Determination of the Corrosion Behavior of Painted Iron 

 and the Inhibitive Action of Paints * 



By R. M. BURNS and H. E. HARING 



THE 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. ' 

 There is, however, an obvious need for dii-ect 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.^ 

 Experience in this and other laboratories has demonstrated that time- 

 potential curves obtained in this manner indicate the corrosion 

 behavior of a metal and the state of its surface.^ In general, it has 

 been found that if the potential of a metal becomes more electropositive 

 (more noble) with time, the formation of a protective film and a 

 retardation or cessation of corrosion is taking place, while conversely, 

 if the potential becomes more negative, solution of a protective film and 

 acceleration of corrosion is indicated. 



* Digest of a paper presented before the Spring Meeting of the Electrochemical 

 Society at Cincinnati, Ohio, April 23-25, 1936, and published subsequently in volume 

 69 of its Transactions. 



1 Schuh, Ind. Eng. Chem., 23, 1346 (1931). 



2 Burns, Bell System Tech. Jour., 15, 20 (1936). 



3 May, Jour. Inst. Met., 40, 141 (1928); Bannister and Evans, Jour. Chem. Soc. 

 (June 1930). 



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