Evaluation of Microstructure, Phase Composition and Properties of Multilayer Zirconia Coating on API5L Steel in Condition of Single and Multitime Cooking

The aim of this study is applied zirconia sol-gel coatings on steel substrate API5L to evaluate the effect of number of layer and type of sintering on the corrosion resistance of these coatings. For this purpose the 4 and 2 layer zirconia sol gel coating using zirconium propoxide as a precursor was applied on API5L carbon steel by dipping method at constant speed 6 cm/min in sol. A group of samples for each cycle of immersion, drying at 110 ˚C for 30 min and sintering for 1h at 450˚C, while others group with repeated cycle of dipping drying, the coating was applied to the desired number of layers and the sintering was accomplished at 450˚C for 1h. Before sintering the thermal analysis of TG-DTA was completed in order to determined to the optimum temperature of gel calcinations or sintering of coated samples. The coating film of zirconia was characterized by FTIR, DTA, XRD and SEM. Properties of coated samples evaluated by measurement of microhardness and electrochemical tests. The results indicate for heat treatment at 550 ˚C mixtures of the monoclinic and tetragonal phases and 450 ˚C crystallization of tetragonal phase. multi level sintering for both 4 and 2 layer more homogeneous microstructure, the corrosion resistance and microhardness is higher than one level sintering. Regardless the type of the sintering, properties of the coating 4 layer is better than the 2 layer.