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The Inlfuence of Rare Earth on Anodic Oxidation of Aluminum Alloy in Sulfuric Acid-oxalic Acid

Author: MengDong
Tutor: XuanTianPeng
School: Hefei University of Technology
Course: Materials Science
Keywords: Aluminium Sulfuric acid-oxalic acid anodic oxidation Corrosionresistance Electrochemical property Rare earth sulfates
CLC: TQ150.1
Type: Master's thesis
Year: 2013
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Abstract


This article studied the basic mechanism of sulfuric acid-oxalic acid anodic oxidationwhich confirmed the optimum craft and formula in sulfuric acid-oxalic acid anodicoxidation and analyzed the effect of rare earth on corrosion resistance, surface morphologyand electrochemical property of anodizing films. The optimum process parameters wasdetermined by orthogonal experiments and the various affecting factors were as follows:sulfuric acid180g/L，oxalic acid5g/L，glycerol15g/L，oxidation temperature20℃，oxidation voltage16V，oxidation time50min. A conclusion had been drawn that theanodizing films mainly consisted of amorphous Al2O3which was composed of twoelements Al and O by observing the surface morphology of the anodizing films by scanningelectron microscopy （SEM） and evaluating the determination of the chemical compositionof the anodizing films by energy dispersive spectroscopy （EDS） and the structure of theanodizing films by X-ray diffraction （XRD） the anodizing films.The optimal dosage of Ceand La was0.2g/L, which was found through immersion corrosion tests of the five solution（acid, alkali, neutral salt, acid salt and alkali salt）, XPS analysis of corrosion products, theneutral salt spray test（NSS） and electrochemical corrosion tests.The corrosion resistance ofanodizing films in the immersion corrosion tests followed the order below: neutral salt>alkali salt> alkali> acid> acid salt.The basic corrosion products of the five solutions wereall Al2O3and Al（OH）3. Both the chemical dissolution of H+and the pitting of Cl-wereexisting at the same time in the acid salt solution. The two forms of corrosion acceleratedthe corrosion rate of anodizing films. And analyzed corrosion products by X-rayphotoelectron spectroscopy （XPS）, it was found that the compound of Cl-existed in theanodizing films. But the compound of Cl-in the alkali salt solution was not found in theanodizing films which showed Cl-was not involved in the corrosion process. Furthermore,after boiled, the porous layer and the porous wall of alumina in the anodizing films wasreacted to generate Al2O3H2O, so as to fill the holes. The corrosion resistance significantlyimproveed and the corrosion rate reduced. In the electrochemical corrosion tests,by usingAC impedance curve and Tafel curves, it could be seen that Ce and La made the radius ofthe capacitance of the anodizing films become large, the corrosion current become smalland the corrosion potential increase. And then the resistance corrosion of the anodizing films enhanced. Electrochemical corrosion resistance of the anodizing films in NaClsolution was the best, followed by HCl solution, and NaOH solution the worst. Rare earthof Ce and La did not change the performance of the anodizing films through gettingdeposited on the anodizing films, but they changed the organizational structure and thesurface morphology of the anodizing films through catalysis which made the barrier layerof the anodizing films become thicker and the porous layer porosity decrease, therebyimproving the performance of the anodizing films.

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