2024-T3鋁合金稀土金屬化成處理與抗蝕性之研究

Abstract

2024-T3鋁合金具有高強度重量比和良好的疲勞強度等優點，是航太產業中載具零件的重要材料，然而，添加銅、鎂等合金元素使機械性質提升的同時，卻也會在合金製造過程中產生異質相使抗蝕能力下降。為了增加其使用壽命，通常會對鋁合金施行表面處理，例如化成處理技術。稀土金屬化成處理為環保無毒、成分簡單的化成系統，是取代六價鉻化成處理的良好方案。 本研究探討了2024-T3鋁合金微米級晶出相在前處理與稀土金屬化成處理的演變，並利用鹽霧測試比較本材料在兩種稀土金屬化成處理（CeCC與CeLa）後的腐蝕機制。 2024-T3鋁合金的晶出相主要可以分為Al-Cu-Fe-Mn-Si相、Al2Cu（ϴ）相與Al2CuMg（S）相三種， 在酸洗1/3/5/10分鐘後晶出相呈現不同的形貌，Al-Cu-Fe-Mn(-Si)相在酸洗10分鐘後輕微去合金化，Al2Cu（ϴ）相產生腐蝕壕溝和蝕坑，而S相則有蝕坑、海綿狀殘骸及露出下方S相的樣貌。前處理參數中以3分鐘硝酸酸洗處理為較適合的參數。 經鈰化成處理(CeCC)與添加鑭之鈰化成處理(CeLa)後，本材料上CeLa皮膜覆蓋率較高，其厚度約為CeCC皮膜的三倍。CeCC和CeLa化成皮膜在Al-Cu-Fe-Mn(-Si)和θ相上較厚是因為這些晶出相作為局部陰極會促進皮膜沉積，在S相上較薄，因其變成蝕坑的形貌，與底材無太大差異。XPS結果顯示化成皮膜以鈰與鑭的氫氧化物/氧化物組成，也有氫氧化鋁及氧化鋁層之存在。 鹽霧試驗結果顯示，2024-T3鋁合金經CeLa化成處理比CeCC化成處理具有更好的抗腐蝕能力。雖然CeCC和CeLa試片表面在4小時鹽霧測試後沒有明顯的腐蝕跡象，但橫截面顯示嚴重的內部腐蝕。腐蝕起始於化成皮膜缺陷、脫水裂紋、惰性晶出相的壕溝處與伽凡尼效應，導致試片表面堆積腐蝕產物，並在合金內部形成沿晶腐蝕和蝕孔，在三種晶出相中，S相受到的腐蝕損傷最小。

2024-T3 aluminum alloy is crucial in the aerospace industry for its high specific strength and excellent fatigue strength. However, the addition of alloying elements like copper and magnesium during manufacturing introduces heterogeneous phases that can diminish its corrosion resistance. Surface treatments, such as conversion coating, are usually applied to enhance its lifespan, with rare-earth metal conversion coating emerging as a non-toxic and environmentally friendly alternative to hexavalent chromium conversion process. This study investigates the evolution of constituent particles in 2024-T3 aluminum alloy during pretreatment and rare-earth metal conversion coating, comparing the corrosion mechanisms after salt spray tests between two types of conversion coating: CeCC and CeLa. The constituent particles in 2024-T3 aluminum alloy mainly include three types: Al-Cu-Fe-Mn-Si, Al2Cu (θ), and Al2CuMg (S). After acid desmutting for 1/3/5/10 minutes, these constituent particles show varying morphologies. Al-Cu-Fe-Mn(-Si) phase undergoes slight dealloying after 10 minutes, θ phase (Al2Cu) develops trenching and pits, while S phase shows pits, sponge-like remnants, and underlying S phase.The preferable pretreatment is 3-minute nitric acid desmutting. Following CeCC and CeLa conversion coatings, CeLa exhibits higher coverage and approximately three times the thickness of CeCC. CeCC and CeLa coatings are thicker on Al-Cu-Fe-Mn(-Si) and θ phases due to their role as local cathodes facilitating coating deposition, whereas they are thinner on S-phase due to its pit morphology, which resembles the matrix. The XPS results indicate that the conversion coating is composed of cerium and lanthanum hydroxides/oxides, as well as the presence of aluminum hydroxide/oxide layer. Salt spray test demonstrates that CeLa on 2024-T3 aluminum alloy exhibits superior corrosion resistance compared to CeCC. While the surfaces of CeCC and CeLa samples show no significant corrosion after 4 hours of salt spray test, cross-sections reveal severe internal corrosion. Corrosion initiates at defects in the conversion coating, dehydration cracks, and galvanic corrosion from inert constituent particles, resulting in accumulation of corrosion products on the surface and intergranular corrosion and pits within the alloy. Among three constituent particles, S phase exhibits the least corrosion damage.