The Research On Preparation And Properties Of Thermal Barrier Coatings On Carbon Steel

Carbon steel is a widely used structural material because of its low prices, good performance, and rich experience. Its surface protection is still an interesting and attractive research field. Recently, great efforts were devoted to the corrosion resistance by deposing a coating on the surface of carbon steel. As a typical thermal barrier coating (TBC) material, 8YSZ (zirconia stabilized by 8wt% Y2O3) coating provides superior thermal performance in high-temperature applications such as diesel engines and gas turbines. However, no study is reported on the protection effect of 8YSZ coating on carbon steel in marine condition. The major objective of this study is to monitor the corrosion resistance of 8YSZ ceramic coating on the surface of carbon steel substrate in seawater to simulate the marine environment.8YSZ ceramic coating with a thickness of about 250μm was air plasma sprayed onto the surface of carbon steel. To discuss the effect of the bond coat, the samples with bond coat with a thickness of about 100μm were obtained. The phase characteristics were carried out by X-ray diffraction (XRD). The surface and cross-section morphology of the samples were analyzed by using scanning electron microscopy (SEM) combined with energy dispersive spectrometry (EDS), and corrosion products were characterized by X-ray photoelectron spectroscopy (XPS). As a nondestructive technique, electrochemical impedance spectrometry (EIS) was applied to monitor the corrosion behavior of the coated samples in seawater,the surface roughness, micro hardness and the adhesion properties were studied during the immersion period.Morphologies of the as-sprayed coatings are very complex, consisting of 8YSZ layers (splats) parallel to the substrate. There exist cracks and voids within the coat, they can develop into channels connecting the metal surface and the outside environment, through which electrolyte and oxygen can reach the metal surface. During the immersion process, the surface roughness increased because of the corrosion production on the surface of 8YSZ top coat, and the adhesion of the samples decreased. For the sample with metallic bond coat, corrosion products were observed between carbon substrate and bond coat, on the 8YSZ coating surface and inside the coating, which has a loose platelike structure and mainly consists ofγ-FeOOH.Equivalent circuits were used to fit the EIS data, carbon steel with an 8YSZ coating showed two time constant, and Warburg impedance appeared later owing to diffusion process near the electrode. The EIS results of bonded coating indicate that three time constants were obtained during the initial immersion period, and then a new time constant appeared due to the formation of rust layer. The corrosion production became thicker when immersed, leading to a degradation of the coating.However, there is no spallation of the coating from the carbon steel even immersed for 100 days, which illustrates that the 8YSZ ceramic coating has good protective performance.