06Ni9DR alloy steel has become a core material for LNG storage and transportation equipment due to its excellent low-temperature toughness (down to -196 ℃) and low thermal expansion properties. The 06Ni9DR alloy steel is heated at 1100-1250 ℃ before hot rolling. However, there are severe oxidation and difficulties in removing the oxide skin, which leads to increased processing costs. The protective coating method has become one of the main means of high-temperature oxidation protection for alloy steels due to its simple preparation process, wide applicability and freedom from shape and size limitations. In the previous stage, a reaction-sintered Al₂O₃-glass composite coating with 73.6% protection after oxidization at 1200 ℃ for 3 h was prepared by our team. The addition of SiC to deplete the oxygen inside the coating is expected to improve oxidation protection. It is worth noting that when there is liquid phase in the coating at high temperature, the SiC will act as foaming and leave pores in it, which will become the oxygen diffusion channel. Therefore, in this work, SiC-Al₂O₃-glass composite coatings with different silicon carbide contents were prepared, and the effect of silicon carbide content on the oxidation protection and its mechanism were revealed.

The crystal structure of the coating samples after heat treatment was by power X-ray diffraction using a diffractometer (10°-90°, X' pert pro, PHILIPS, Netherlands). The XRD spectra were recorded at 40 mA and 40 kV using Cu K_α radiation (λ=0.1542 nm). The apparent porosity (AP) and bulk density (BD) of the coating samples were measured by Archimedes drainage method. The microstructure and element analysis of the coating samples and steels with coatings were evaluated by using field emission scanning electron microscopy (SEM, EVO-10, ZEISS, Germany), equipped with an X-ray energy dispersive spectrometer (EDS, Xplose30, Oxford, Britain) at 16 kV. The average pore size inside the coating was counted by Image Pro Plus 6.0. The mass loss rate (β) of GC steels with coatings after oxidation test was referred to the Chinese industry standard for the determination and calculation method for oxidation burning loss of slab (YB/T 4773-2019). The pore size distributions and average pore diameter of coatings samples after heat treatment for 3 h were measured by mercury piezometer (5 nm-800 μm, AUTOPORE V9620, Micromeritics Instruments, America).

The composition of the physical phase showed that the SiC in the composite coatings could continuously oxidize to generate the square quartz phase, and the amount of generation increased with the content of SiC and the heat treatment duration. The average pore size of all coating samples after 10 min of oxidation was only about 2.5 μm. The number of macropores in the composite coatings with low SiC content increased significantly, and the average pore size also increased to 30.6 μm with the oxidation duration. The average pore size decreased with the SiC content in the composite coatings after heat treatment for 180 min. The excellent oxidation protection was observed for all steels with coatings after oxidation for 10 min, which was more than 96%. The oxidation protection ratio of the composite coating with the low SiC content decreased to 40.9% for 180 min, and the oxidation protection ratio of the composite coating with the high SiC content remained almost unchanged, up to 96.1%.

The main conclusions of this paper are summarized as following. The mechanism of "foaming attenuation" in composite coatings with low SiC content was revealed. There was a high content of liquid phase in it. During the oxidation process, the oxidation products of SiC, such as CO and CO₂, will be wrapped by the liquid phase. The amounts of gases wrapped inside the coating increased with the oxidation duration, resulting in a significant "foaming" phenomenon in the coating and the formation of large-size pore channels, which significantly reduced the oxidation protection. The mechanism of "oxygen depletion enhancement" in composite coatings with high SiC content was also revealed. The liquid phase content in composite coatings with high SiC content was low, and the generation of square quartz from the oxidation of SiC increased the viscosity of the liquid phase. The foaming effect of SiC was inhibited, resulting in the formation of a microporous structure with an average pore size of about 2.7 μm, effectively extending the oxygen diffusion path. At the same time, the oxygen in the diffusion process will be continuously consumed by the SiC in the composite coating, so that the excellent oxidation protection was provided at 1200 ℃ for different oxidation durations.