Technical Digests---Determination of the Corrosion Behavior of Painted Iron and the Inhibitive Action of Paints

HE value of paints for the protection of metal surfaces depends upon their effectiveness as physical barriers against the corrosive elements of the surrounding environment and upon the electrochemical activity of the primer pigments in rendering the surfaces passive. Physical testing methods have been developed which furnish valuable information concerning the quality and rate of aging of paint films.1 There is, however, an obvious need for direct methods of determining the condition and behavior of the metal surface beneath the paint film, the rate of penetration of corrosive agents through the film, and the mechanism of the inhibitive action afforded by the film. The present paper describes an electrochemical method of obtaining this information. It is well established that the process of corrosion in the presence of moisture is electrolytic in character--that it occurs by means of the operation of small galvanic cells at the surface of the metal. It should be possible, therefore, to determine the corrosion behavior of a metal by measuring the electrical characteristics of these individual corrosion cells; but the multiplicity and minute size of the anodic and cathodic areas makes such measurements impracticable. However, it is readily possible to determine the resultant of all of the polarized potentials of the anodic and cathodic areas on the metal surface, and to follow the change in this potential (of the electrode as a whole) with time. 2 Experience in this and other laboratories has demonstrated that timepotential curves obtained in this manner indicate the corrosion behavior of a metal and the state of its surface.