6061-T6 鋁合金鋯化成處理後於0.1MNaCl溶液中之抗蝕性研究

Abstract

鋁合金憑藉其優異的導電與導熱性質，結合低密度與高比強度的機械性質，已廣泛應用於汽車工程及航太工業等關鍵領域。然而，6061-T6鋁合金在含有氯離子的嚴苛環境中極易發生局部腐蝕，進而影響其結構可靠度與使用壽命。傳統工業上多仰賴鉻酸鹽化成處理來提供防護，但鑑於六價鉻對人體與生態環境的高危害性，開發環保且無毒的鋯系化成處理（Zirconiumconversioncoating）以取代傳統製程已成為重要趨勢。 本研究旨在探討6061-T6鋁合金經鋯化成處理後，於0.1M NaCl溶液環境中抗蝕性能之提升效益。本研究透過動電位極化曲線（Potentiodynamicpolarization,PDP）與浸泡試驗，針對未處理基材(GRI)與鋯化成試片(FZA)之抗蝕性差異進行定量與定性評估。在電化學測試方面，極化曲線量測結果顯示，經過鋯化成處理之試片(FZA)，其腐蝕電流密度（Icorr）較未處理的裸材(GRI)呈現大幅度的下降。腐蝕電位（Ecorr）較負，此因鋯化成皮膜抑制了陰極反應，此現象亦由陰極塔弗斜率獲得驗證；同時，推測因皮膜仍存有局部缺陷，導致其崩潰電位與未處理試片無明顯差異。 為進一步檢視實際防護效果與釐清腐蝕機制，本研究執行了0.1M NaCl溶液的長時間浸泡試驗。微結構觀察指出，未處理6061-T6基材(GRI)的嚴重腐蝕主要由Mg₂Si的極性反轉引發；反轉後的極度鹼化與後續孔蝕內的酸化產氫機制，最終演變為嚴重的晶間腐蝕。定量數據顯示，未處理試片(GRI)浸泡12小時即開始有腐蝕環的產生，最終浸泡24小時後平均直徑達到744.74µm的腐蝕環，24小時後表面腐蝕環與蝕孔數量分別高達15.8個/cm²與17.0個/cm²。相較之下，鋯化成皮膜在浸泡初期能提供優異的物理屏障，使試片在24小時浸泡後表面破壞數量皆為0。長時間浸泡觀察亦顯示，鋯化成試片(FZA)直至第9天才生成明顯腐蝕環，且浸泡14天後平均直徑僅124.44µm。 綜上所述，本研究結合了動電位極化曲線的定量數據與浸泡試驗後的微結構形貌對比，確認了鋯化成處理能大幅延緩巨觀局部腐蝕的發生時間，並顯著限制腐蝕環向外擴展的速率，有效提升6061-T6鋁合金於0.1M NaCl環境下的化學穩定性與耐蝕表現。此研究成果客觀記錄了不同試片受腐蝕損傷程度的微觀差異，為無鉻環保型表面處理技術之應用提供了具體的實驗佐證。

Aluminum alloys are widely applied in critical fields such as automotive engineering and the aerospace industry due to their excellent electrical and thermal conductivity, combined with low density and high specific strength. However, the 6061-T6 aluminum alloy is highly susceptible to localized corrosion in severe chloride-containing environments, which compromises its structural reliability and service life. Traditional industries heavily rely on chromate conversion treatments for protection, but given the high toxicity of hexavalent chromium to the human body and the ecological environment, developing eco-friendly and non-toxic zirconium conversion coatings (ZrCC) to replace conventiona processes has become an important trend. This study aims to investigate the enhancement of corrosion resistance of 6061-T6 aluminum alloy treated with ZrCC in a 0.1 M NaCl solution environment. This study quantitatively and qualitatively evaluated the corrosion resistance differences between untreated substrates(GRI) and ZrCC-treated specimens(FZA) through potentiodynamic polarization (PDP) curves and immersion tests. In terms of electrochemical testing, the polarization curve results showed that the corrosion current density (Icorr) of the ZrCC-treated specimens decreased significantly compared to the bare substrate. The corrosion potential (Ecorr) shifted negatively because the ZrCC film inhibited the cathodic reaction, a phenomenon verified by the cathodic Tafel slope. However, due to the presence of local defects in the film, the breakdown potential of the ZrCC-treated specimens showed no obvious difference from that of the untreated specimens. To further examine the practical protective effects and clarify the corrosion mechanisms, long-term immersion tests in a 0.1 M NaCl solution were conducted. Microstructural observations indicated that the severe corrosion of the untreated 6061-T6 substrate(GRI) is mainly triggered by the polarity reversal of the Mg₂Si phase. The extreme localized alkalinization following this reversal, along with the subsequent acidification and hydrogen evolution mechanisms within the pits, eventually evolves into severe intergranular corrosion. Quantitative data showed that untreated specimens(GRI) began to develop corrosion rings after 12 hours of immersion, with the average diameter reaching 744.74 μm after 24 hours. Furthermore, the density of surface corrosion rings and pits reached as high as 15.8 rings/cm² and 17.0 pits/cm², respectively, after 24 hours. In contrast, the zirconium conversion coating provided an excellent physical barrier in the early stages of immersion,resulting in zero macroscopic surface damage after 24 hours. Long-term immersion observations also revealed that obvious corrosion rings did not appear on the ZrCC-treated specimens(FZA) until the 9th day, and the average diameter was restricted to only 124.44μm after 14 days of immersion. In summary, combining the quantitative data of potentiodynamic polarization curves and the microstructural morphological comparisons after immersion tests, this study confirmed that ZrCC treatment can dramatically delay the onset time of macroscopic localized corrosion and significantly restrict the outward propagation rate of corrosion rings. It effectively enhances the chemical stability and corrosion resistance of 6061-T6 aluminum alloy in a 0.1 M NaCl environment. These findings objectively record the microscopic differences in corrosion damage among different specimens, providing concrete experimental evidence for the application of chromium-free, eco-friendly surface treatment technologies.