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標題: | 鋁合金7075-T6之三價鉻化成處理與抗耐蝕性研究 The Corrosion Resistance of Trivalent Conversion Coating on Aluminum Alloys 7075-T6 |
作者: | Siao-Ying Chen 陳筱螢 |
指導教授: | 林招松(Chao-Sung Lin) |
關鍵字: | 鋁合金7075-T6,酸鹼洗前處理,三價鉻皮膜化成處理,抗蝕性,晶出相, AA7075-T6,alkaline and pickling pretreatments,trivalent chromium conversion coating,corrosion resistance,constituent particle, |
出版年 : | 2018 |
學位: | 碩士 |
摘要: | 鋁合金7075-T6為析出強化型合金,因其具有良好的強度-重量比,廣泛運用在航太載具上。在加工過程中,7075-T6會產生三種異質相:晶出物、分散相和析出強化相,異質相與基材會有迦凡尼腐蝕使其抗蝕力降低。所以,為確保元件壽命,會對7075-T6鋁合金進行防蝕處理,例如六價鉻化成處理。三價鉻是一種取代有環保疑慮六價鉻的製程之一,商用三價鉻藥水以三價鉻、鋯鹽為主要成份,再添加了增厚劑、界面活性劑、穩定劑等物質,使化成液與化成皮膜有較好的性質。
本論文探討7075-T6中的鎂矽及鋁鐵銅晶出相,於酸鹼洗後之組成和樣貌變化與在三價鉻化成中的表現;以及比較只含主劑成分之三價鉻、鋯鹽藥水(後以CZ代稱)與商用藥水(SurTec 650)之化成皮膜差異。實驗發現經氫氧化鈉鹼洗與硝酸酸洗後,鎂矽相發生去合金化,鎂會選擇性溶出,使其成為富矽相。鋁鐵銅晶出相周圍有腐蝕壕溝之形貌,其也會遭受去合金化,鋁與鐵會優先溶出,表面形成海綿狀結構。酸洗過後試片表面孔洞之樣貌,應為晶出相溶解或脫落所造成。 CZ之化成膜在真空環境下有剝落現象,而隨7075-T6鋁合金基材粗糙度增加,剝落現象漸為減緩。若比較未脫落之區域,其與SurTec化成膜有相似之形貌。仍可觀察到富矽相,EDS線性掃描顯示其鉻與鋯訊號與基材相近。化成後之表面無法直接觀測到鋁鐵銅晶出相,然而有眾多凸起樣貌之膜厚區,其比基材有較多鉻、鋯及鐵訊號,推測下方為鋁鐵銅晶出相。相較於前處理與研磨拋光試片,SurTec與CZ試片有較小陰陽極極化電流密度。陽極極化皆有鈍化區產生,然而CZ拋光試片之陰極極化行為可分為兩類,CZ-1和CZ-2。CZ-2與SurTec曲線表現相似,皆有出色陰極抑制效果;CZ-1有較高的陰極電流密度與較負塔弗斜率,顯示抗蝕性不如SurTec和CZ-2曲線。有關陽極極化行為,SurTec試片有較高的崩潰電位,在相同絕對電位下也有較佳的陰極抑制效果。鹽霧測試結果顯示,相同的化成液下,基材粗糙度較高之化成試片有較少的蝕孔;相同基材粗糙度時,CZ試片擁有較少之蝕孔。此結果與極化曲線之結果矛盾,仍待進一步探討。 Aluminum alloy 7075-T6 is a precipitation-strengthening alloy. In light of its high strength-to-weight ratio, 7075-T6 alloy has been widely used in the aircraft industry. Three heterogeneous phases are commonly found in commercial 7075-T6 alloys, including constituent particle, dispersoid, and precipitation-strengthening phase. The galvanic effect between the matrix and heterogeneous phases markedly reduces its corrosion resistance. Conversion coating treatment is thus indispensable for aluminum alloys, such as the hexavalent chromium treatment. Trivalent chromium is one of alternatives to hexavalent chromium, which is known as a carcinogen substance. Commercial trivalent chromium solutions are generally composed of Cr(III) and zirconate with the addition of thickener, surfactant, stabilizer to improve the solution stability and the corrosion resistance of the resulting conversion coating. The study focuses on the compositions and morphology change of Mg-Si and Al-Fe-Cu constituent particles after alkaline and pickling pretreatments, as well as on the conversion coating formation behaviors on constituent particles in a trivalent chromium conversion solution. The difference between commercial solution (SurTec 650) and the solution solely composed of Cr(III) and zirconate (denoted as CZ herein) was also studied. After sodium hydroxide cleaning and nitric acid pickling, the Mg-Si particles underwent a dealloying process. Magnesium was selectively dissolved in the pretreantment solution, resulting in Si-rich particles. Moreover, typical trencheing was observed around the Al-Fe-Cu particles. The Al-Fe-Cu particle also suffered dealloying, by which aluminum and iron dissolved, resulting in a sponge structure on the surface of the particle. The pitting morphology on the alloy surface after pickling pretreatment was caused by the dissolution or shedding of the particles. The CZ conversion coating tended to flake off under vacuum. The coating on the substrate with larger roughness showed less film peeling. A comparison on the area where the coating remained adherent revealed that both the SurTec sample and CZ sample had the similar appearance. The Si-rich phase was still found in the SEM easily, and the EDS line scan mapping showed that the Cr and Zr intensity was at the same level with the matrix. In contrast, the Al-Fe-Cu particle was not be found readily in the SEM image. However, there were some regions with thicker films at which more Cr, Zr, and Fe EDS signals were detected. It is likely that the Al-Fe-Cu particles were below these thicker coating regions. The potentiodynamic polarization results showed that the SurTec and CZ conversion coatings displayed smaller cathodic kinetics than the samples after alkaline cleaning and pickling. Both SurTec and CZ conversion coatings exhibited a passivation region in anodic polarization curves. The cathodic polarization curve of the CZ sample can classified into two types, CZ-1 and CZ-2. The CZ-2 curve was similar to that of the SurTec sample; both showed cathodic inhibition effect. The CZ-1 curve had higher cathodic current density and more negative Tafel slope, indicating a corrosion resistance lower than that of the CZ-2 or SurTec samples. Compared to the CZ sample, the SurTec sample had higher break down potential and smaller cathodic current density at the absolute cathodic potential (-0.85 V vs SCE). Finally, the Cr(III)-coated samples with higher substrate roughness had fewer corrosion spots after NSS test; and, for the substrate with a same surface roughness level, the CZ samples had fewer isolated spots than the SurTec sample. The NSS and polarization results were contradictory. More studies are necessary to understand this discrepancy. |
URI: | http://tdr.lib.ntu.edu.tw/jspui/handle/123456789/77524 |
DOI: | 10.6342/NTU201803317 |
全文授權: | 未授權 |
顯示於系所單位: | 材料科學與工程學系 |
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