2024-T3鋁合金前處理與鋯化成處理之研究

Abstract

鋁合金由於密度低、比強度高，為目前工業產品追求輕量化的常用材料。此外，其可回收性也符合現今社會對於環保的要求。一般而言，鋁合金表面存在著原生的緻密氧化層，然而此種合金本身的異質微結構仍然容易使其發生局部腐蝕，因此為了滿足實際應用，必須進行表面處理以提升其抗蝕性。有鑑於以往的六價鉻化成處理存在著具毒性、致癌性，以及對環境有害的疑慮，近年來鋯化成的研究成為一大亮點。 本研究探討2024-T3鋁合金的微米級二次相(或稱為一次析出物)在前處理與鋯化成處理後的變化，並利用動電位極化測試的結果來討論本材料在不同表面處理過後所發生的腐蝕行為差異。 2024-T3鋁合金微結構中的微米級二次相可分為三類，即S相(Al2CuMg)、θ相(Al2Cu)，以及Al-Cu-Fe-Mn相。S相與θ相的形狀與尺寸相似，而Al-Cu-Fe-Mn相常以多相型顆粒的形式存在，其尺寸也通常較大。在前處理之後，Al-Cu-Fe-Mn相顆粒的形貌並無明顯變化，而θ相顆粒與鋁基地的交界處則會形成腐蝕壕溝。至於S相則可分為三種形貌，包括蝕坑、海綿狀殘骸與特殊形貌S相。 在化成處理之後，海綿狀殘骸的鋯含量最多，而Al-Cu-Fe-Mn相、θ相與特殊形貌S相顆粒表面也都能發現奈米級顆粒，顯示鋯化成膜亦覆蓋在二次相顆粒之上。在無前處理化成之後，各種二次相表面的鋯含量皆較有前處理化成來得多，其中S相表面的鋯含量是三種二次相中最多的。 動電位極化測試結果顯示，有前處理化成膜具有最好的陽極抑制能力，而無前處理化成膜則具有最佳的陰極抑制效果。然而，這兩種條件的鋯化成膜在2024-T3鋁合金表面的形成機制，仍需進一步的研究。

Because of their low density and high specific strength, aluminum alloys are commonly employed materials for industrial products in pursuit of lighter weight. Furthermore, their recyclability lives up to the environmental requirements of today’s society. Generally, there exists a native compact oxide layer on the surface of an aluminum alloy, but the heterogeneous microstructure of this alloy makes it highly susceptible to localized corrosion. In view of the toxicity, carcinogenicity, and environmental concerns brought about by the conventional hexavalent chromium conversion processes, studies on zirconium conversion coatings have gained increasing attention in recent years. This study investigates the changes of micron-sized second-phase particles (or so-called constituent particles) of AA2024-T3 after pretreatment and zirconium conversion processes. Also, the difference in corrosion behavior of this material after different surface treatments is discussed based on the results of potentiodynamic polarization tests. The micron-sized second-phase particles of AA2024-T3 can be classified into three types, namely S phase (Al2CuMg), θ phase (Al2Cu), and Al-Cu-Fe-Mn particles. S-phase and θ-phase particles have similar shapes and sizes, while Al-Cu-Fe-Mn particles usually exist in the form of multiphase particles with larger sizes. After the pretreatment, the shape of Al-Cu-Fe-Mn particles does not alter significantly, while trenching occurs at the periphery of θ-phase particles. At this stage, the S-phase particles observed can be categorized into three types according to their morphologies, including etch pits, sponge-like remnants, and special S-phase particles. After the conversion process, sponge-like remnants show the highest zirconium content, while nanoscale particles can be found on the surfaces of Al-Cu-Fe-Mn particles, θ-phase particles, and special S-phase particles, indicating that the zirconium conversion coating also covers these second-phase particles. After the conversion process without pretreatment, the zirconium content on the surface of each type of second-phase particle is higher than that of the conversion coated counterpart which underwent pretreatment prior to the conversion process. What’s more, the zirconium content on the surface of the S phase is the highest among the three types of second-phase particles under this condition. The results based on the potentiodynamic polarization tests reveal that the conversion coating after pretreatment exhibits the best anodic inhibition, whereas the other coating without pretreatment shows the best cathodic inhibition. However, the formation mechanisms of the zirconium conversion coatings on 2024-T3 aluminum alloy under these two conditions still warrant further investigation.