This paper studies the individual and combined effect of adding Cu (0.26–1.06 wt%), Cr (0.54 wt%) and Ni (0.92–2.92 wt%) alloying elements to carbon steel with regard to mechanical behaviour and atmospheric corrosion resistance in two marine atmospheres with different chloride deposition rates (20 and 71 mg Cl−/m2 · d). Mechanical properties are assessed in terms of yield strength, ultimate strength, elongation, hardness and weldability, while atmospheric corrosion resistance is evaluated by determination of corrosion losses, rust layer structure, and nature of the corrosion products formed. The best atmospheric corrosion resistance is obtained with weathering steel (3 wt% Ni), which would be suitable for use unpainted in marine atmospheres of moderate salinity (≤20 mg Cl−/m2 · d), though a longer cooling time would be required during its manufacture in order to assure adequate weldability.