鎂合金之硝酸/己二酸系統化學拋光

Abstract

本研究以AZ31、AZ91及LZ91鎂合金為基材，於5℃之0.2M硝酸/0.005M己二酸溶液中進行化學拋光，探討不同基材與製程時間對於光澤度提升之影響。研究中使用OM、SEM進行巨觀及微觀形貌觀察，搭配光澤度、粗糙度量測及ESCA、OCP、極化曲線等分析方法，探討化學拋光機制及基材顯微結構對光澤度提升之效應。 實驗結果顯示，AZ31於化學拋光後之光澤度大幅提升.，LZ91僅略為提升.，AZ91則反而下降。由SEM觀察結果發現，化學拋光後AZ31呈現完全平整之形貌；LZ91表面雖然平整但出現晶界腐蝕痕跡；AZ91則出現嚴重雙相效應，呈現alpha相向下陷落而beta相外露之崎嶇形貌。粗糙度之量測顯示，AZ31與LZ91間之粗糙度相去不遠，AZ91粗糙度則遠高於前述兩種基材。以alpha-stepper掃描之結果同時顯示AZ31及LZ91之拋光深度隨製程時間增加而線性增加，且表面形貌由多而淺之刮痕轉為少而起伏較大之形貌。此外，ESCA分析結果指出，化學拋光後三種基材表面生成一層以氧化物及氫氧化物為主之皮膜，其中AZ31及LZ91之表面皮膜厚度應介於100-200nm之間，AZ91則無法以此結果估計。ESCA縱深分析結果亦顯示表層有己二酸反應產物吸附之跡象。極化曲線量測結果顯示，該氧化物/氫氧化物皮膜無法提供確實之鈍化效果，其中AZ31及AZ91之皮膜能些許提升抗蝕性，LZ91經拋光後抗蝕性反而下降。 綜合以上結果可推論出鎂合金化學拋光之機制及己二酸於其中扮演之角色。鎂基材於接觸拋光液後迅速溶解釋出鎂離子並促使介面pH上升而產生氫氧化鎂沉積，此後皮膜之生成與再度溶解之間逐漸達到一動態平衡狀態。己二酸於拋光過程中吸附於表面突起處並促使該處加速溶解，進而達成整平效果，但未併入皮膜之中。

In this research, the chemical polishing of AZ31, AZ91 and LZ91 magnesium alloys were studied for understanding the relationship of different matrix and gloss performance. The chemical polishing was performed at 5℃, in the solution composed of 0.2 M nitric acid and 0.005 M adipic acid with various time, and the mechanism of chemical polishing and the role of adipic acid were discussed by using OM, SEM, alpha-stepper, gloss meter, OCP, ESCA, and polarization curves. Experimental results showed that the gloss of polished AZ31 increased significantly due to flatten surface. Gloss of AZ91 dropped dramatically because of very high roughness after chemical polishing due to severe partial dissolution of alpha phase. The surface of LZ91 was also flatten but with local corrosion at grain boundary, thus the gloss did not increase apparently. The results of alpha-stepper scanning indicated that both the depth of polished region and the roughness increased linearly with increasing time. The results of ESCA analysis demonstrated that a MgO/Mg(OH)2 surface film with a thickness of 100-200 nm formed after polishing, and the results of polarization curves indicated that such a film did not improve the corrosion resistance apparently. The ESCA depth profile analysis also indicates that adipic acid exists only at the surface of the film, suggesting the adsorption of adipic acid during chemical polishing From above experimental results, a mechanism was proposed. The adipic acid dissociated and adsorbed preferentially at the edge of surface scratches. The negative-charged C_4 〖〖H_8 (COO)〗_2〗^(2-) might attract positive H+ and thus facilitate the dissolution of adsorbed substrate. As a result, the scratches was removed and gloss of the substrate was greatly increased.