The relevance of this study pertains to coating material performance in desalination and power plants and particularly during cathodic protection (CP) of coated steel piping network. Organic coatings find various uses in many, industries and such coatings are used extensively in conjunction with CP by the petroleum and water industry for corrosion protection. One of the most common technical problems encountered by the inspector or engineers is that of micro-defects (pinholes) in coating and optimization of material selection. The nature of the surface electrochemical reactions associated with defects in these protective coatings and the ability of the Scanning Potential Mapping (SCM) to detect them is the concern of this investigation. The results indicate that this method can monitor the qualitative effectiveness of the cathodic depolarization CP. A demonstration problem is elaborated which gives information on the surface potential distribution on the exterior surface of the coal tar epoxy coated steel substrate with predefined defects. The results demonstrate the evolution with time of the SCM for the coated specimen, which shows initial increase of the surface potential. As the polarization potentials are concerned, the cathodic potential affected mainly their cathodic regions.