Exploring the corrosion behavior of materials in a marine atmosphere and studying their corrosion resistance mechanisms can provide a basis for developing new weathering steels. This paper primarily introduces the initial corrosion behavior and corrosion resistance mechanism of 10CrNi3MoV steel in a marine atmosphere and explores the effects of Ni on the corrosion resistance. The corrosion resistance of 10CrNi3MoV steel and Q235 steel was compared using the weight loss method. The results show that the corrosion rate of 10CrNi3MoV steel was approximately half that of Q235 steel. The rust layer morphology and elemental distribution of 10CrNi3MoV steel were observed via scanning electron microscopy and energy-dispersive X-ray spectroscopy. The composition of the rust layer was analyzed using X-ray diffraction and X-ray photoelectron spectroscopy, and the protective effect of the rust layer was further tested using an electrochemical test. The reason why Fe2NiO4 is beneficial for improving the corrosion resistance of the rust layer was explained using the Vienna Ab initio Simulation Package. The results reveal that the morphology, composition, element distribution, and electrochemical properties of 10CrNi3MoV steel were considerably stronger than those of Q235 steel because of the formation of Fe2NiO4 in the rust layer. Fe2NiO4 refines the rust layer particles and promotes the transformation of corrosion products to α-FeOOH. In addition, it imparts ion selectivity to the rust layer, effectively blocking the invasion of Cl−.