Study Of Al₂O₃-CeO_2Y2O₃Composite Protective Coating

Metallic materials with desired oxidation resistance at high temperature normallydepend on whether a compact protective oxide layer such as Cr₂O₃or Al₂O₃can beformed with high thermal stability and slow growth rate, thus the oxide layer is takenas a barrier to completely isolate the metallic substrate with environment. However,the commercial alloys usually form poor protective oxide layers where the metallicelements in the oxide layers are basically dominated by the principal alloyingcomponents in the materials. Typically, a thermally grown oxide layer such as Cr₂O₃or Al₂O₃is normally grown at high temperature as a protective coating to improvingthe resistance to high temperature corrosion. Stainless steels have excellent corrosionresistance at room temperature and in dry air, but they may suffer serious corrosion athigh temperature, in high humidity and with very corrosive gases. To protect stainlesssteels, an inorganic coating applied on the stainless steel surface can improve itscorrosion resistance. The Al₂O₃-CeO₂–Y₂O₃composite coatings have been identifiedto possess excellent corrosion resistance, and the coatings have become the focus ofresearch in recent years for its potential applications.In this study, the Al₂O₃-CeO₂–Y₂O₃composite coatings was deposited on SUS304stainless steel surface using the sol-gel method and electrodeposition method. Thecomposition, structure and texture of the coatings were analyzed by X-ray Diffraction（XRD）, EDS and scanning electron microscopy （SEM）. The corrosion resistance ofcoatings were assessed by electrochemical techniques.A composite sol was made with Ce（NO₃）₃·6H₂O, Al（NO₃）₃·9H₂O andY（NO₃）₃·6H₂O as precursor. Through this process, we investigated the influence ofadding solvent speed, sol temperature, pH value and the amount of PVP on theformation of the composite coatings. Compared to the dip-coated method, thespin-coated method were testified to be more appropriate in achieving the compositecoatings with satisfying corrosion resistance. In this study, a composite layer wasformed by spin-coating5times in a heating rate0.5°C/s and heat treatment soaking attemperature of550°C. The composite layer thus made presents high corrosionresistance with average porosity as low as24%.Al–Ce–Y alloy coatings were prepared by electrodeposition method. Afterdeposition, the alloy coatings were heat-treated in open air at temperature of550°C.After heat treatment, Al₂O₃-CeO₂–Y₂O₃composite coatings were formed due to the pH value at2.5³.5and temperature between30⁴5°C. The deposition currentdensity was about2.2².5A·cm^-2and deposition time last for20to30min. The alloycoatings thus deposited are relatively uniform and compact. The composite oxidelayer formed after heat treatment is excellent to resist corrosion, as examined usingelectrochemical techniques.