Abstract: External reactor vessel cooling (ERVC) is a key technology in the severe accident mitigation strategy of third generation pressurized water reactor nuclear power plants, and the critical heat flux (CHF) determines the pressure vessel integrity. Studies on ERVC boiling heat transfer enhanced by coating show that coating surface can significantly increase CHF, in order to investigate the aging effect of the real material of nuclear reactor pressure vessel SA508 steel surface on CHF characteristics of coating surfaces, experimental studies on boiling heat transfer characteristics of SA508 steel bare surface and TC4 titanium alloy porous coating surface based on SA508 steel under different aging conditions were carried out. The substrate of SA508 steel bare surface was made of copper block and 3 mm thick SA508 steel by explosive welding. The TC4 titanium alloy porous coating surface was prepared by spraying particles on the bare surface of SA508 steel by supersonic cold gas spray technology. The micro-scale (the diameter of particles is 10-50 μm) TC4 titanium alloy particles were accelerated to a high speed of 400-500 m/s by the propulsion from a stream of high-pressure inert gases with a much lower temperature than thermal spraying. Four kinds of aging experiments were carried out on the SA508 steel bare surface and three kinds of aging experiments on the TC4 titanium alloy porous coating surface at the inclination angle of 5°. The results show that the SA508 steel bare surface oxidizes rapidly in water, and the CHF tends to increase with the deepening of aging, within 20%. The scanning electron microscopy and EDS characterization were used to analyze the surface characteristic. The CHF enhancement is due to the generation of Fe₃O₄ magnetic micro-nano particle layers that improve surface wettability and added the nucleation sites by oxidation of SA508 steel. Nevertheless, the CHF increase due to oxidation is limited. Compared with the SA508 steel bare surface, the CHF of TC4 coating surface without oxidation increases more than 36.9%. The TC4 coating surface changes from super-hydrophilic to hydrophobic with the deepening of oxidation, the oxide generated by substrate SA508 steel is easy to clog the coating pores, and the CHF shows a deterioration trend. The application of porous coating technology to enhance the performance of ERVC needs to consider adding oxidation resistance layer between the SA508 steel substrate and the porous coating surface, and the problem of bonding strength and alternating stress of multilayer coating should be solved. This study helps to deepen the understanding of enhanced heat transfer technology for ERVC and lays the foundation for subsequent research.