This paper presents the results of laboratory investigation on the corrosion of oil alkyd-coated steel plates with unpainted defects under marine environment. Oil Alkyd paint-coated divided steel plates, (30mm × 30mm) composed of 9 pieces of 10mm × 10mm steel elements with different types of defects, were submerged in 3% NaCl solution after which the corrosion behavior of the steel plates was investigated for a period of 4 months. Macrocell corrosion, polarization resistance and double layer capacitance were measured in the laboratory in order to assess the corrosion behavior of paint coated steel plates with defects. The test results show that the presence of defects enhanced the degradation at the surrounding sound portion of the coated steel plates by causing a reduction in its polarization resistance while enhancing the increase in double layer capacitance. Furthermore, macrocell corrosion occurred between the painted part which acted as cathode and the defect part which acted as anode. Macrocell corrosion was seen as a contributor to the delamination of the surrounding painted portions paving way for the generation and progress of microcell corrosion at the surrounding painted parts.