酸洗前處理對鋁合金2024-T3三價鉻化成膜性質之影響

Abstract

鋁合金2024-T3質輕強度高，是航空載具的重要金屬材料，添加合金元素可以提升鋁的機械強度卻也因鋁和這些合金元素生成較活性的Al-Cu-Mg相與較為鈍性的Al-Cu-Fe-Mn相而產生伽凡尼腐蝕使抗蝕能力降低。為了避免腐蝕對飛行安全產生影響，一般會為鋁合金進行表面處理。六價鉻化成系統可以提供鋁合金優良的抗蝕能力，但六價鉻本身有毒性，對生物和環境有害，使用上受到限制。三價鉻化成系統為目前廣泛使用的六價鉻化成替代方案，兩者的價態不同對於人體及環境的影響也不相同。三價鉻系統本身的氧化力較弱且沒有六價鉻擁有的自我癒合能力，仍須調控製程條件來達到最良好的表現，尤其以酸洗前處理影響化成膜性質最顯著。析出相在酸洗前處理中的變化為本研究的重點。 析出相主要有兩支:(1) Al-Cu-Mg析出相(2) Al-Cu-Fe-Mn析出相，前者較底材活性後者則較鈍態。在硝酸酸洗處理下，Al-Cu-Mg相有去合金化的現象，處理時間越長則越明顯，對於Al-Cu-Fe-Mn相的影響不太大，大部分析出物依然保持完整；Deoxidizer 6酸洗處理對兩種析出物展現很強的侵蝕能力，Deoxidizer 6處理過後Al-Cu-Mg相幾乎被侵蝕殆盡而Al-Cu-Fe-Mn相則形成蜂窩狀的表面，試樣表面有一層Cr氧化物生成。硝酸處理三價鉻化成較Deoxidizer 6處理的三價鉻化成能抵抗鹽霧測試。以SEM觀察鹽霧測試後的試片，兩者有相似構造的腐蝕產物，而Deoxidizer 6處理過後有較大規模的腐蝕跡象。

Aluminum alloy 2024-T3 has widely been utilized in the aircraft industry because of the high mechanism strength and low-weight. Adding alloy elements into aluminum substrate improve the mechanism strength but reduce the corrosion resistance. To improve corrosion resistance, conversion coating has been conducted on aluminum alloy 2024-T3. Hexavalent chromium conversion coating once is a choice of conversion coating; However, Hexavalent chromium is a toxic substance and harm both human and environment. Therefore, substitutions are necessary. Trivalent chromium conversion coating is a potential substitution for hexavalent chromium; However, Trivalent chromium conversion isn’t as good as Hexavalent chromium in corrosion resistance and doesn’t exhibit self-healing behavior. Modification of the processing procedure is hence necessary to improve the anti-corrosion property of trivalent chromium conversion coating. There are two series of precipitates in aluminum alloy 2024-T3: Al-Cu-Mg and Al-Cu-Fe-Mn, former is more active than matrix and the latter is more noble than matrix. After the acidic pretreatment of nitric acid, Al-Cu-Mg precipitates dealloy. The extent of dealloying is more obvious when the processing time goes. Al-Cu-Fe-Mn precipitates have little influence during the procedure. Both type of precipitates are eroded severely especially Al-Cu-Mg precipitates during acidic pretreatment of Deoxidizer 6, and grew a thin layer of Cr oxides on the surface. The specimens treated by nitric acid exhibit better corrosion resistance than those treated by Deoxidizer 6 in salt spray test. Dome-like corrosion product can be observe on the specimens treated by both parameters but the large scale corrosion only can be observed in specimens treated by Deoxidizer 6.