以田口法探討6066陽極層之耐蝕性

Abstract

6XXX系列鋁合金具有良好成型性，例如常用的6061-T6在常溫下可達到17 %的伸長率，而6066-T6也有12 %。在鍛造用鋁合金中，雖然強度不如2XXX系列和7XXX系列鋁合金，但6066鋁合金由於比6061鋁合金添加了更多的合金元素，在經過T6處理後，強度可達400 MPa左右，比常用的6061-T6鋁合金大了將近100 MPa，但所須成本卻差不多，對於在一些中高強度的使用上為一個不錯的選擇，因此本實驗選用6066鋁合金，並伴隨陽極處理在材料表面形成一層緻密保護層。實驗部分，為了在發生問題時能直接往回追溯，因此一開始是購買4 in.的鑄錠，先經過熱輥、製程退火、冷輥、及T6處理這些前段製程後，再將試片製備成固定尺寸後進行陽極處理。陽極處理部分，我們利用L9直交表，以硫酸鋁添加量、硫酸濃度、電流密度、及工作時間為影響陽極層耐蝕性的四個因子，每個因子取三個水準，進行二部分的實驗，並配合表面陽極層性質之分析。最後，可得到6066-T6鋁合金陽極層耐蝕性之最佳條件為在硫酸鋁添加量9 g/l、硫酸濃度17 wt%、電流密度1.2 A/dm2、及工作時間10 min下。經極化曲線分析，其腐蝕電流密度可達10-10 Scale，相較於未經陽極處理之原材，可提升3 ~ 4個，且經過1392 hr中性鹽霧試驗後，其腐蝕面積約只有2.7 %，確實具有良好耐蝕性，而硬度可達到554 Hv。依實驗結果研判，這些陽極層都已經具有相當好的絕緣性，但卻仍會有腐蝕發生。因此，除了陽極處理的理想條件外，試片本身的物理缺陷也佔了相當大的因素，若要獲得更好的陽極膜層，那麼在陽極處理前的試片，應該盡量減少其雜質的含量，以減少在陽極處理過程中在這些除了鋁之外的雜質處所造成的電荷集中，使陽極層產生裂縫而大大地降低了耐蝕性。

6XXX series aluminum alloys have excellent formability. For examples, the commonly used 6061-T6 alloy has maximum elongation of 17 %, and 6066-T6 alloy has maximum elongation of 12 % at room temperature. Although 6XXX alloys have lower strength than 2XXX and 7XXX alloys in as-wrought state, the strength of 6066-T6 alloy can be inceresed to near 400MPa after T6 treatment. Compared to 6061-T6, 6066-T6 has 100 MPa more increment in strength due to higher fraction of alloy elements, while these two alloys cost about the same. Therefore, it would be a good selection for aluminum plants. For this reason, we chose 6066 aluminum, and coated a protective film with anodic treatment. At first, we purchased 4 in. billet in order to solve problems directly during experiment. Experiments were proceeded in sequence with hot rolling, process annealing, cold rolling, and T6 treatment. Then, we prepared specimens identically for the following anodic treatment. For anodic treatment, we selected four important process parameters that influence the product quality (corrosion resistance) very much as four control factors in Taguchi's method. These four parameters are Al2O3•14 ~ 18H2O(s) addition, H2SO4(aq) concentration, current density, and working time. Furthermore, by selecting three appropriate levels for each factor, we can construct the L9 (34) matrix experiment. Then we proceeded two parts experiment and analysis of surface properties of anodic films. Eventually, we obtained an optimum condition of 6066-T6 aluminum anodic film for corrosion resistance at 9 g/l Al2O3•14 ~ 18H2O(s) addition, 17 wt% H2SO4(aq), 1.2 A/dm2 current density, and 10 min working time. Results from polarization curve and 1392-hour neutral salt spray test show that corrosion current density can reach 10-10 scale (3 ~ 4 order increased compared to raw material) and has just 2.7 % corrosion area. It can provide excellent corrosion resistance certainty and hardness of 554 Hv. On our results, the anodic films should have excellent corrosion resistance, but they were still corroded. Thus, if we would like to obtain better anodic film, we should also consider physical defects except for an optimum condition in practice. We should reduce the content of impurities in our specimens before anodic treatment, For example, lower content limit of impurities is required for better control of charge during anodic process. Hence we can obtain excellent anodic film with less cracks. Keywords：6066 aluminum, anodic treatment, corrosion resistance, Taguchi method