2024-T3鋁合金鋯化成膜之腐蝕行為

Abstract

2024-T3鋁合金具有比強度高、易加工性等優點，是常用於航太產業之輕量化材料。2024-T3鋁合金中添加了銅、鎂等合金元素以提升機械性質，然而，異質的微結構卻使其容易受到局部腐蝕，進而降低使用壽命，因此須透過表面處理以提高耐腐蝕性。由於傳統使用的六價鉻化成處理已因為其毒性和致癌性而被限制使用，鋯化成處理因此成為熱門的替代方案之一。 本研究探討鋯化成膜對於提升2024-T3鋁合金耐腐蝕性之效果，利用浸泡實驗和微結構觀察，比較僅經過前處理和經過化成處理的2024-T3鋁合金腐蝕行為與機制，並藉由動電位極化曲線探討異質微結構對於鋁合金電化學行為的影響。 2024-T3鋁合金的晶出相可分為Al-Cu-Fe-Mn相、Al2Cu相(θ相)和Al2CuMg相(S相)共三種，在前處理和化成處理後，大多數的晶出相顆粒相對於鋁基地為陰極，鋯化成膜因陰極位置還原反應旺盛而在此沉積較厚。 微結構影像和即時攝影結果顯示，前處理2024-T3鋁合金浸泡1小時內即產生腐蝕環並伴隨著晶間攻擊，環外的晶出相周圍皆嚴重腐蝕，環內的晶出相則受到陰極保護，浸泡10小時後環內出現肉眼可見腐蝕點。化成處理2024-T3鋁合金的晶出相僅在鋯化成膜缺陷處遭腐蝕，雖發生晶間攻擊，但腐蝕環在浸泡15小時後才出現，並在48小時內無出現肉眼可見腐蝕點。鋯化成膜降低了約90%的浸泡6小時腐蝕事件數量。 由於2024-T3鋁合金上的鋯化成膜存在許多缺陷，從動電位極化曲線難以顯示其保護效果，此外，鋯化成膜雖然擴大了純鋁陽極極化曲線的鈍化區間，但其陰極電流密度卻最大。鋯化成膜之組成成分和均勻性仍須進一步研究以釐清其對於電化學行為之影響。

2024-T3 aluminum alloys are widely used in the aerospace industry as lightweight structural materials due to their high specific strength and good formability. The addition of alloying elements such as copper and magnesium enhance their mechanical properties. However, the resulting heterogeneous microstructure makes them highly susceptible to localized corrosion, which shortens their service life. Therefore, surface treatments are required to improve their corrosion resistance. Due to the toxicity and carcinogenicity of conventional hexavalent chromium conversion coatings, zirconium conversion coatings have emerged as a promising alternative. This study investigates the effectiveness of zirconium conversion coatings in enhancing the corrosion resistance of 2024-T3 aluminum alloy. Through immersion tests and microstructural observations, the corrosion behavior and mechanisms of pretreated and conversion-coated samples were compared. Potentiodynamic polarization curves were also used to examine how the heterogeneous microstructure affects the electrochemical behavior of aluminum alloys. The constituent particles in 2024-T3 aluminum alloy include Al-Cu-Fe-Mn phase, Al2Cu phase (θ phase), and Al2CuMg phase (S phase). After pretreatments and conversion coating, most constituent particles behave as cathodic sites relative to the aluminum matrix. The zirconium conversion film tends to deposit more thickly at these cathodic sites due to more intense local cathodic reactions. In pretreated samples, corrosion rings and intergranular attack appeared within 1 hour of immersion. Severe corrosion was observed around the constituent particles outside the corrosion rings, while constituent particles inside the rings were cathodically protected. After 10 hours of immersion, macroscopic corrosion pits developed inside the rings. In contrast, for the conversion-coated samples, corrosion occurred only at defect sites within the zirconium coating. Although intergranular attack still occurred, corrosion rings did not appear until after 15 hours of immersion, and no macroscopic pits were observed within 48 hours. The zirconium coating reduced the number of corrosion events after 6 hours of immersion by approximately 90%. Due to defects in the zirconium conversion film on 2024-T3 aluminum alloy, its protective effect was not clearly reflected in the polarization curves. In addition, the anodic polarization curve of pure aluminum with zirconium conversion coating showed an extended passivation region, despite exhibiting the highest cathodic current density. Further studies on the composition and uniformity of the zirconium conversion film are needed to clarify its influence on electrochemical behavior.