Calcareous deposit was formed on Q235 carbon steel surface in nature seawater by applying cathodic protection, afterwards, the corrosion test of the steel covered with and without calcareous deposit was carried out in f/2 culture medium containing Amphora. Then the morphology and chemical composition of corrosion products were characterized by means of XRD, SEM, FTIR and EIS. The result revealed that the calcareous deposit prepared by a current density of -30 µA/cm² showed an even- and uniform-surface morphology composed of nice crystalline phase. The Amphora and its metabolite could adhere to the steel surface forming a biofilm, which could suppress the mass transfer of corrosive species from the medium to the steel surface to some extent, but not obviously that of the oxygen. Calcareous deposit was apt to combine with Amphora forming a composite film on the steel surface, which then could rather effectively suppress the migration of oxygen. The corrosion process of Q235 steel beneath the composite film can be described as the following three stages: as the corrosive media penetrated the composite film on to the steel surface, the steel is corroded leading to the formation of corrosion products, while, with which the combination of the existed biofilm, thus the composite film could act as barrier enabling the corrosion rate to be decreased to some extent; later as damages occurred within the composite film, thereby its protectiveness deteriorated, the corrosion rate of the steel increased again.