<p>The laser cladding and diamond-like carbon (DLC) composite coatings were prepared on the surface of 304 stainless steel (SS) using CO₂ lasers and plasma-enhanced chemical vapor deposition (PECVD) technologies. The corrosion behaviors of the composite coatings in high temperature and pressurized water (290&#xa0;°C, 10&#xa0;MPa) have been investigated. The result demonstrated that the addition of yttrium oxide during laser cladding and the addition of nitrogen during PECVD can significantly affect the oxidation behavior of composite coatings in high temperature and pressurized water. Yttrium oxide can affect the microstructure of the cladding and reduce the number of defects, and the corrosion resistance of the laser cladding layer was improved. The inert DLC film prepared on the surface of the laser cladding layer can effectively mitigate the localized corrosion damage caused by the columnar crystal organization in the cladding layer. In addition, the nitrogen-doped in the DLC film can affect the local solution’s hydrochemistry environment due to the formation and hydrolysis of NH₄⁺ during the service of the composite coating, which in turn affects the corrosion behavior of the coating. The influencing mechanisms of yttrium oxide and nitrogen on the corrosion behavior of the composite coatings are also discussed.</p>