In order to replace the hazardous chromate-based surface treatment, a new cerium chemical conversion coating was developed on 316L stainless steel through a mixed solution of hydrated cerium nitrate, citric acid, and hydrogen peroxide. The chemical composition was characterized by energy-dispersive spectroscopy, X-ray photoelectron spectroscopy and atomic force microscope. The dense conversion coating is composed of CeO2 with a small amount of Ce2O3 and has small grain size lower than 50 nm. Its thickness is about 47.4 nm as determined by spectroscopic ellipsometry analysis. Potentiodynamic polarization was used to study the corrosion behavior of the coatings in the concentrated artificial seawater at 72 °C. In comparison with the conventional nitric acid-chromate passivated specimens, the cerium conversion coatings show much higher pitting potentials. It is suggested that the cerium conversion treatment is more effective than the nitric acid-chromate passivation to improve the pitting resistance of 316L stainless steel used in the hot seawater environments.