鋯成膜速率和抗蝕特性，在後續討論以下主題，二、添加陽離子對FZA化成特性的影響，以及三、添加自由氟離子對FZA化成特性的影響。
6061鋁合金在經過鹼洗酸洗後，TEM橫截面影像顯示表面會披覆僅7至8 nm的氧化層，且均勻覆蓋在鋁基材和晶界上，該層主要由氧化鋁和氫氧化鋁組成，經過鹼洗脫脂和酸洗活化表面的試片才能接續進行鋯化成處理，EIS結果顯示自配FZA化成系統可以與X4707匹配，在鹽霧試驗結果中，鹼洗酸洗試片在七天測試即有嚴重脫色現象，FZA化成膜在鹽霧環境十四天後仍保有金屬光澤，由於10至15 nm厚的FZA膜層主要由氧化鋁、氫氧化鋁、氟化鋯、二氧化鋯組成，可見鋯物種併入確實有助於提升抗蝕性。
後續選擇以添加銅離子和鋅離子於FZA化成系統，期望藉此提升成膜速率以符合工業需求，然而銅離子會直接還原成銅顆粒遍布於鋁基材，伽凡尼效應與銅上較高的析氫速率都會加速鋁在腐蝕測試液中的溶解，鋅離子則會以鋅和氫氧化鋅的混合沉積於惰性一次析出物α-AlFeSi上，由於鋅的溶解度高，因此無法有效抑制鋁鐵矽相中鋁的溶解。歸納上述結果，可以知道添加陽離子並無助於提升FZA皮膜的抗蝕性，因此最後改為以添加自由氟離子做測試。
在添加氟離子於FZA化成系統的實驗中，50 ppm氟的添加後能使皮膜阻抗較FZA皮膜提升，且皮膜大概能增厚至15至20 nm，氟化鋯的沉積比例也會較FZA高，藉由氟的添加，能更有效率移除鋁的氧化層，進而更加驅動鋯成膜反應 使膜層更厚且均勻，然而實驗中也顯示氟的過量添加反而會抑制鋯沉積反應進行，使抗蝕性下降。

關鍵字

6061鋁合金 ； 鋯化合物化成處理 ； 微結構 ； 電化學交流阻抗 ； 鹽霧試驗

並列摘要

In light of low density and high strength/weight ratio, aluminum alloys are the main lightweight materials. Although the native oxide layer on aluminum alloys is corrosion resistant, a conversion coating layer is still essential to enhance the corrosion resistance and adhesion of subsequent organic paintings in practical applications. However, as traditional hexavalent chromium conversion coatings (CCCs) are toxic and carcinogenic, many studies have found zirconium conversion coating as alternatives to CCCs.
This research aims at understanding the properties of 6061 aluminum alloys (AA6061) after pretreatment and zirconium conversion coatings. Firstly, the properties of Gardobond X4707 commercial coating solution was investigated. Additionally, a hexafluorozirconic acid coating system, which was denoted as “FZA” , was designed based on the ICP-OES analysis data of X4707. Secondly, the effect of cation (e.g. Cu2+ and Zn2+) in the FZA was explored. Thirdly, the effect of fluoride ion in the FZA was investigated. The latter two parts focused on enhancing the coating formation rate and corrosion resistance.
The TEM cross-sectional images show that a 7-8 nm Al2O3 and Al(OH)3 oxide layer is uniformly covered on the AA6061 substrate and grain boundary after pretreatment. The zirconium conversion coating treatment can be conducted after alkaline degreasing and acid deoxidation pretreatments. The EIS results reveal that the corrosion resistance of the FZA coating is comparable to that of X4707. In the salt spray test, black corrosion products were seen on most of the pretreatment specimen after 7-day exposure. However, the FZA-coated specimen can remain undamaged after 14-day exposure. From the results of TEM EDX mapping and XPS analysis, the FZA coating is about 10-15 thick, with the composition of Al2O3, Al(OH)3, ZrO2 and ZrF4. Thus it can be inferred that zirconium species deposited in the aluminum oxide can indeed improve the corrosion resistance of AA6061.
In the next part, Cu2+ or Zn2+ was added in the FZA for enhancing the coating formation rate. However,the results show that Cu particles spread all over the AA6061 surface, resulting in the serious galvanic corrosion effect. Moreover, there is higher hydrogen exchange current density on Cu particles. Both result in accelerating the dissolution of AA6061 matrix. The Zn2+ addition, leads to the deposition of Zn and Zn(OH)2 mainly on noble constituent particles, α-AlFeSi. Due to the high solubility of zinc, it cannot effectively inhibit corrosion attacking on the AA6061 matrix from α-AlFeSi. As a result, the addition of Cu2+ or Zn2+ cannot improve the corrosion resistance of FZA coatings.
The last part of this research is about the effect of the presence of fluoride ion in FZA. The EIS results show that 50 ppm fluoride ion addition can effectively increase the polarization resistance of FZA coating, which was denoted as “50 ppm F”. Moreover, the 50-ppm-F coating can be thickened to 15-20 nm. From the XPS results, the deposition proportion of ZrF4 is increased compared to the FZA coating. By adding fluoride ion, the aluminum oxide layer can be removed more efficiently. Therefore, the zirconium species deposition is more comprehensive, resulting in a more uniform and thicker coating film. However, the excessive addition of fluoride ion (> 100 ppm) inhibits the zirconium deposition reaction, which in turn decreases the corrosion resistance of FZA coating.

並列關鍵字

6061 aluminum alloy (AA6061) ； zirconium conversion coating ； microstructure ； electrochemical impedance spectroscopy ； salt spray test

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