Chemical Polishing of Magnesium Alloys in Nitric/Adipic Acid Solution
Date Issued
2010
Date
2010
Author(s)
Hsih, Hsao
Abstract
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.
Subjects
Magnesium alloys
chemical polishing
gloss
Nitric acid
Adipic acid
Type
thesis
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