Abstract
Metallic materials with desired oxidation resistance at high temperature normally depend on whether a compact protective oxide layer such as Cr2O3 or Al2O3 can be formed with high thermal stability and slow growth rate, thus the oxide layer is taken as a barrier to completely isolate the metallic substrate with environment. However,the commercial alloys usually form poor protective oxide layers where the metallic elements in the oxide layers are basically dominated by the principal alloying components in the materials. Typically, a thermally grown oxide layer such as Cr2O3 or Al2O3 is normally grown at high temperature as a protective coating to improving the resistance to high temperature corrosion. Stainless steels have excellent corrosion resistance at room temperature and in dry air, but they may suffer serious corrosion at high temperature, in high humidity and with very corrosive gases. To protect stainless steels, an inorganic coating applied on the stainless steel surface can improve its corrosion resistance. The Al2O3–CeO2–Y2O3 composite coatings have been identified to possess excellent corrosion resistance, and the coatings have become the focus of research in recent years for its potential applications.
In this study, the Al2O3–CeO2–Y2O3 composite coatings was deposited on SUS 304 stainless steel surface using the sol-gel method and electrodeposition method. The composition, structure and texture of the coatings were analyzed by X-ray Diffraction (XRD), EDS and scanning electron microscopy (SEM). The corrosion resistance of coatings were assessed by electrochemical techniques.
A composite sol was made with Ce(NO3)3·6H2O, Al(NO3)3·9H2O and Y(NO3)3·6H2O as precursor. Through this process, we investigated the influence of adding solvent speed, sol temperature, pH value and the amount of PVP on the formation of the composite coatings. Compared to the dip-coated method, the spin-coated method were testified to be more appropriate in achieving the composite coatings with satisfying corrosion resistance. In this study, a composite layer was formed by spin-coating 5 times in a heating rate 0.5°C/s and heat treatment soaking at temperature of 550°C. The composite layer thus made presents high corrosion resistance with average porosity as low as 24%.
Al–Ce–Y alloy coatings were prepared by electrodeposition method. After deposition, the alloy coatings were heat-treated in open air at temperature of 550°C.After heat treatment, Al2O3–CeO2–Y2O3 composite coatings were formed due to the pH value at 2.5~3.5 and temperature between 30~45°C. The deposition current density was about 2.2~2.5 A·cm-2 and deposition time last for 20 to 30 min. The alloy coatings thus deposited are relatively uniform and compact. The composite oxide layer formed after heat treatment is excellent to resist corrosion, as examined using electrochemical techniques.
Key words: sol-gel method; Al2O3–CeO2–Y2O3 composite coating; Al–Ce–Y;electrochemical deposition method
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