鋁合金三價鉻化成處理研究

Abstract

鋁合金2024-T3為航空載具結構件的重要材料，為了避免材料腐蝕影響航空飛行的安全，通常會在鋁合金2024-T3表面進行化成處理。經六價鉻化成處理的鋁合金擁有優異的抗蝕能力，但六價鉻具有致癌性，也會給環境帶來嚴重汙染，國際指令因此嚴格限制六價鉻物質的使用，目前大量研究試著尋找替代六價鉻的環保製程。 三價鉻化成處理是現今廣泛應用的替代六價鉻環保製程，但三價鉻本身氧化能力較六價鉻弱，且沒有六價鉻皮膜特有的自我癒合能力，故仍需透過製程條件的調控來改善抗蝕能力，現今國內航空產業所使用的化成液大多來自國外廠商，配方和操作窗口也受到航空規範的限制，製程上能夠做的調整幅度也有限，在不對三價鉻化成處理本身做改良的情形下，本研究探討不同酸洗前處理對鋁合金2024-T3三價鉻化成皮膜抗蝕能力的影響，研究發現兩種不同酸洗前處理結果對於鋁合金表面形貌影響差異大，經Deoxidizer 6酸洗後底材表面改變較大，表面佈滿蝕孔且平整性差，經硝酸酸洗的底材僅在表面出現些微蝕孔，最終硝酸酸洗前處理的三價鉻皮膜能夠通過航空產業所要求的168小時鹽霧試驗規範。 在TEM橫截面觀察下，兩酸洗前處理條件的三價鉻化成處理後，皆在表面生成三價鉻鈍化膜，硝酸酸洗前處理之皮膜較Deoxidizer 6酸洗前處理之皮膜緻密且厚。藉由TEM-EDS元素分析，三價鉻皮膜為一雙層結構，外層的Cr(III)、Zr含量較內層高，內層的Al含量則較外層高，XPS分析皮膜組成中含有Cr(III)、Zr和Al的氧化物及氫氧化物。在浸泡含有六價鉻的Deoxidizer 6酸洗液後，試片表面會留有一層薄薄鈍化層，此鈍化層會降低三價鉻化成處理時的反應性。基於考量到皮膜連續性及化成反應性因素，硝酸酸洗前處理較Deoxidizer 6酸洗前處理適合Metalast TCP-HF三價鉻化成製程，動電位極化曲線結果顯示腐蝕電流較小且較能阻擋鹽霧試驗下蝕孔的擴大。

Aluminum alloy 2024-T3 has widely been used in the aircraft industry. To avoid an air crash caused by corrosion of the alloy, conversion coatings have commonly conducted on aluminum alloy 2024-T3. Hexavalent chromium conversion coatings provide excellent anti-corrosion properties; however, hexavalent chromium is a toxic element and harmful to species and environment. International directives have strictly banned the use of hexavalent chromium. Therefore, many efforts have been made to find the process alternative to hexavalent chromium conversion process. Trivalent chromium conversion coating is a potential alternative process to hexavalent chromium conversion coating. However, the oxidizing capability of trivalent chromium is inferior to hexavalent chromium. Moreover, trivalent chromium conversion coating does not have self-healing capability. Modifications of the processing procedure are thus essential to improve the anti-corrosion properties of trivalent chromium conversion coating. The trivalent chromium conversion solution we now use in Taiwan is imported from foreign manufacturers and the modifications we can make are limited by aerospace specifications. Under that premise, this study is to understand the effect of two different acidic pretreatments, nitric acid and Deoxidizer 6, on the microstructure and properties of the subsequent trivalent chromium conversion coating on aluminum alloy 2024-T3. The results show that the coating treated with nitric acid pretreatment performs better in potentiodynamic polarization test and are able to pass 168-hr of the salt spray test. Pits attacked by F ions become larger and then coalescence with each other, resulting in a rough surface after pretreatment in Deoxidizer 6. The uneven surface may also influence the uniformity of the trivalent chromium coating. However, there are only a few pits on the surface pretreated in nitric acid and the surface seems smoother than that pretreated in Deoxidizer 6. Cross-sectional characterization reveals the presence of trivalent chromium conversion coating on aluminum alloy 2024-T3 after immersion in Metalast TCP-HF regardless of the acidic pretreatments. The trivalent chromium conversion coating on aluminum alloy 2024-T3 pretreated in nitric acid is denser and thicker compared to that on aluminum alloy 2024-T3 pretreated in Deoxidizer 6. A two-layered structure is clearly observed, where Cr(III) and Zr content in the outer part of the coating is higher than that in the inner part. On the contrary, the inner part of the coating is rich in Al. The trivalent chromium conversion coating on aluminum alloy 2024-T3 is composed of oxide and hydroxide of Cr(III), Zr, and Al, as revealed by XPS. Due to the presence of chromate ions in Deoxidizer 6, there is already a thin film containing Cr(III) on the surface after the acidic pretreatment. This thin film serving as a passivation layer deteriorates the reactivity during the following TCP. With respect to the uniformity of the coating and reactivity of conversion reaction, the pretreatment in nitric acid is more suitable for the trivalent chromium conversion process using the Metalast TCP-HF solution.