Marine durability of 23-year old reinforced concrete beams (cracked and uncracked) of size 15×30×180 cm exposed to marine tidal and atmospheric environments is presented here. The study mainly focused on corrosion of steel bars with respect to orientation in concrete, carbonation and chloride-ion induced corrosion of steel bars, quantitative evaluation of corrosion based on the electrochemical data, the influence of stimips on macrocell formation. The experimental works covered electrochemical investigations (half cell potential, polarization resistance, solution resistance), chloride ion profile and carbonation depth measurements, microscopic observation of steel-concrete interfaces, visual observation of steel bars corrosion, etc. The study concluded that chloride-ion induced corrosion significantly influenced by the orientation of steel bars in concrete. Cracks as well as interfacial gaps between steel and concrete should be taken into account in order to predict the corrosion of steel bars in concrete, i.e. the durability of reinforced concrete structures, especially under tidal environment. Macrocell corrosion was a governing process of corrosion in tidal environment. Conversely, microcell corrosion was a governing process of corrosion in atmospheric environment. The depth of corrosion can be estimated roughly by using polarization resistance data, provided a suitable pit concentration factor is used.