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The effect of chromic sulfate concentration and immersion time on the structures and anticorrosive performance of the Cr(III) conversion coatings on aluminum alloys |
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| Authors: | Wei-Kun Chen Chung-Ming Liu Ming-Der Ger |
| Institution: | a Graduate School of Defense Science, Chung Cheng Institute of Technology, National Defense University, Tao-Yuan 335, Taiwan, ROC b Department of Applied Chemistry & Materials Science, Chung Cheng Institute of Technology, National Defense University, Tao-Yuan 335, Taiwan, ROC c Research Center for Biomedical Devices, Taipei Medical University, Taipei 110, Taiwan, ROC d Department of Chemical and Materials Engineering, Lunghwa University of Science and Technology, Tao-Yuan 3305, Taiwan, ROC e Department of Materials Science and Engineering, National Taiwan University, Taipei 106, Taiwan, ROC |
| Abstract: | The main purpose of this study is to develop trivalent chromium, Cr(III), conversion coatings on aluminum alloys. The influence of Cr(III) concentration and immersion time on structures and anticorrosive performance of the coatings has been investigated. Corrosion behaviors of the coatings were evaluated in a 0.5 M H2SO4 aqueous solution at room temperature using potentiodynamic polarization. The structure and valence state of the coatings were examined by scanning electron microscopy (SEM), transmission electron microscopy (TEM), and X-ray photoelectron spectroscopy (XPS). The addition of Cr(III) ions to the conversion bath considerably changes structures and compositions of the coatings. The coatings with Cr oxides possess a denser and thinner structure. Moreover, the corrosion resistance of Cr(III) coatings tends to decline with increasing immersion time due to the dissolution of coatings in the dipping period. According to XPS analysis, the Cr(III) conversion coatings are composed of Cr2O3, Cr(OH)3, Al2O3, Al(OH)3, ZrO2, Zr(OH)4, AlF3, and ZrF4, but no hexavalent chromium component in the coatings. The result indicates that the coatings prepared in the solution with 0.01 M Cr(III) for 5 min have the smoothest and densest structure and the best anticorrosive performance among all of conversion coatings in this work. |
| Keywords: | Trivalent chromium conversion coating Aluminum alloys Corrosion resistance Chromic sulfate Immersion time |
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